Neutrons can be detected only indirectly by nuclear reactions where charged particles are produced. For the purposes of spectrometry, the energy of these charged particles must be related to the energy of the neutron. Two kinds of reactions can be used in neutron detectors: (1) Exothermic nuclear reactions resulting in secondary charged particles, e.g. 3 He(n,p) 3 H + Q (Q = 764 keV). In such detectors, a neutron of energy E n produces an electric signal at the detector output, the height of which is proportional to E n + Q. (2) Elastic scattering of neutrons with nuclei of the filling gas, i.e. production of recoil protons in hydrogen (or a hydrogen-containing gas such as CH 4 ) or of alpha particles in 4 He filling. The maximum energy transfer (in the case of a head-on collision) from the neutron to the recoil nucleus of mass M is given by E max = [4M/(M + 1) 2 ] E n . The result is a distribution of the energies of the recoil nuclei between 0 and E max where each energy value is generated with the same probability.
As radiation passes through the fill gas, ionization occurs. When a high voltage is placed between two areas of the gas filled space, the positive ions will be attracted to the negative side of the detector (the cathode) and the free electrons will travel to the positive side (the anode). These charges are collected by the anode and cathode which then form a very small current in the wires going to the detector. By placing a very sensitive current measuring device between the wires from the cathode and anode, the small current measured and displayed as a signal. The more radiation which enters the chamber, the more current displayed by the instrument.
At very low voltages the field is insufficient to prevent recombination and the collected charge is less than that of the original ion pairs. At higher voltages, ion saturation occurs when the field is high enough to suppress recombination and all charges are collected. This is the normal range of operation for ionization chambers. At still higher voltages, gas multiplication starts to occur, amplifying the charge represented by the original ion pairs created within the gas. Pulses are therefore, considerably larger than those from ion chambers used under the same conditions. In addition, proportional counters can be applied to situations in which the number of ion pairs generated by the radiation are too small to permit satisfactory operation in pulse type ion chambers. Further increases in voltage lead to a region of limited proportionality, wherever non-linear effects start to occur and then finally to the Geiger counter region, where each output pulse from the detector is of the same amplitude and no longer reflects any properties of the incident radiation. While ionization chambers and proportional counters have found an important place in neutron measurement, Geiger M üller counters have not, primarily because of the difficulty in discrimination between photons and neutrons.
Ionization chamber, or “ion chamber”, based instruments are designed to measure exposure rates of ionizing radiation. The detector is usually cylindrical, filled with air and fixed to the instrument. When radiation interacts with the air in the detector, ion pairs are created and collected generating a small current. The amount of ionization charge deposited in air and the measurement of this ionization current will indicate the exposure rate. Ion chamber have a very rapid response time, so they are particularly useful for instruments that may be used in high dose rate applications.
In 1958 Francis Shonka developed what came to be known as A-150 tissue equivalent plastic. To a large extent, A-150 plastic has become the material of choice for constructing the ionization chambers and proportional counters used to measure the absorbed dose to tissue from neutrons and protons. These two ionization chambers were built by Shonka in the 1960s. The larger of the two is 4" in diameter while the smaller chamber is 2" in diameter. A-150 Muscle Tissue Hydrogen 10 % 10 % Nitrogen 3.5 % 3 % Carbon 76.1 % 11 % Oxygen 5.2 % 74 % A-150 plastic is a good but not perfect match for tissue. The hydrogen and nitrogen contents of A-150 and tissue are identical but their carbon and oxygen contents are almost reversed. Fortunately, the oxygen and carbon neutron kerma factors are very similar. And because the kerma factor for hydrogen is so much larger than that of carbon and oxygen, the neutron kerma factors for A-150 and muscle tissue are nearly identical. With regard to the chamber's response to gamma rays and x-rays, the effective atomic numbers of A-150 and tissue are reasonably close.
Proportional counters operate in the proportional region of the gas multiplication curve. Therefore, the pulse height is proportional to the number of ions resulting from a charged particle interaction. Protons and alpha particles produce larger pulses than beta particles or photons. This allows electronic discrimination to reject photons and betas, which makes proportional counters especially useful for detection of high LET particles such as alphas. BF 3 and 3 He fill gases are used for thermal measurement in moderator-based instruments, while H 2 , CH 4 , 4 He, etc. are used for spectrometry.
In 6% of the reactions the resulting alpha particle and 7 Li recoil nucleus share the reaction energy of 2.792 MeV. In the remaining 94% of the reactions the resulting particles share only 2.314 MeV. The main peak is due to the 2314 keV full-energy dissipation in the counter gas. The small distribution (peaked at channel 513) above the main peak is due to full-energy dissipation of particles resulting from transitions to the ground state. The pulse-height distribution below the full-energy peak is due to the partial escape of the alpha particle and/or 7 Li recoil nucleus; the lower limit of the neutron induced events corresponds to the full escape of the alpha particle, implying the full energy dissipation of the 7 Li recoil nucleus (841 keV, channel 155) in the gas. The pulse-height distribution below channel 100 is due to gamma events and noise. The separation of the neutron-induced events from pulses due to noise or gamma ray induced events can be achieved simply by introducing a discrimination threshold just below the lower limit of the neutron induced pulse-height distribution.
3 He counters, which contain nominally 8 kPa of 3 He, provides a system with a response roughly twice that of a 4 mm by 4 mm LiI system. Although it less sensitive to gamma rays than 6 LiI scintillators, the low gas amplification can make it difficult to set a discriminator level which allows all the neutron-induced events to be counted while excluding noise and gamma ray events. The detecting reaction, 3 He(n,p)T (Q = +764 keV), has a cross section for thermal neutrons of 5321 barns. The full-energy peak at about channel 430 corresponds to the reaction energy of 764 keV, the lower-amplitude pulses are due to the partial escape of the proton and/or triton; the lower limit of the neutron induced events, at about channel 100, corresponds to the full escape of the proton (573 keV) implying the full energy dissipation of the triton (191 keV) in the gas. About 100 kPa of krypton is included with the 3 He gas to increase the stopping power. A simple threshold placed at about channel 80 allows the separation of the neutron induced events from noise and gamma ray induced events.
Proportional counters which are operated as recoil detectors can be filled with hydrogen (H 2 ) or a hydrogen-containing gas (CH 4 ) using elastic (n,p) scattering or with 4 He gas resulting in (n,α) scattering. The SP2 recoil proton counters consist essentially of a spherical stainless steel shell about 4 cm diameter, acting as a cathode, and a central tungsten anode wire. In general, the counters are filled with 100 to 1000 kPa (1 to 10 atm) hydrogen, sometimes in a mixture with 2 to 10% of methane or argon that acts as a quenching gas. Some experimenters choose to add traces of 3He to be used as an internal energy calibration source. Alternatively, a weak 239 Pu source may be deposited as an alpha particle emitter in the centre of the counter on the anode wire, which acts as an internal energy calibrator
The advantages of H 2 - and CH 4 -filled recoil proton proportional counters reside in the fact that the counters use the (n,p) scattering cross section: This is well known and changes monotonically with energy. It is isotropic in the centre-of-mass system for neutron energies less than at least 5 MeV. Because of the near equality of neutron and proton masses, it can be described by simple scattering theory so that response calculations are relatively straightforward. It has a reasonably large absolute value so that the counters have a useable efficiency although measurements of several hours are still required in radiation protection applications. It produces recoil protons having a constant mean energy loss of W = 36 eV per one ion pair produced above 3 keV.
Schematic diagram for a spherical proton recoil counter and the processes which occur in it when either a neutron scatters off a hydrogen nucleus or a photon produces secondary electrons. The advantage of CH 4 filling gas is its high stopping power for recoil protons. Above 250 keV the tracks of recoil protons are more than 3.5 times shorter than in hydrogen, so these counters are more suitable at the high-energy end of proportional counter application. One of their main disadvantages is the production of carbon recoils, which contribute to the measured spectra and must thus be taken into account. The maximum energy transfer of a neutron to the carbon nucleus of atomic mass M = 12 is 4M/(1 + M) 2 = 0.28. Moreover, the carbon recoils produce only 75% of the ionization compared with protons, i.e. only 0.28 x 0.75 = 0.21 1/5 of the neutron energy can be detected with the consequence that the distribution of carbon recoils ends at 1/5 of the corresponding neutron energy. The effect of carbon recoils must be taken into account in the determination of the response matrix of the detector and subsequently corrected for.
When a neutron is incident on such a proton recoil counter, various processes can occur: The incident neutron, if scattered by a hydrogen nucleus, produces a recoil proton. The electrons produced along the proton track due to ionizations drift towards the anode wire along an electric field line. Near the wire, the electron gains enough energy for the gas atoms to be ionized. The ionization electrons can themselves produce further electrons and thus an avalanche is produced (gas amplification).
When a neutron is incident on such a proton recoil counter, various processes can occur: The charges are collected at the anode and a signal is produced. The signal is proportional to the amount of ionization and thus to the energy of the recoiling proton (not of the incident neutron). For each neutron energy E n , proton energies E p in the range 0 E p E n can be obtained depending on the scattering angle between the neutron and the proton.
Spherical detectors have isotropic properties. The detector response is nearly independent of the direction of the incident neutrons and hence these spherical detectors are preferably used in multidirectional neutron fields. However, great expertise is required for the production of high-quality detectors with good energy resolution. The purity of the gas filling as well as the constancy of the electric field over the full length of the anode wire are of great importance. The latter must be optimized, relative to the detector diameter, depending on: diameter of the insulator, diameter of the anode wire, diameter of the wire holder, length of the anode wire holder and distance between the insulator and the end of the anode wire.
Cylindrical detectors are not so complicated in their construction and can even be manufactured with large volumes in order to improve their efficiency and extend the neutron energy range of their application. In some cases an upper energy limit of 6.5 MeV has been obtained with large cylindrical proton recoil proportional counters. The active volume of the detector, in which the electric field is perfectly uniform over the length of the anode wire, can be exactly defined with special precautions at both ends of the anode wire (so called field tubes). The disadvantage of cylindrical detectors is their anisotropy with respect to the direction of neutron incidence, which causes problems in multidirectional neutron fields.
The attributes of well made spherical proportional counters such as the SP2 counters include: High energy resolution (E/E in the order of a few per cent) for neutron spectrometry. Isotropic response because of their spherical shape. Work in high thermal and epithermal fields. Tried and tested counters and expertise in their use is available. Can cover the 50–1500 keV energy range where fluence to dose equivalent coefficients vary rapidly with energy. If electronic n/γ discrimination techniques are applied even lower energies can be measured.
Since the counters have a thin anode wire they are highly microphonic. This problem may be solved by enclosing the counters in a firm metal box that would have an optimum thickness in terms of acoustic noise damping and neutron field attenuation. The box could perhaps be made of aluminum or even cadmium. The latter would serve as a shielding against thermal neutrons as well for those counters that may contain traces of 3He for energy calibration. A sheet of lead would reduce the influence of gamma rays on the counter. Since two or three counters are usually to be used in succession, in order to cover the total energy range, longer measuring times are required. They have a relatively low efficiency due to the low nuclear density of the gas filling compared with that of solid or liquid organic scintillators.
For neutron energies below about 2 MeV, and for a spherical proton recoil counter filled with hydrogen at a pressure of 0.1 to 1MPa, the neutron mean free path is about 1 m, which is very much larger than the diameter (4 cm) of the counter. Thus, the counter is almost transparent to neutrons, the predominant reaction process being at most a single elastic scattering event of the incident neutrons by protons. Since the scattering cross section is isotropic in the centre-of-mass system, it follows that for an incident monoenergetic neutron fluence of energy E, the resulting proton recoil energy distribution P(E) would ideally have the characteristic rectangular shape. Provided all proton recoils are stopped within the counter volume, and that the ionization is proportional to the energy Ep of the protons.
Under real conditions this is not the case because of the distortion effects which result from the fact that (although W values are independent of proton energy above a few keV) the number of ions collected at the anode does not give a measure of the proton recoil energy. The reasons for this are: not all recoil protons lose their entire energy within the counter before hitting the wall (wall distortion effects) and gas amplification is not constant over the entire volume, as the electric field strength drops at the ends of the anode wire (gas amplification distortion effects).
The response functions for monoenergetic spectra are calculated using the Monte Carlo code RESP and resolution broadened using either GAUS or RSPG codes from the PTB SPHERE/HEPRO packages. The responses are accordingly optimised by adjusting the counter parameters such as the gas mixture, pressure, resolution parameters, etc. The response functions are considered optimum when calculations and measurements agree well at a few monoenergetic neutron energies.
The upper energy limit is set by wall effect distortions of those recoil protons that cannot deposit their entire energy in the active counter volume and are stopped by the counter wall. The shape of the neutron response function, being rectangular in the ideal case, is considerably distorted for increasing neutron energies and the ‘step’ at the upper end of the distribution, which contains essential information regarding the energy of incident neutrons, progressively vanishes. Wall effects can be reduced by large volume counters and/or filling gases with high stopping power, i.e. mainly high pressure. In practice, large counter volumes are limited by geometrical limitations at the measuring place as well as by increasing problems in quality assurance in the manufacture of large detectors. Reasonable gas pressures go up to the order of 10 MPa.
It is obvious that there is an overlap between neutron- and gamma ray-induced pulses for very low energies which does not allow the pulse shapes to be separated by a simple threshold setting as in the scintillation technique. Due to this overlap, a separate measurement of the gamma background by maintaining identical electronic adjustment is required using a pure gamma source. Since for the usual gamma sources the induced secondary electrons can easily pass through the detector volume, the shape of the measured background spectrum is nearly independent of the special gamma source used ( 137 Cs, 60 Co or others). In the evaluation procedure the measured gamma background spectrum is subtracted after appropriate normalization from the (n+ γ ) distribution in each energy channel and the remaining recoil proton pulses for this energy are integrated.
It is obvious that the requirements for a low influence of photons and low wall effect distortions of recoil protons are contradictory. Consequently, a set of proportional counters with different volumes (if available) and/or gas filling parameters (mixture content and/or pressure) must be used to cover the total energy range from about 50 keV to 1500 keV. In order to decrease the energy limit down to 10 keV or even less, electronic n/ γ discrimination techniques are required in any case. Usually, three counters (e.g. spherical SP2 type), used in overlapping energy ranges, are sufficient (the indicated energy limits are examples only and may vary to some degree).
The maximum fluence and dose rate limits are given by the highest tolerable count rate of the spectrometer, usually being of the order of 5,000 to 10,000 counts s -1 (cps), provided that pile-up rejection and dead-time correction are made correctly in the electronics system. Thus, a 1% proportional counter efficiency means that a corresponding integral fast neutron fluence rate of (0.5 -1) x 10 6 cm 2 s -1 or a dose equivalent rate of 500 - 1000 mSv h -1 , respectively, is an acceptable upper limit.
The minimum fluence and dose rate limits are determined by the statistical uncertainty of the measurement, i.e. by the statistics of the proton recoil spectrum in connection with the affordable measuring time. Particularly, too small a number of counts in the high-energy region of the exponentially decreasing recoil proton spectrum may cause strong oscillations in the unfolded neutron spectrum due to poor statistics. Assuming a required integral number of counts in the proton recoil spectrum of at least a few tens of thousands of events (with special attention to the statistics in the high-energy region), the necessary count rate for a 2 h measurement can be calculated to about 5 cps, corresponding to an integral fast neutron fluence rate of about 5x10 2 cm -2 s -1 or a dose equivalent rate of 500 Sv h -1 for fast neutrons. It is obvious that the lowest level of dose equivalent rates to be measured is a compromise between statistical accuracy and affordable measuring time; thus a definite value cannot be given (e.g. for measurement which is 10 times longer a dose rate limit 10 times lower is acceptable, getting the same statistical uncertainty).
Scintillation detectors employ an optical detection mechanism. Radiation enters detector, and interacts with the detector material, causing a light flash or scintillation. The scintillation is detected by a photomultiplier, or PM tube a optically coupled to the scintillator. This produces a signal that is processed by the processed by electronic system attached to the PMT. A pulse whose height is proportional to the energy deposited in the crystal can be processed. The radiation detected depends on material used and other deign features of the counter.
The 6 LiI(Eu) scintillators are primarily used for moderated detector (multisphere) measurements. The reaction peak appears in a pulse-height spectrum as a broad quasi-Gaussian full-energy peak. 6 LiI crystals are usually connected to a photomultiplier with a light pipe. The reaction energy - 4.780 MeV – is shared by the resulting alpha particle and triton. Where high sensitivity is required larger scintillator crystal sizes can be used, e.g. 4 mm high by 8 mm diameter(3), 8 mm by 8 mm, or 12.7 mm by 12.7 mm, but the subtraction of gamma events becomes increasingly more difficult.
The shape of this peak is determined by the poor pulse-height resolution and the edge effects when alpha particles or tritons partly escape. The quasi-Gaussian distribution due to thermal neutrons is superimposed on an exponential distribution of pulse heights due to gamma events. As the light output of the scintillator for heavy charged particles is much lower than for electrons of the same energy, pulses due to photoelectrons and Compton electrons induced by gamma rays have pulse heights comparable with the neutron induced pulses. A separation between neutron and gamma induced events can be made by fitting the pulse-height spectrum with a Gaussian combined with an exponential function
For neutron energies >1 MeV, organic scintillation detectors are preferable to proportional counters for neutron spectrometry. The energy range of interest can be covered using an appropriate size of one of the different scintillation detectors available -plastic, anthracene, stilbene or liquid scintillators.
In all systems, advantage is taken of a high detection efficiency owing to the high n–p scattering cross section and the higher density of scintillation detectors compared with gas filled proportional counters. The neutron response can be well calculated on the basis of evaluated cross sections, at least for neutron energies of up to 20 MeV. All organic scintillation detectors are equally sensitive to photons and neutrons, and neutron fields are in general contaminated by photons, produced either in the source or by interaction of the neutrons in the shielding and the environment. Photon energies of up to 10 MeV must be taken into consideration in mixed fields.
Plastic scintillators are not suitable for spectrometry in mixed fields because they do not exhibit any n/ γ discrimination capability, but they are frequently employed for neutron Time-of-flight (T.O.F) spectrometry owing to their excellent sub-nanosecond time resolution. Stilbene crystals exhibit excellent n/ γ discrimination properties, but light production by the secondary charged particles depends on the direction of the ions with respect to the axis symmetry of the crystal. The response functions for neutrons depend on the angle of incidence and must therefore be determined for the actual directional distribution of the neutrons. This type of organic scintillator can therefore only be used for the investigation of well-localized neutron sources.
NE213 or BC501A liquid scintillators do not have the directionality drawback of stilbene, and their n/ γ discrimination properties are similarly good. Since xylene is the basic liquid, the container must be carefully prepared, taking into consideration the chemical properties and the rather large expansion coefficient of xylene.
The glass containers used in the early days have been successfully superseded by robust aluminum capsules wrapped up in polyethylene expansion tubes such that the detector can be used in the temperature range from 5°C to 35°C. Any size and shape can be designed and constructed for coupling to one or two phototubes as appropriate for optimal response. Unfortunately, steel cylinders equipped with bellows for expansion, designed for use of the liquid scintillator in any orientation, are not commercially available. Liquid scintillators encapsulated in aluminum are well suited for most applications. Since BA1-type NE213 scintillators are no longer commercially available, liquid scintillators of type BC501A (model MAB-1F) are now the only scintillators available, but these models, too, are a good choice because they are identical to NE213 scintillators in both design and chemical composition.
Range of charged particles produced in an organic scintillation detector by interaction with photons and neutrons.
In principle, the scintillator should be coupled as closely as possible to the phototube. The best pulse height resolution at low amplitudes has, indeed, been achieved when the phototube touched the liquid. Detailed investigations of light transport from the scintillator to the phototube have, however, shown that a partially coated light guide should be inserted between the scintillator and the phototube in order to avoid the need for strange capsule shapes. If the length of the scintillator is not much greater than its diameter, the positional dependence of the light transport can be considerably reduced such that the pulse-height resolution is substantially improved at high amplitudes. In addition, the ground level of the detector capsule and the photomultiplier housing can well be separated from the negative high voltage (~1500–2000 V) at the photocathode and the partially inhomogeneous quantum efficiency of large area phototubes is well averaged out. Last, but not least, this design allows a light-emitting diode (LED) to be coupled such that the LED light can be directed into the scintillator. In this way, gain stabilization can be achieved to compensate for the gain variation due to temperature, magnetic fields and the variable load of the phototube (count rate effect).
Unfolding of the measured pulse-height spectrum selected for neutrons by pulse-shape analysis requires a matrix of response functions for neutron energies in the energy range of interest. The response matrix can be determined experimentally, by calculations or, as is commonly done, by combination of both methods. Taking advantage of multi-parameter data acquisition and neutron TOF measurements, the complete matrix can be established experimentally if the spectral neutron fluence of the calibration field can be determined with an independent reference detector or by calculation. On the other hand, calculation of the response functions, for example by Monte Carlo simulation, requires reliable input data such as detector geometry, chemical composition, neutron cross sections for the interaction with the components of the scintillator (H, C) and the capsule (Al), the light output in dependence on the energy of the charged particles produced and the pulse-height resolution function dL/L(L). These characteristic functions must be determined experimentally by iteratively analyzing all response functions R(E n ) measured for neutron energies from threshold (chiefly determined by the limitation of n/ γ discrimination at low amplitudes) up to the highest energies (at least limited by the ranges of charged secondaries which are higher than the linear dimensions of the scintillator).
Response functions calculated with the NRESP7 code for an NE213 scintillation detector and neutrons in the energy range between 1 MeV and 19 MeV (normalized to unity neutron fluence)
Semiconductor based personal neutron dosimeters work by the same principles as most passive detectors: they both detect charged particles generated in neutron-induced nuclear reactions in the detector itself or charged particles generated in specially selected converter layers mounted close to the detector. Most active detectors will not detect neutrons below 1 MeV unless they contain hydrogenous material.
Active personal neutron dosimeters employing semiconductor detectors work by the same principles as most passive detectors: they both detect charged particles generated in neutron-induced nuclear reactions in the detector itself or charged particles generated in specially selected converter layers mounted close to the detector. Usual semiconductors will not detect neutrons below 1 MeV since they do not contain hydrogenous material.
For measurement in the neutron energy range up to 10 keV, where the greater portion of neutron dose equivalent is produced by 580 keV protons from the 14 N(n,p) reaction, capture reactions in nuclides such as 6 LiF or 10 B can be used. As with passive dosimeters, albedo neutrons can be detected with this type of converter. Such converters are layers upon or incorporated into charged particle detectors such as simple photodiodes, surface barrier detectors or PIPS (passivated implanted planar silicon) detectors. The relatively large energy deposition by the secondary charged particles generally allows discrimination against intrinsic noise and photon events.
At neutron energies above several tens of keV recoil protons from elastic scattering in hydrogen play the important role in generating dose equivalent in tissue. For measurement of dose equivalent, recoil protons from hydrogenous converters can be detected in this energy range. It has been demonstrated that a (CH 2 ) n (polyethylene) converter of thickness of 20 mm will suffice to achieve an acceptable dose equivalent response, if the adjacent charged-particle detector is capable of counting all protons emerging from the converter with energies greater than 10 keV.
This illustrates the use of hydrogenous radiators - polyethylene, CH 2 - together with silicon detectors for neutron dose measurement. Two different thicknesses of polyethylene are used to achieve different energy responses for high and low energy neutrons. In addition, a 10 B radiator is used for thermal neutron measurement.
Semiconductor detectors (SCD) have to be covered with a layer of hydrogenous radiator. The use of commercial SCDs does not allow this layer to be placed directly on top of the chip surface as the bonding wires will be damaged. However, some RAM chips are covered by a thin layer of polyamide to be sheltered against alpha radiation from the ceramic chip carriers. Despite the fact that these layers are too thin to create charged particle equilibrium for a medium energy neutron spectrum they function as a converter. The sensitive part of the SCD is the depleted zone of a reverse biased p-n junction. As there is no free charge existing (other than a very small amount due to thermal fluctuation) all electrons generated in the slowing down of a charged particle can be collected at one electrode of the diode. Beside this desired detecting mechanism, there are two effects which may lead to an error in the interpretation of the results. First, charged particles which stop near and below the sensitive part release a high density of electrons in its vicinity. These charge carriers diffuse partly towards the collecting electrode thus increasing the measured signal. Secondly, it may happen that there is an interaction of the primary neutrons with the silicon substrate which extends approximately 300 μ m further down. Recoil protons from these reactions can also reach the detecting part, which gives misleading information, as in this case the silicon acts as converter.
The energy dependence of the dose equivalent response of the devices being currently developed is lowered at neutron energies below a few hundred keV, because the range of low-energy (recoil) protons is short and thus any insensitive surface layer ("dead layer") between converter and sensitive layer of the detector will reduce the response to these protons. Dead layers, typically ranging from 50 nm to 300 nm, may result from the construction of junction devices which may need a surface electrode or from the basic physics of devices which may result in a surface undepleted layer.
Furthermore, typical noise levels are 3000 electrons and this corresponds to energy depositions of 10 keV to 20 keV in silicon. At low proton energies, the energy in each pulse is similar to that deposited by interactions of photons. Typical sensitive layer thicknesses are of a few tens to a few hundreds of μ m. The ranges in silicon of 50 keV and 100 keV electrons are, respectively, 24 mm and 78 mm, and their LETs 1.2 keV mm -1 and 0.76 keV mm -1 . Thus, there will be many non-neutron induced pulses of a few tens of keV.
Whereas the measurement of low-energy neutrons with converters such as 6 LiF, employing an exoergic reaction, does not exhibit any problems because of the high energies of the secondary charged particles, there are problems of discrimination against photons for the counting of recoil protons. One commercially available device with an electronic threshold to cut off photon-induced signals showed a strong energy dependence of its dose equivalent response at intermediate energies (over-response in the range of 2 keV to 100 keV). Two-diode devices are used to subtract the photon component with paired detectors only one of which has a hydrogenous converter layer. With solid state detectors such as p n junctions this allows the detection threshold for fast neutrons via recoil protons to be reduced, in principle, to about 200 keV from about 500 keV using just an electronic threshold. Photon discrimination is also possible, in principle, with pulse shape analysis which, however, requires specially designed electronics.
Another approach to neutron/photon discrimination for recoil proton counting involves measurements in very small volumes since neutron-induced events are generally confined to smaller volumes than photon-induced events. Small volumes can be effected by arranging strip or even pixel structures with dimensions of a few μ m as arrays on one silicon chip and allowing an anti-coincidence measurement between neighboring elements in order to suppress photon events which usually deposit their energy in more than one element. This way the threshold should be reduced to the order of 100 keV, with acceptable noise level. Ideally, charge-coupled devices (CCDs) would exhibit the smallest volumes and low noise, however, there may be problems with the dead layers involved.
The fast neutron sensor is a 10x10 mm 2 p-type silicon on which an amorphous silicon hybrid is deposited. The fast neutron sensor is in contact with 80-mm-thick polyethylene radiator to produce recoil protons, and can measure neutrons with energy >1 MeV. The slow neutron sensor is also a 10x10 mm 2 p-type silicon on which a natural boron layer is deposited around an aluminum electrode to detect a and Li ions from the 10 B(n, α ) 7 Li reaction, and sensitive to neutrons of energy<1 MeV.
Direct Ion Storage is basically a small ion chamber. In a nonvolatile solid-state memory cell, information is stored in the form of an electronic charge trapped on the floating gate of a MOSFET transistor. The amount of charge in each memory cell could now be made fully variable at will and therefore the memory cell could be used to store analog information such as non-digitized speech directly. This is a standard Analog-EEPROM memory cell.
The charge on the floating gate can be set to a predetermined level by tunneling electrons through the oxide layer. The charge is then permanently stored on the gate because in the normal operating temperature range the electrons have a very low probability of exceeding the energy barriers in the metal-oxide and oxide-silicon interfaces. The stored information is read without disturbing the charge stored, by measuring the channel conductivity of the transistor. In order for ionising radiation to have an effect on the charge stored, either a new charge would have to be brought to the gate or some existing charge removed. Ionising radiation incident on the oxide layer will produce electron-ion pairs but due to the very low mobility of charge carriers in the oxide, recombination occurs with high efficiency and most of the free charge is neutralised before it has a chance to cross the metal-oxide interface.
If the oxide layer surrounding the floating gate is provided with an opening allowing the surface of the floating gate to be in direct contact with the surrounding air (or any other gas), any ionising radiation incident in the air or gas space now produces electron-ion pairs with extremely high mobility and, if there is an electric field surrounding the floating gate, these charge carriers can very efficiently be transferred to the gate before recombination occurs. The result is a change in the stored charge proportional to the amount of radiation incident on the detector.
A new type of personal dosemeter has been developed by RADOS Technology Oy. This system is based on a novel detector type, an ionization chamber with so-called direct ion storage (DIS). Ionization chambers are sensitive to both neutrons and photons. Therefore, the application of the DIS principle to neutron dosimetry requires a double-chamber system for separating photon from neutron dose components. A chamber with high and a chamber with low neutron sensitivity must be used, and their signals have to be differentiated. The energy dependences for photons of the two chamber types have to be almost equal to obtain good photon discrimination and to lower the risk of false neutron dose measurements in mixed neutron/photon fields.
Prototype dosemeters with a high response for fast neutrons were built with tissue equivalent wall materials A-150 and polyethylene (PE) for detection of recoil protons. For the measurement of thermal neutrons A-150 containing different contents of boron nitride (BN) and PE containing LiNO 3 were used. Thermal neutrons can then be detected by the secondary charged particles of the (n, α ) reaction with 10 B and 6 Li, respectively. Teflon and graphite, which are materials with low interaction probabilities with neutron radiation, were used for the construction of dosemeters with low neutron sensitivity.
Superheated droplets are suspended in a firm elastic polymer. Neutrons trigger droplets giving rise to visible vapor bubbles trapped at formation sites. The number of bubbles is a measure of the neutron dose.
Superheated emulsions operate like the bubble chamber, long used in high-energy particle physics. Bubble chambers utilise a pressurised homogeneous liquid that is sensitized for brief periods of time when it is brought to a transient superheated state by dropping the applied pressure. Conversely, superheated emulsions are continuously sensitive since the liquid is kept in steady-state superheated conditions, i.e. above its boiling point. Neutron induced charged particles generate trails of submicroscopic vapour cavities inside the droplets; when these cavities reach a critical size, the expansion becomes irreversible and the whole droplet evaporates. The amount of energy and the critical size required for bubble nucleation depend on the composition and on the degree of superheat of an emulsion.
Several light halocarbons borrowed from the refrigeration industry are typically used in the preparation of superheated emulsion. These highly volatile liquids must be homogeneously dispersed in a compliant material such as a soft polymeric or aqueous gel. This emulsifier matrix must be suitably immiscible and inert, so that the droplets neither dissolve nor lose their properties due to chemical or interface reactions. The emulsification procedure is rather complex and the difficulties increase when a monodispersed emulsion is sought, i.e. a dispersion of droplets of the same size. Two laboratories, at Yale University and at AECL Chalk River, developed superheated emulsion manufacturing technology, which prompted the creation of two commercial companies. The two versions of the emulsions are slightly different because host media are, respectively, an aqueous gel in superheated drop detectors and a stiffer polyacrylamide compound in bubble (damage) detectors. Detectors containing homogeneous and stable suspensions of 10 4 -10 5 droplets of 20–100 m diameter are currently available in both versions.
Two different types of bubble detectors have been produced. The version developed by Bubble Technology is passive. The number of bubbles formed from neutron interactions is counted after irradiation. The Apfel detector is active. Each bubble formation is detected acoustically using transducers that are coupled to the detector. When the bubble forms, a small “pop” is emitted and detected with the acoustical device.
Has been developed in passive or active versions. In the active version, bubble formation is detected by acoustic pickup. The detectors are extremely sensitive to neutrons (down to µSv range), and are generally completely insensitive to gamma rays, depending on the specific formulation. They can also be formulated with neutron energy thresholds from <20 keV to several MeV.
Active methods of neutron detection
International Atomic Energy Agency NEUTRON DOSIMETRY AND MONITORINGActive Methods of Neutron Detection
Active Methods of Neutron Detection - Unit ObjectivesThe objective of this unit is to present a summaryof the active detection mechanisms that can beapplied to personal and area monitoring, andcalibration instrumentation used for neutrondosimetry.At the completion of this unit, the student shouldunderstand how the detection mechanisms foractive methods employed in current neutronmonitoring problems function. The studentshould also have a general understanding of theadvantages, disadvantages, and areas ofapplication of each of these methods. International Atomic Energy Agency
Active Methods of Neutron Detection - Unit Outline Introduction Gas Filled Detectors Ionization chambers Proportional counters Scintillators Thermal neutron detection Fast neutron spectrometry Semiconductor Detectors Silicon Diode Based Detectors Direct Ion Storage Detectors Superheated Emulsion (Bubble) Detectors International Atomic Energy Agency
INTRODUCTION International Atomic Energy Agency
Neutron detection Neutrons can be detected only indirectly by charged particles from nuclear reactions. For spectrometry applications, the energy of these charged particles must be related to the energy of the neutron. Two kinds of reactions can be used in neutron detectors: Exothermic nuclear reactions Elastic scattering of neutrons with detector nuclei International Atomic Energy Agency
Exothermic reactions Result in secondary charged particles, e.g. 3He(n,p)3H + Q (Q = 764 keV). A neutron of energy En produces an electric signal at the detector output, the height of which is proportional to En+Q. International Atomic Energy Agency
Elastic scattering Elastic scattering of neutrons with nuclei of the filling gas, i.e. production of recoil protons in hydrogen (or a hydrogen-containing gas such as CH4) or of alpha particles in 4He filling. The maximum energy transfer (in the case of a head-on collision) from the neutron to the recoil nucleus of mass M is given by Emax = [4M/(M + 1)2] En International Atomic Energy Agency
GAS FILLED DETECTORS International Atomic Energy Agency
Gas filled detectors Voltage supplyIncident Electricradiation current or pulse Anode+ measuring device Cathode-Fill gas, e.g. air, CH4, etc. International Atomic Energy Agency
Gas filled detectors Region Process* I II III IV V VI Log detected charge, Q0 I Recombination II Ionization III Proportional p+ Limited IV proportionality e- V Geiger VI Breakdown* Red indicates useful for Voltage neutron applications International Atomic Energy Agency
Ionization Chambers International Atomic Energy Agency
Ionization chambers The ionization current indicates exposure rate. Very rapid response time. Dual chambers used for neutron measurement. Air equivalent walls and air fill gas for photons A-150 Tissue Equivalent plastic walls and T.E. fill gas for neutrons + photons Difference = neutrons Relatively insensitive Neutron applications - Used mainly for calibrations. International Atomic Energy Agency
Ionization chambers International Atomic Energy Agency
Proportional Counters International Atomic Energy Agency
Proportional counters Pulse height is proportional to the number of ions resulting from a charged particle interaction. Proton and alpha particles produce larger pulses than a beta particle or photon. Discriminator can reject photons and betas. BF3 and 3He fill gases used for thermal measurement. H2, CH4, 4He, etc. used for spectrometry. International Atomic Energy Agency
Pulse-height spectrum with BF3 gas International Atomic Energy Agency
Pulse-height spectrum with 3He gas counter International Atomic Energy Agency
Proportional counters Operated as recoil detectors Filled with H2 or CH4, using elastic (n,p) scattering, or 4 He gas resulting in (n,α) scattering. SP2 proportional proton recoil counter International Atomic Energy Agency
Proton recoil proportional countersUse the (n,p) scattering cross section: Well known and changes monotonically with energy. Isotropic in the centre-of-mass system for neutron energies less than at least 5 MeV. Described by simple scattering theory - calculations are relatively straightforward. Reasonably large absolute value so that the counters have a useable efficiency. Recoil protons have a constant mean energy loss of W = 36 eV/ ion pair produced > 3 keV. International Atomic Energy Agency
Spherical proton recoil counter International Atomic Energy Agency
Processes occur in a proportional counter The incident neutron, if scattered by a hydrogen nucleus, produces a recoil proton. Electrons produced along the proton track due to ionizations drift towards the anode wire along an electric field line. Near the wire, the electron gains enough energy for the gas atoms to be ionized. Ionization electrons can produce further electrons and thus an avalanche is produced (gas amplification). International Atomic Energy Agency
Processes in a proportional counter Charges are collected at the anode and a signal is produced. Signal is proportional to the amount of ionization and thus to the energy of the recoiling proton (not of the incident neutron). For each neutron energy E , proton energies E n p in the range 0 ≤ Ep ≤ En can be obtained depending on the scattering angle between the neutron and the proton. International Atomic Energy Agency
Spherical proton recoil counters Spherical detector response is nearly independent of the incident neutron direction. Fill gas purity and constant electric fields are important. Electric field constancy depends on: Diameter of the insulator, Diameter of the anode wire, Diameter of the wire holder, Length of the anode wire holder and Distance between the insulator and the end of the anode wire. International Atomic Energy Agency
Cylindrical proton recoil detectors Not as complicated in their construction. Can be manufactured with large volumes for increased sensitivity and energy range. The active volume of the detector can be exactly defined with special precautions at both ends of the anode wire – “field tubes”. Disadvantage – anisotropy. Problems in multidirectional neutron fields. International Atomic Energy Agency
Attributes of SP2 proportional counters * High energy resolution (ΔE/E in the order of a few per cent) for neutron spectrometry. Isotropic response. Work in high thermal and epithermal fields. Tried and tested counters - expertise in their use is available. They cover the 50–1500 keV energy range where fluence to dose equivalent coefficients vary rapidly with energy. With electronic n/γ discrimination lower energies can be measured.* Characteristic of most, well made spherical proportional counters International Atomic Energy Agency
Proton recoil proportional counter disadvantages May be highly microphonic. Solution - Enclosing counters in a firm metal box for acoustic noise damping with minimum neutron attenuation. May be made of aluminum or cadmium. A sheet of lead reduces gamma rays. Since 2 or 3 counters may be used in succession to cover the energy range, longer measuring times are required. Low efficiency due to the low fill gas density compared with solid scintillators. International Atomic Energy Agency
Ideal response function of a hydrogenrecoil counter to monoenergetic neutrons For a monoenergetic neutron fluence of energy E, the proton recoil energy distribution P(E) would ideally have the characteristic rectangular shape. International Atomic Energy Agency
Response of a hydrogen recoil counter Not the case because of distortion effects - the number of ions collected at the anode does not give a measure of the proton recoil energy: Not all recoil protons lose their entire energy within the counter before hitting the wall - wall distortion effects - and Gas amplification is not constant over the entire volume (electric field strength drops at the ends of the anode wire) - gas amplification distortion effects. International Atomic Energy Agency
SP2* counter response to 144 keV neutrons* 100 kPa International Atomic Energy Agency
Proportional counter response functions * Type SP2* Type SP2 International Atomic Energy Agency
n/γ discrimination International Atomic Energy Agency
Typical SP2 application energy ranges International Atomic Energy Agency
Proton recoil counter maximum sensitivity Maximum fluence and dose rate limits usually of the order of 5000 to 10,000 counts s-1(cps). Pile-up rejection and dead-time correction must be made correctly. Given a 1% proportional counter efficiency - the corresponding integral fast neutron fluence rate of (0.5 - 1) x 106 cm2 s-1 ⇒ dose equivalent rate of 500 - 1000 mSv h-1, respectively, is an acceptable upper limit. International Atomic Energy Agency
Proton recoil counter limit of detection Determined by the statistical uncertainty and acceptable measuring time. Assume a required number of counts is at least a few tens of thousands of events. Necessary count rate for a 2 h measurement is about 5 cps: Fast neutron fluence rate ≈ 5x102 cm-2 s-1, or Dose equivalent rate of 500 µSv h-1. Definite values cannot be given. International Atomic Energy Agency
SCINTILLATION DETECTORS International Atomic Energy Agency
Scintillation detectors Radiation enters detector material. Interaction causes light flash (scintillation). Scintillation detected by photomultiplier. Signal processed by electronics Pulse height proportional to energy deposited. International Atomic Energy Agency
Scintillation detection Photocathode Window Reflector P.M. Photoelectron tube from photocathode Scintillator γ β+Source α β- Neutron p U.V. photons produced Dynode (secondary Anode from local excited electron emission) states following ionization International Atomic Energy Agency
Thermal Neutron Detection International Atomic Energy Agency
Thermal neutron detector – 6LiI(Eu) Used for moderated detector (multisphere) measurements. Reaction energy - 4.780 MeV - shared by the resulting alpha particle and triton. Appears in a pulse-height spectrum as a broad quasi-Gaussian full-energy peak. LiI crystal usually connected to a 6 photomultiplier with a light pipe. Crystal sizes typically: 4 mm x by 8 mm, 8 mm x 8 mm, or 12.7 mm x 12.7 mm. International Atomic Energy Agency
6 LiI pulse-height spectrum4mm x 4mm detectora-b. Range of fitted datac. Gaussian peakd. Photon backgrounde. Sum of fitted componentsf. Lower discriminator limit International Atomic Energy Agency
Fast Neutron Detection International Atomic Energy Agency
Organic scintillation detectors Organic scintillators are best neutron spectrometry at higher energies (>1 MeV). Scintillation materials include: Plastic Anthracene Stilbene Liquid scintillators International Atomic Energy Agency
Organic scintillation detectors High detection efficiency due to: High n-p scattering cross section, and Higher density of scintillation detectors compared with gas counters. Neutron response well calculated from cross sections, up to 20 MeV. All organic scintillation detectors are equally sensitive to photons and neutrons. Photon energies of up to 10 MeV must be taken into consideration in mixed fields. International Atomic Energy Agency
Organic scintillator characteristics Plastic scintillators not suitable for mixed fields – no n/γ discrimination. Good for neutron time-of-flight spectrometry - excellent sub-nanosecond time resolution. Stilbene crystals have excellent n/γ discrimination. Light production by the secondary charged particles depends on the ion direction. ∴ neutron response functions depend on angle of incidence - must be determined for the actual neutron directional distribution. International Atomic Energy Agency
Liquid scintillator characteristics NE213 or BC501A liquid scintillators do not have directionality drawback. n/γ discrimination properties are also good. Xylene is the basic liquid, so container must be carefully prepared. Concerns: Chemical properties Rather large xylene expansion coefficient. International Atomic Energy Agency
Liquid scintillator characteristics Aluminum capsules in polyethylene expansion tubes - can be used from 5°C to 35°C. Any size and shape can be constructed for coupling to one or two phototubes as appropriate for optimal response. Liquid scintillators encapsulated in aluminium are well suited for most applications. NE213 scintillators no longer available BC501A (model MAB-1F) are identical to NE213 in design and chemical composition. International Atomic Energy Agency
Charged particle ranges organic scintillators International Atomic Energy Agency
NE213 organic scintillator assembly NE213 Plexiglas scintillator lightguiden/γ field Light emitting diode (LED) Photomultiplier International Atomic Energy Agency
Calculated NE213 response function 10 MeV neutrons International Atomic Energy Agency
Calculated NE213 response functions International Atomic Energy Agency
SEMICONDUCTOR DETECTORS International Atomic Energy Agency
Silicon Diode Based Detectors International Atomic Energy Agency
Detection mechanisms for SSD Use same principles as passive dosimeters. Detect charged particles in detector, or Use converter layers (e.g. polyethylene). Silicon diodes. Direction ion storage. International Atomic Energy Agency
Semiconductor based detectors Semiconductor detectors detect charged particles generated in neutron-induced nuclear reactions: In the detector itself, or Charged particles generated in converter layers mounted close to the detector. Conventional semiconductors will not detect neutrons below 1 MeV since they do not contain hydrogenous material. International Atomic Energy Agency
Detection of low energy neutrons Radiators such as 6LiF or 10B can be used. Albedo neutrons can be detected with this type of converter. Converters are layers upon or incorporated into charged particle detectors. Secondary charged particle energy deposition allows discrimination against intrinsic noise and photons. International Atomic Energy Agency
Detection of higher energy neutrons Above several tens of keV recoil protons from elastic scattering in hydrogen play the important role in generating dose equivalent in tissue. Recoil protons from hydrogenous converters can be detected in this energy range. A 20mm (CH2)n converter will provide an acceptable dose equivalent response. International Atomic Energy Agency
Illustration of a Si diode neutron dosimeter International Atomic Energy Agency
Cross section of a silicon detector n n γ n γ ≥2 mm Radiator ~2 mm Air gap Dead layer ~4 μm SiO2 Positive traps Holes Electrons~300 μm Compton electrons Protons Silicon substrate International Atomic Energy Agency
Energy dependence Range of low-energy recoil protons is short. Detector “dead layer" between converter and sensitive layer reduces the proton response. Dead layers (~ 50 nm to 300 nm) result from: Construction of junction devices requiring a surface electrode, or Basic physics of devices which may result in a surface undepleted layer. Typical noise levels correspond to energy depositions of 10 keV to 20 keV in silicon. International Atomic Energy Agency
Energy dependence At low proton energies, pulse energy is similar to that deposited by photon interactions. Typical sensitive layer thicknesses are of a few tens to a few hundreds of µm. Silicon ranges of 50 keV and 100 keV electrons are, respectively, 24 mm and 78 mm ⇒ LETs are 1.2 keV mm-1 and 0.76 keV mm-1. Result many non-neutron induced pulses of a few tens of keV. International Atomic Energy Agency
Neutron-photon discrimination Two-diode devices can be used to subtract the photon component with paired detectors. With S.S. detectors, the fast neutron detection threshold via recoil protons can be reduced to ~200 keV using just an electronic threshold. Pulse shape analysis can be used for photon discrimination, but needs special electronics. International Atomic Energy Agency
Alternate approach to n/γ discrimination Use of very small volumes ⇒ neutron events confined to smaller volumes than γ events. Small volumes: Arrange strip or pixel structures ~ few μm dimensions as arrays on one silicon chip. Anti-coincidence between neighboring elements suppresses photons. Threshold should be reduced to ~ 100 keV, with acceptable noise level. Charge-coupled devices (CCDs) would have smallest volumes and low noise, but dead layers may cause problems. International Atomic Energy Agency
Example of a silicon based detector International Atomic Energy Agency
Direct Ion Storage Detectors International Atomic Energy Agency
DIS detectors are small ion chambers Information stored as charge trapped on the floating gate of a MOSFET transistor. Charge in each memory cell can be made fully variable. Result memory cell used to store analog information. Control gate Silicon oxide Oxide Electron tunneling Floating gate Analog-EEPROM paths memory cell Source Drain Si International Atomic Energy Agency
DIS detectors are small ion chambers Charge on floating gate set by tunneling electrons through the oxide layer. Charge is permanently stored on the gate. Stored information is read without disturbing the charge stored, by measuring the channel conductivity of the transistor. Radiation incident on the oxide layer produces electron-ion pairs but most of the free charge is neutralised before it has a chance to cross the metal-oxide interface. International Atomic Energy Agency
Cross section of DIS Fill gas Opening Floating gateOxide Electron tunneling path Source Drain Si Modified transistor with ion chamber International Atomic Energy Agency
DIS detectors for neutron dosimetry Ion chambers can be made sensitive to neutrons and photons. DIS for neutron dosimetry requires two chamber system. One chamber with high neutron sensitivity. One chamber with low neutron sensitivity. Signals must be differentiated. Photon energy dependence of the chambers must be almost equal. International Atomic Energy Agency
Dual ion chamber DIS system Neutron sensitive detector Neutron insensitive detector thermal fast neutrons neutrons photons electrons photonsα particles protons electrons A-150 plastic with BN Graphite or Teflon Ion chamber with A−150/PE Ion chamber with Teflon or containing BN/LiNO3 graphite International Atomic Energy Agency
SUPERHEATED EMULSION (BUBBLE) DETECTORS International Atomic Energy Agency
Bubble Damage Polymer Detector Superheated droplets are suspended in a firm elastic polymer. Neutrons trigger droplets giving rise to formation sites. Number of bubbles is a measure of the neutron dose. International Atomic Energy Agency
Bubble formation steps in superheated emulsions International Atomic Energy Agency
Halocarbons used in superheated emulsions Boiling Critical Empirical point point Chemical name formula T (°C)* Tc (°C) b 1,2-dichlorotetrafluoroethane C2Cl2F4 3.65 145.7 Octafluorocyclobutane C4F8 -6.99 115.22 Dichlorofluoromethane CCl2F2 -29.76 111.8 1,1,1,2-tetrafluoroethane C2H2F4 -26.07 101.2 Hexafluoropropylene (HFP) C3F6 -29.40 85.0 Monochloropentafluoroethane C2ClF5 -39.17 79.9 Octafluoropropane C3F8 -36.65 71.95* At atmospheric pressure (101 kPa) International Atomic Energy Agency
Superheated Drop detectors use different detection mechanisms Bubble Technology TM APFEL Liquid Matrix Bubble Dosimeter Superheated Drop Detector Cap Glass or Plastic Event Event Tube Acoustical Acoustical Elastic Polymer Transducer Transducer (Gel) Trapped Bubbles ~1 mm diam. Superheated Anti-Coincidence Counting and Liquid Drops Circuitry Display Circuitry ~0.025 mm diam. Noise Acoustical Transducer International Atomic Energy Agency
Bubble damage polymer detector Passive readout – optical bubble detection. Active readout – acoustical detection of bubble formation. Extremely sensitive to neutrons (in µSv range). Completely insensitive to gamma rays. Can be made with neutron energy thresholds from <20 keV to several MeV. International Atomic Energy Agency
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