The experiment tested two boron trifluoride proportional gas tube neutron detectors and a REM sphere dose meter. The gas tubes detected slow neutrons below 0.5 eV using the 10B(n,α)7Li reaction, while the REM sphere could detect fast neutrons above 0.5 eV after they were slowed. Count rate and pulse height data were taken for the gas tubes at varying voltages to determine optimal operating voltages. Spectral measurements found that gamma rays were effectively blocked. Neutron fluxes were significantly attenuated outside shielding containing boron and cadmium. REM sphere measurements from 1-16 feet from a californium source yielded experimental dose rates from 1.07-77.2 μSv/hr

1. Experiment #10: Neutron
Detection
By
Jared Crown
Laura Drumm
Dan Maierhafer
Rich Woodruff
EES611
April 25, 2001

2. I.Abstract
This paper details the operating characteristics of two Boron Tri-fluoride
Proportional Gas Tube Neutron Detectors and a REM Sphere dose meter.
The BF3 detectors are used to detect slow neutrons below 0.5 eV, while
the REM sphere can be used to detect fast neutrons above 0.5 eV
because it has an integral neutron slowing material, which slows the
neutrons, so they are detectable with the proportional gas tube detector.
The mechanism of detection is the 10B (n,α) 7Li reaction with the Boron
inside the gas tube. Data on tube voltage was taken along with count rate
and pulse height in order to find the counting plateau for each of the two
gas tubes.
The spectrum was taken using the larger gas tube detector, which was
placed at various locations inside and outside the shielding enclosure. It
was found that the neutron flux was significantly attenuated outside of the
shielding enclosure due to the slowing effect of the Boron impregnated
plastic inside the enclosure, and the subsequent absorption of neutrons in
the Cadmium skin.
The REM sphere was moved at one-foot increments between 1 and 16
feet from an unshielded 4.11 MN/s 252Cf source. The experimental Dose
equivalent ranged from 1.07 uSv/Hr at 16 feet away to 77.2 uSv/Hr at 1
foot away. The calculated theoretical Dose Equivalent ranged from 1.81
uSv/Hr to 464 uSv/Hr. This difference was due mainly to detector
efficiency, and loss of counts due to the “Wall Effect.”
II.Summary
Neutrons are somewhat similar to gamma rays in that they carry no
charge, and do not interact with matter through Coulombic forces.
Interactions with neutrons occur at the nucleus of an atom and may
scatter the neutron, or cause the nucleus to emit secondary radiations
(Knoll, p.55).
Neutrons can be divided into 2 major classifications: Slow and Fast. The
dividing line between slow and fast neutrons occurs at about 0.5 eV, which
is the same energy of the abrupt drop in neutron capture cross section of
the element Cadmium. It is very difficult to detect fast neutrons, because
the cross section of most materials is very small for fast neutrons, so they
pass right through the material. Slow neutrons need to be slowed down
by elastic collisions with something of similar mass, like the proton in a
2

3. hydrogen atom. After slowing, they can be absorbed in an (n,α) reaction
that will convert the neutron into an alpha particle, so that it is detectable.
A good neutron capture reaction occurs with 10B (n,α) 7Li. This reaction
has two Q-values: 2.792 MeV when the resulting Li atom is at ground
state, and 2.310 MeV, which occur when the resulting Li atom is at an
excited state. The excited state occurs 94% of the time, while the ground
state occurs 4% of the time. The cross section of this reaction is 3840
barns (3840 x 10-24 cm2) (Knoll, P.56-57).
A sealed gas proportional tube was used as the neutron detector in this
experiment. Gas detectors which detect charged particles may contain
P-10 gas, which is a mixture of 90% Argon and 10% Methane. For
neutrons, however, the gas used containing Boron is 10BF3 (Knoll, P.511).
If the detector volume were infinitely large, all neutron energy would be
absorbed and reconverted to alpha particle energy. This would give a
single peak at each of the two emission Q-values of the 10B (n,α) 7Li
reaction. Sometimes the energy of the alpha particle or the Li atom
escapes the detector volume. For this case, the detected energy will be
less than the peak value. There are two of these cases for the 10B (n,α)
7
Li reaction, corresponding to the alpha particle escaping the detector, or
the Li atom escaping the detector. This is called the “Wall Effect.”
A device known as the REM Sphere can detect fast neutrons. This device
first slows the neutron down and then uses a gas proportional tube to
detect it. It turns out that a 12-inch diameter sphere can instantly detect
the equivalent dose delivered by the neutron.
The neutron detection experiment was carried out using the equipment
and samples listed in Table 1.
3

4. Table 1. List of Lab Equipment Used for Neutron Detection
Type Manufacturer Model Number
Neutron 5.17ug 252Cf (3/31/1997) SR-Cf-334-A
Source
Radioactive 1 uCi 137Cs (662 KeV) N/A
Sample
Preamp Canberra 2006E
(X0.5)
Amplifier Canberra 2012
HV Supply Tennelec TC945
Oscilloscope Tektronix TAS250
Large LDR, Inc. 20256
Neutron
Detector
Small N-Wood Counter Lab, Inc. G-5-1
Neutron
Detector
First count rate and pulse height data versus tube voltage was recorded
for the large neutron detector. The count rate and pulse height versus
tube voltage was plotted. The Count Rate curve clearly shows a flat
region between 200 –1200V, within which a stable operating tube voltage
lies. The Pulse Height Curve narrows this down even more, and shows a
constant slope proportional region between 700-1400V. The operating
voltage chosen was 1000 volts because the Count Rate curve was flat,
and the Pulse Height curve looked like it was in the Proportional Region.
Next, the count rate and pulse height data versus tube voltage was
recorded for the small neutron detector. The count rate and pulse height
versus tube voltage was plotted. The Count Rate curve shows a relatively
flat region between 400-1300V, within which a stable operating tube
voltage lies. The Pulse Height Curve narrows this down even more, and
4

5. shows a constant slope proportional region between 800-1500V. The
nameplate-counting plateau was defined as the range from 1550 to 1750
volts, so we chose an operating voltage between these two points.
After this, a 137Cs gamma ray source was placed next to each detector.
The energy spectrum was taken to identify the energies emitted by the
source. Since natural gamma rays would interact with the tube wall and
emit a relatively low energy Compton, this detected peak energy level
(662 KeV) can be ignored because neutron interaction energies would be
significantly higher (in the MeV range). An amplitude discrimination was
then placed on the MCA, so these gamma ray energies would be ignored.
The detector was then moved to different positions around the inside and
the outside of the neutron source enclosure. The skin of the enclosure
was made of Cadmium, which was a good neutron absorber, and the
enclosure was filled with Boron impregnated plastic. The function of the
plastic was to slow down the neutrons to energies at which the Cadmium
skin could absorb. Some low energy neutrons may have undergone a 10B
(n,α) 7Li reaction also.
A REM Sphere was used to detect the neutron emission rate at distances
from 1 to 16 feet from the source. The conversion factor in Equation 1
was used to obtain experimental dose equivalent rate knowing gross
count rate:
Equation 1: 41cpm = 1mrem / hr
The experimental dose equivalent rate was then converted from mrem/hr
to Sv/hr and varied from 1.07 uSv/hr at 16 feet to 77.2 uSv/hr at a one-foot
distance.
In order to determine theoretical dose equivalent rate, the mass of the
source was converted into a neutron emission rate using Equation 2.
Equation 2: S _ dot ( N / s ) = 2.3 * 10 6 ( N / s / ug ) * mass(ug )
This source was decay corrected, and the corrected neutron emission rate
was used in Equation 3 to calculate neutron flux.
S _ dot ( N / s )
Equation 3: Φ ( N / cm * s) =
2
4*Π *r2
5

6. Next, neutron flux was used in Equation 4 to find the Dose Equivalent
Rate.
Φ ( N / cm 2 * s) * DF (rem / hr ) /( N / cm 2 * s)
Equation 4: H ( Sv) =
100
The results are tabulated in Table 2.
Table 2. Spreadsheet used to calculate Equivalent Dose Rate from
Neutron Source using REM Sphere
Exp. Dose Exp. Dose Neutron Flux Theoretical Dose
Distance (cm) Ns+b (cts) ts+b (min) S+B (cpm) Equivalent (mrem/hr) Equivalent (Sv/Hr) (N/cm^2*s) Equivalent (Sv/hr)
487.68 219 5 43.8 1.0683 1.06829E-06 1.3735 1.81E-06
457.2 186 5 37.2 0.9073 9.07317E-07 1.5628 2.06E-06
426.72 196 5 39.2 0.9561 9.56098E-07 1.7940 2.37E-06
396.24 189 5 37.8 0.9220 9.21951E-07 2.0806 2.75E-06
365.76 210 5 42 1.0244 1.02439E-06 2.4418 3.22E-06
335.28 244 5 48.8 1.1902 1.19024E-06 2.9060 3.84E-06
304.8 288 5 57.6 1.4049 1.40488E-06 3.5162 4.64E-06
274.32 362 5 72.4 1.7659 1.76585E-06 4.3410 5.73E-06
243.84 462 5 92.4 2.2537 2.25366E-06 5.4941 7.25E-06
213.36 321 3 107 2.6098 2.60976E-06 7.1760 9.47E-06
182.88 452 3 150.6666667 3.6748 3.6748E-06 9.7673 1.29E-05
152.4 614 3 204.6666667 4.9919 4.99187E-06 14.0649 1.86E-05
121.92 337 1 337 8.2195 8.21951E-06 21.9765 2.90E-05
91.44 487 1 487 11.8780 1.1878E-05 39.0693 5.16E-05
60.96 987 1 987 24.0732 2.40732E-05 87.9059 1.16E-04
30.48 3165 1 3165 77.1951 7.71951E-05 351.6237 4.64E-04
6

7. The theoretical dose equivalent rate varied from 1.81 uSv/hr for a 16-foot
distance from the source to 464 uSv/hr for a distance of one foot away.
These data are plotted in Figure 1.
Plot of Neutron Dose Equivalent Rate versus Distance for REM Sphere
Exp. Dose Equivalent (Sv/Hr)
Theoretical Dose Equivalent (Sv/hr)
Power (Exp. Dose Equivalent (Sv/Hr))
Power (Theoretical Dose Equivalent (Sv/hr))
1.0E-03
Neutron Dose Equivalent Rate (Sv/Hr)
1.0E-04
-2
y = 0.4312x
2
R =1
1.0E-05
-1.6682
y = 0.0222x
2
R = 0.9901
1.0E-06
1.0E-07
10 100 1000
Distance between REM Sphere and Cf-252 Source (cm)
Figure 1. Neutron Dose Equivalent Rate versus Distance between
252
Cf Neutron Source and REM Sphere Detector
As expected, the theoretical dose equivalent rate was higher than the
experimental dose equivalent rate because the efficiency must be less
than 100% in the gas tube. The loss of counts is due to the efficiency and
to the “Wall Effect” described earlier.
7

8. The Experimental Dose Equivalent Rate was divided by the Theoretical
Dose Equivalent Rate, and plotted in Figure 2.
Ratio of Measured Equivalent Dose/Calculated Equivalent Dose for REM Sphere
0.7
0.6
0.5
Ratio of Hmeasured/Hcalculated
0.4
0.3
0.2
0.1
0
0 100 200 300 400 500 600
Distance between REM Sphere and Cf-252 Source (cm)
Figure 2. Ratio of Measured Dose Equivalent divided by Theoretical
Dose Equivalent Rate versus Distance between 252Cf Neutron
Source and REM Sphere Detector
The Hmeasured / Hcalculated ratio was very low at short distances to the
source. This may have been because the closer the detector was to the
source, the more an error in position mattered in the detected neutron
8

9. count rate. At middle distances, the ratio was about 0.3, whereas the ratio
started to increase sharply at about 400 cm distance and more. This may
have been the result of positioning not being such a large factor in neutron
count rate.
References:
Knoll, Glenn F., Radiation Detection and Measurement. Third Edition.
John Wiley & Sons, Inc., 2000.
9