An Experimental Study of the Relative Response of Plastic
Scintillators to Photons and Beta Particles
A Kumar1 and A.J. Waker 1
1)

Faculty of Energy System and Nuclear Science, UOIT, 2000 Simcoe St. N, Oshawa ON, Canada, L1H 7K4.

Abstract
A scintillation counting system has been constructed with the use of BC-400 and EJ-212 series plastic
scintillators along with a subminiature photomultiplier tube to investigate the effect of increasing plastic
scintillator thickness on system integrated counts. Measurements have been carried out using four
different gamma sources with different energies ranging from 6keV to 1.332MeV and a Ni-63 beta
source of maximum energy of 66keV. A simulation was also carried out in MCNP4a to verify the
number of H-3 beta particles with max energy 18.6keV that would reach the plastic scintillator in a
vacuum setting as well as in an air medium. Scintillator thicknesses ranged from 10μm to 2500μm. The
response of the system was determined by measuring the integrated counts as a function of scintillator
thickness. The results of these measurements showed a positive linear correlation between scintillator
thicknesses and integrated counts for all the gamma sources while the slopes of the correlations of eachgamma source was a function of the source energy. The beta particle response showed an initial increaseof counts with scintillator thickness followed by a slight decrease. The MCNP simulation confirmed ananalytical calculation of the fraction of H-3 beta particles for a given air concentration that would reachthe scintillator. These results in conjunction with the experimental findings will be used to assess thepotential of a plastic scintillator system forming the basis of a tritium monitor for the detection of tritiumin high energy gamma backgrounds for Canadian nuclear power workers.
Keywords: Plastic scintillator, Tritium, MCNP, gamma photons, beta particles

1.

Introduction
Mixed field Dosimetry incorporating low energy beta particle together with high energy gamma

radiation has been a challenge in the past continues to be the subject of current investigations. Tritium inparticular presents a hazard in nuclear power plants as it is emits low level beta radiation with an averageenergy of the beta particles being approximately 5.6keV.1 The beta particles emitted from tritium have amaximum energy of 18.6keV which is considered to be low energy. Although these beta particles cannot
penetrate the outer layer of human skin, internal exposure can occur through inhalation and absorption
through the skin and can contribute as much as 30% to the occupational exposure of Canadian nuclear
energy workers.2 This exposure is high enough to warrant the development of personal detection
methods to monitor and control exposures in high energy gamma backgrounds such as nuclear power
plants.

1

The purpose of this research project is to study the relative response of plastic scintillators to photons
and beta particles in order to determine the feasibility of building a tritium monitor that can operate in ahigh energy gamma background. For this research four gamma sources were used, Iron-55(Fe-55),
Americium-241(Am-241), Cesium-137(Cs-137) and Cobalt-60(Co-60). The beta source used forphysical experimentation was Nickel-63(Ni-63) while a computer simulation was also created to test therange of tritium particle in air and vacuum settings using the Monte Carlo N-Particle (MCNP) code,
version 4A.

2. Materials and Methods
2.1 Experimental Setup
The plastic scintillators chosen for the experiments were the BC-400 series from Saint Gobain
Crystals and EJ-212 series from Eljen Technologies. The scintillators absorb the incoming radiationenergy emitted from the source and reemit it in the form of light which is then picked up by aphotomultiplier tube (PMT). The PMT multiplies the current produced by the light via multiple dynodestages to enable photons to be electronically detected. Both the EJ-212 and the BC-400 series plastic
scintillators are used for general purpose type scintillation detection and are sensitive to both gamma
photons and beta particles.
The experimental setup consisted of the Hammatsu R7400U miniature PM, chosen due to its
vertical mount and easy placement of plastic scintillators directly into the well of the PMT. The PMTfeatured a 16mm diameter entrance window with a 12mm seated length and the E5780 base connector
with three connecting wires. One of these wires was used as a ground, another for the high voltage
supply and the third for the outgoing signal which was connected to a preamplifier. A cylindricalaluminum holder was also constructed for proper alignment of the PMT, scintillator and source. To
minimize scattering, optical grease was applied onto the well of the PMT and the scintillators. The EJ550 optical grease was purchased from Eljen Technologies was effective in enhancing the results
obtained for each trial.

2

Figure 1: Schematic of detector set up with connections to signal processing electronics
Gamma photon interaction with matter depends on the energy and atomic number of the absorber andresults in attenuation while beta particles are essentially electrons which lose energy based on the
distance travelled. Considering the range of beta particles and the attenuation of gamma photons, an
optimal scintillator thickness is required which limits attenuation for high energy gamma photons yet isthick enough to register beta particles emitted from a specific source. Based on these principles it was
hypothesized that the optimal scintillator thickness would need to be thin enough to still register counts
from beta particles while being insensitive to most gamma photons.

2.2 Description of Simulation Setup
MCNP 4a was used to simulate the number of tritium beta particles that would interact with thescintillator over a 5 minute period. This was done by building a simple setup which tallied the number ofbeta particles reaching the PMT. The source was defined as a volumetric source surrounding the PMT
inside a given volume. The number of H-3 beta particles detected would then be compared to the numberof high energy gamma photons recorded in an identical 5 minute period.
The MCNP4A simulations were compared to the calculated theoretical maximum range of H-3 beta
particles which was found through equation 3 given below.
3

where

……………………………………. (1)Assuming the density of air to be 1.2041kg/m3

Using the above formula, the maximum range of a tritium beta particle is calculated to beapproximately 5mm in air. This range is used as the height to obtain the volume of air in which H-3 beta
particles are able to reach the PMT and was found to be approximately 550mm3 given the radius of 6mm
which resembles the diameter of the scintillation disks. The final step was to determine the number ofcounts over a 5 minute period which was calculated to be 49.5counts for a concentration of 1 DAC(300kBq/m3)4.
This expected number of counts represents a situation where all the H-3 beta particles travel
towards the PMT. A realistic assumption would include dividing the counts in half to account for the factthat the beta particles are emitted isotropically and on average only a half of the emitted betas will be
travelling towards the PMT. The resulting counts would thus yield approximately 24.7counts over a 5
minute period. In order to confirm these results, an MCNP simulation was needed which would measure3the number of H-3 beta particles that would reach the scintillator in an air medium. The MCNP
simulation is shown in Figure 2.

Figure 2: MCNP setup for the determining the number of H-3 beta particles reaching the detector for agiven tritium concentration

3. Results and Discussion
3.1 Gamma Sources
The results with the gamma sources were in agreement with the hypothesis and are illustrated in
Figure 3. The measurements were repeated for each thickness yielding an uncertainty of 2.6% on eachtrial.

Figure 3: Shows a linear increasing trend with the lowest energy gamma source (Fe-55) having thesteepest slope and the highest energy gamma source (Co-60) having the smallest slope.

4Figure 3 illustrates an increasing linear correlation as scintillator thickness increases. The
integrated counts on the y-axis indicate the number of counts registered by the detector which is thenumber of photons emitted from each source that interact with the polymer matrix of the scintillators.
Since Co-60 has the highest energy it also has the lowest attenuation coefficient (0.0794cm-1) andtherefore most of the photons emitted from Co-60 pass through the scintillator undetected. Cs-137 has
the second highest energy which corresponds to the second lowest attenuation coefficient (0.1082cm-1)
among the four isotopes tested thus explaining its slope being comparable with Co-60. Am-241 has aneven greater slope than either Co-60 or Cs-137 which coincides with it having the second highest
attenuation coefficient (0.255cm-1) while Fe-55 has the steepest slope due to its attenuation coefficient
being higher still (23.0251cm-1).

3.2 Beta Source
The beta source used for the experiment was a Ni-63 sealed source having a maximum energy of
65.87keV. The max range for this source was found to be 63.45μm calculated using equation 1. Basedon this range, the expected result was to observe a rapid rise in the slope of the counts up to 64μm
followed by a plateau at a constant level. At this thickness all the beta particle energy from the source
would be converted into scintillation light and registered by the PMT and sufficient to induce a pulse
measureable by the multichannel analyzer. Following an initial rise in counts where most of the beta
particles are detected, there would not be expected a significant increase or decrease in the integrated
counts as none of the beta particles being emitted by the source have an energy or range greater than
65.87keV or 63.45μm respectively. The results are shown in Figure 4.

Figure 4: Ni-63 Results for Integrated Counts Vs Increasing Scintillator thickness showing an initial
increase in counts up to 50μm followed by a gradual decrease as scintillator thickness increases.
Figure 4 shows an increase in Ni-63 counts up to a thickness of 50μm followed by a constantgradual decrease in the number of counts. Any extra material that is added beyond the maximum range
for Ni-63 beta particles acts as an obstruction material limiting the number of light photons reaching the5photocathode of the PMT. This explains the gradual decrease in the number of counts as the scintillation
thickness increases.
Once a complete set is recorded for Co-60 and Ni-63 through each scintillator thickness an
optimal scintillator thickness can be established. Upon analysis, the initial hypothesis of a thin
scintillator thickness being optimal was found to be incorrect. This was due to Co-60 counts being
distributed over the range of pulse heights as the thickness of the scintillators increased. The distribution
in the pulse height spectrum changes as the scintillator thickness changes. To find the optimal scintillator
thickness, Co-60 counts were compared to Ni-63 for each scintillator thickness, to find the highest ratio
between the counts of the two sources.

Figure 5: Co-60 spectrum at 500μm scintillator thickness

Figure 6: Co-60 at 100μm scintillator thickness
Figure 5 and 6 allow for comparisons between two trials measured for Co-60 at 100μm
scintillator thickness and 500μm scintillator thickness. These two trials show how the pulse heightspectrum shifts as the thickness of scintillator increases. Co-60 emits photons of energies at 1.332MeV
and 1.172MeV, which mainly interacts through Compton scattering. Depending on the thickness of thescintillator, the incoming photons will either generate a single Compton electron with average energy
and a range that is greater than the scintillator thickness or will create multiple Compton electrons
through more than one scatting interaction. In either case, it is expected that more electron energy would
be deposited in the scintillator from a single photon interaction with increasing scintillator thickness.
This increase in deposited electron energy per interacting photon will produce pulses of greater heightand so the pulse height spectrum is observed to extend to higher channel numbers with increasing
scintillator thickness. These shifts in the pulse height spectrum pose a problem in determining optimal
scintillator thickness due to the fact that beta emitters result in a constant spread over the spectrum
channels.

6Figure 7: Ni-63 at 500μm scintillator thickness ends at channel 128

Figure 8: Ni-63 at 100μm scintillator thickness ends at channel 135
Figure 7 and 8 shows the distribution of pulse heights for Ni-63 trials illustrating that there is no
significant shift in the pulse heights as the scintillator thickness increases. Taking into account the
spectrum shift for high energy gamma photons (Co-60), as well as the consistent channel spread for beta
particle emissions from Ni-63, an overlapping of the channels was utilized as a method for obtaining the
optimal scintillator thickness. Each scintillator thickness for both sources over the same channel intervals
was analyzed by calculating and obtaining a ratio of Ni-63counts to Co-60 counts. The ratio yields thedifference for each scintillator thickness between beta particles and high energy photons that result froma given exposure to both sources.

Figure 9: Overlap of Co-60
and Ni-63 counts

7

Figure 9, shows an overlap of Ni-63 counts over Co-60 counts at a scintillator thickness of
1000μm. The white-dot scatter plot on the figure indicates the pulses from a Co-60 source, while theyellow-dot scatter plot indicates those from the Ni-63 source. The shaded pink region shows the regionof interest which extends from channel 11 to channel 149 where counts from both sources are observed.
This methodology was used for each scintillator thickness with the results presented in Table 1 andFigure 10.
Thickness(μm) Co-60
Ni-63
50
22975
46552910021976
488332250
19157
474018500
18494
4613871000
25351
4402891500
24213
4178402000
23574
403716
Table1: Co-60 and Ni-63 count comparison

Ratio of Average Ni-63 counts to
Co-60 counts

30
25

Ratio

20
15
10
5
0
0

500

1000
Thickness

1500

2000

Figure 10: Ratio of Average Ni-63 counts to Co-60 counts
From Figure 10, a ratio of 25 corresponds to two scintillation thicknesses at 250μm and 500μm.
The thinner 250μm scintillator is chosen as the optimal thickness since it may be better suited to theexperiment as it would provide an increased insensitivity to gamma photons in comparison to the 500μm
scintillator thickness and therefore an overall reduction in count rate.
Once this optimal thickness was established, the exposure rate for the Co-60 source overa 5 minute time period was required. This was done using equations 2 and 3 below as well as values forthe mass stopping power for plastic scintillators and air from the ICRU Report 44. The activity of the
Co-60 source was estimated from the activity at the time of purchase to be8.

Exposure at 1m:------------------------------------------------------------ (2)Using the activity of the source, equation 2 and the inverse square, the exposure rate at a distance of
was calculated as.
…………...…………………………………………………….. (3)Finally equation 3 and the above calculated values were used to estimate the dose rate as
Therefore 19,157 counts over a 5 minute period was found to be equivalent to 237μGy/hr

3.3 Tritium (MCNP4a) Results
The MCNP4a results were in agreement with the calculated results. Two scenarios were attempted in
MCNP4a which featured a vacuum and air medium respectively with particles in both scenarios being
emitted isotropically. The tally for the vacuum scenario yielded a surface tally of 0.15893, the ratio of
the number of beta particles that crossed the PMT surface to the total number of beta particles emitted.
In order to obtain the number of counts over a 5 minute period, the ratio obtained from the MCNP run is
now used. A volumetric H-3 beta source was simulated with a diameter of 21mm. The photocathodediameter of the PMT was 16mm and an extra 5mm was added in order to ensure the source was a bit
larger than the PMT diameter allows for beta particles to reach the PMT from a solid angle of 2π. Using
this radius, the volume of the source was found and multiplied by the surface tally and the DAC fortritium to calculate an activity of 50.5 counts over a 5 minute period for 100,000 histories in a vacuum
scenario.
The second simulation run for beta particles in a medium of air resulted in the tally through the plastic
scintillator to be 0.06728. Using this ratio, the previously calculated volume and the DAC, the activity
over a 5 minute period was calculated to yield 21.4 counts.
These counts are in reasonable agreement with the calculated values where beta particles were assumed
to travel isotropically in air (24.7 counts).
The simulated value deviates from the calculated value by 15% which can possibly be explained due tothe scatter that would have occurred when air is present. The assumption of only half the beta particles
reaching the detector was an overestimation as the MCNP version shows that slightly less than half reachthe scintillator.

4. Conclusion
The purpose of this research was to obtain experimental data on the gamma ray sensitivities ofplastic scintillators as a function of scintillator thickness as well as to gain a better understanding of theinteraction of beta particles with these detector materials. A correlation was found between the slope ofthe number of detected photons as a function of scintillator thickness. The attenuation of gamma rays as

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a function of energy also supported the experimental data as the attenuation coefficient decreased, fewer
photons were stopped for a given scintillator thickness and vice versa. Another objective of the research
was to analyze the system’s response to beta sources as a function of scintillator thickness and Ni-63 wasused for this purpose. An optimal scintillator thickness for greatest photon-beta discrimination was foundto be 250μm by comparing integrated counts for the highest energy gamma source (Co-60) to the low
energy beta source (Ni-63) over the same number of pulse-height channels. At this thickness, the highenergy photon background would result in approximately 19,000 counts being registered by the detector
for a dose of 237μGy/ hour over a 5 minute period. For the same photon dose rate, the number of H-3
betas detected would have to be greater than this value for the tritium signal to be discernable over thesame interval period of 5 minutes. Using MCNP4a to model the interaction of tritium beta particles withthe detector system and any approximate analytical calculations a tritium concentration of 1DAC was
calculated to producing approximately 25 counts over a 5 minute interval. Since the H-3 counts werevery low in comparison to 19,000 counts from Co-60 a tritium monitor may not be possible; however the
optimization of scintillator thicknesses might be applied to other beta-photon mixed field dosimetry
situations as well as for low energy gamma radiation such as Fe-55 in a high energy gamma background.

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