2. Modes of Radiation Detectors
Current Mode: Radiation is converted into a current, and the current is manipulated
and detected. This is how smoke detectors operate.
Pulse Mode: Radiation is converted into a voltage pulse, and the voltage is
manipulated and detected. This is more useful for spectroscopy.
Ionization chambers can be operated with either modes.
4. A Common Application for Ionization Chambers
(Current Mode Ionization
Chamber)
Smoke Detectors: They utilize an air chamber and an Am-241 source. Am-241
undergoes alpha decay.
The half life of Am-241 is approximately 432 years and the alpha energy is 5.5 MeV.
This produces a constant current in the ionization chamber.
When smoke particles enter the detector, they can interact with the alpha particles,
making them neutral, and disrupting the current which causes the alarm to go off.
6. SRIM Features Utilized
Using gas targets.
The projected range of an ion in the gas for different energies.
Ionization or Stopping Power for the Ions and the recoiling target atoms.
Since SRIM is a Monte Carlo code, the simulation is done until the projected range
converges.
For gases, the gas is considered to be at standard temperature and pressure. Also
only a limited number of gases can be simulated accurately using SRIM. This is due
to lack of experimental data and general theory. Gases that were used for this
project that are simulated accurately in SRIM: Ar, He, H, N, air, O, and methane.
7. Assumptions used in Modeling the
Chamber
The chamber is at standard temperature and pressure.
Only alpha particles are simulated.
The source is placed inside the chamber. This is to ensure that radiation deposits its
energy inside the gas of the chamber.
Recombination inside the chamber is negligible. This is justifiable if a sufficient
voltage is applied.
9. Decay of Interest for the Majority of the
Presentation
.1 micro Ci Po-210 source
Only undergoes alpha decay with energy of 5.3 MeV
Half Life of 138.4 days
10. Goals of the Simulations
Determine an ideal gas to use in the chamber.
The projected range of ions in this gas is small allowing the chamber to be more
portable.
This gas produces a large amount of ion pairs under incident radiation. This allows
for a larger signal pulse (in the form of a voltage).
Energy loss by the recoiling gas particles is minimized.
11. Stopping Power and Bethe Formula
-Linear Stopping Power is defined as the differential energy loss for a charge particle
moving through a material divided by a differential path length
S= −
ⅆ𝐸
ⅆ𝑥
- The value along the trajectory of the particle is referred to as its specific energy loss
- This is described by the Bethe Formula
12. Stopping Power and Bethe Formula
𝑆 =
4𝜋ⅇ4 𝑧2
𝑚0 𝑣2 𝑁𝐵
Where B is:
𝐵 = 𝑍[ln(
2𝑚 𝑜 𝑣2
𝐼
) – ln(1 -
𝑣2
𝑐2 ) -
𝑣2
𝑐2]
N is the density of the material while Z is the atomic number of the material.
𝑚 𝑜, 𝑒 is the mass and charge of the electron while z and v are the charge and velocity of
the particle respectively.
I is the Ionization Potential of the material.
Since we are only concerned with gases (their low density at standard temperature and
pressure) and alpha particle at non-relativistic velocities (this causes the second and
third term of B to become negligible), the most important term is the atomic number of
the gas particles. The higher the atomic number of the gas particles, the larger the
specific energy loss.
13. Stopping Power in SRIM
Stopping Power is referred to as Ionization in SRIM and is giving in units of
(eV/angstrom).
Data can be extracted for the stopping power of the ion and recoiling gas particles.
From SRIM calculations, it is clear that the specific energy loss of recoiling gas
particles is negligible compared to ions.
14. Bragg Curve for 5.3 MeV Alpha Particle
in Air and Air Recoil Ionization
15. Main Characteristics of The Bragg
Curve
For the majority of the ions trajectory, specific energy loss rises steadily but slowly.
Towards the end, the particles specific energy loss rises rapidly and reaches a peak
(this is due to the square of the velocity in the denominator and that the ion is
effectively in the vicinity of the gas particles for a relatively longer period of time).
At the end of the trajectory, for the case of an alpha particle, the alpha particle
picks up 2 electrons becoming neutral and its specific energy loss drops to 0.
16. Average Ionization for Different Gases
for 5.3 MeV Alpha
0.00E+00
2.00E+04
4.00E+04
6.00E+04
8.00E+04
1.00E+05
1.20E+05
1.40E+05
Ar He H2 N2 Air O2 CH4
ionization(eV/mm)
Gas
Average Ionization
17. Peak Ionization for Different Gases for
5.3 MeV Alpha
0
50000
100000
150000
200000
250000
300000
350000
Ar He H2 N2 Air O2 CH4
PeakIonization(eV/mm)
Gas
Peak Ionization
18. Projected Range for Different Gases for
5.3 MeV Alpha
0
50
100
150
200
250
Ar He H2 N2 Air O2 CH4
ProjectedRange(mm)
Gas
Projected Range
19. Experimental Conclusion from
Ionization Simulations
Smaller peak ionization (specific energy loss) or smaller average ionization (specific
energy loss), lead to larger projected ranges.
This is self-explanatory since ionization (specific energy loss) is energy loss per
distance traveled.
20. W-Values
The energy that is loss by the alpha particle is used to ionize the gas particles.
For different types of incident radiation (beta and alpha particles) and different
gases, different amounts of energy is needed to form an ion pair.
These values are called W-Value.
Their units are in eV/ion-pair.
These values were taken from Knoll.
21. W-Values for Different Gases from
Alpha Particles
26.3
42.7
36.4 36.4
35.1
32.2
29.1
0
5
10
15
20
25
30
35
40
45
Ar He H2 N2 Air O2 CH4
W-Values(eV/ionpair)
Gas
W-Values (From Knoll)
22. Calculations Done
For the Ionization curve showed above, a Reiman Sum was taken. This gives the
total energy loss of the total particle.
A Reiman Sum was also done for the recoil ionization curve, in order to prove that
the ionization of the recoil particles are negligible.
It is then assumed that all the energy that was loss by the ion, is used to ionize gas
particles.
This energy is divided by the W-Value for the gas. This yields the total ion pairs
formed.
23. Energy Loss by Ions and Recoil Particles
5280000
5282000
5284000
5286000
5288000
5290000
5292000
5294000
5296000
Ar He H2 N2 Air O2 CH4
EnergyLoss(eV)
Gas
Reiman Sum Energy Loss by Ions and Recoil Particles
Ions Recoil
25. Converting Ion Pairs into Voltage
In a given gas, the ionized atoms move much slower than electrons under the
direction of an electric field. (due to the mass difference). For a typical chamber
dimension, the time it takes for an ionized atom to reach its corresponding
electrode is on the order of milliseconds while for electrons its on the order of
microseconds.
Therefore we are only going to take into account the signal due to electrons. This
can be done by making the RC time constant that is between the electron travel
time and the ion travel time.
The electrons create a current when they come close to the electrodes. This current
can be used to charge up a capacitor.
26. Converting Ion Pairs into Voltage
This leads to a peak voltage across the capacitor as a function of the capacitance
and the number of electrons.
𝑉𝑚𝑎𝑥 =
𝑛∗1.6𝑥10−19
𝐶
Arbitrarily, a capacitance of 100 pF is chosen.
In reality though, the voltage pulse will have a slightly smaller peak voltage due to
geometrical characteristics of the chamber and entrance of the ion into the
chamber.
27. Peak Voltages Calculated for a 5.3 MeV
Alpha Particle in Different Gases
0
0.00005
0.0001
0.00015
0.0002
0.00025
0.0003
0.00035
0.0004
Ar He H2 N2 Air O2 CH4
Voltage(V)
Gas
Peak Voltage
29. Conclusions
Argon yields the largest peak voltage for a given alpha energy.
This is because Argon has a relatively large atomic number (Z= 18) compared to
the other gas elements and has a small W-value.
Argon also has the smallest amount of ionization due to the recoiling gas particles.
5.3 MeV alpha particles also have approximately a 4 cm range in Argon gas. This
easily allows for smaller, portable ionization chambers.
30. Corresponding Different Alpha
Energies to Voltages
If we pretend that we live in a fictitious universe where there is only alpha decay, it
is possible then to create a plot of peak voltage versus energy and range versus
energy.
This would allow us to determine the alpha energy by observing the peak voltage
of a pulse. From here we can use the energy to determine the source of the alpha
particle.
We of course do not live in a fictitious universe, and much more complicated
processing is necessary since other decays are possible and can ionize gas in the
chamber.
