Tons of radioactive wastes that pose great challenges to environmental safety are produced globally in nuclear facilities. It is necessary to characterize these radioactive wastes accordingly to ease the process of treatment. Characterization helps in decision-making and estimating the radiological hazards of such radioactive materials at decommissioning sites (Kang et al., 2020, p. 106). However, characterizing and measuring radioactive materials that emit beta particles have always proved to be difficult due to their low energy emission. And conventional detection methods such as the Geiger-Muller counter and ionization chambers have been ineffective. Thus, other measurement techniques, including liquid scintillation counting, have been employed to detect such low energy beta particles (Kang et al., 2020, p. 1).

Low energy beta particles are difficult to measure because they cannot penetrate the thick medium that separates the detector and the isotope. The solution to this problem is to attach the beta emitter to the detector. The common way of achieving this is to use the NaI scintillation detector. Scintillation detectors detect the amount of radiation by measuring the light produced by the material through ionization (Bourland, 2016, p. 85460). NaI scintillation detector uses a sodium iodide coupled photomultiplier tube to amplify the detected signal from the crystal. NaI detectors are used to detect low energy emitting particles because they are very sensitive (Bourland, 2016, p. 85460). The iodine present in the NaI detector is 127I which is non-reactive. However, exposing the iodine (127I ) to a beam of neutrons activates it, converting it to iodine 128I. 128I. Is a radioactive material that decays to emit beta particles and forms a stable xenon atom-54. The half-life of the 128I is 25 minutes. Therefore, this experiment investigated methods for obtaining energy spectrum from beta decay and further outlined techniques for determining endpoint energy.

Theory

Beta particles are generated from the ionization of the low atomic mass neutrons (Kang et a., 2020). Beta particles are like electrons. They have the same mass as electrons, and they are either positively or negatively charged (L’Annunziata, 2007, p. 132). However, beta particles differ from electrons in different ways. One way is that they originate from the nucleus of an atom (L’Annunziata, 2007, p. 132), while electrons occupy the shells around the nucleus (L’Annunziata, 2007). The mass of the beta particles is small when compared to that of protons or neutrons. Research shows that the mass of a beta particle is 1/1838 of that of the neutron (L’Annunziata, 2007).

As mentioned, beta particles originate from the nucleus of the decaying atom (L’Annunziata, 2007). Therefore, they can be generated through ionization of the low atomic mass neutrons. Beta particles are produced when the gamma radiation converts into energy that is of the same mass as beta particles. Beta particles can be produced in pairs, one positive and the other negative, depending on the type of gamma rays produced (L’Annunziata, 2007). Additionally, beta particles lose their energy in the same way as gamma rays, but with some significant differences (CHOPPIN et al., 2002, p. 137). Beta particles can lose a fraction of their energy in a single collision because of their small masses. Consequently, they are greatly scattered out of their beam path and undergo wide-angle deflection when they collide with an impenetrable surface (Choppinn & Raydberg, 2002).

Most of the beta particles' energies are lost during the ionization. The velocity of beta particles is higher than that of alpha particles. This is due to their smaller masses as compared to heavy particles. Because of their high velocity, the specific ionization of beta particles is lower than that of other heavy ions (CHOPPIN et al., 2002, p. 137). Consequently, their momentum is considerably high compared to orbital electrons.

Materials and methods

The experiment was performed on a virtual computer software system available on the university website. The only requirement was the computer software, which had all the data set ready for the experiment. To access the software, one had to log in to the university website, navigate to the enhanced virtual workspace, click on the menu, and scroll down to MAESTRO. In the maestro for windows menu, the software, maestro, had all the data entered. It was just to click each file to load them into the buffer. Using the software, the experiment is divided into three parts; spectrum calibration, the half-life of 128I, and calibration.

Spectrum calibration

The software had a file named Chn, which had the spectrum of the detected NaI crystal ionization saved. The spectrum was measured from 137Cs and 60Co. The 137Cs spectrum had a single gamma-ray emission recorded at 662 KeV while that of the 60Co had two gamma rays emission recorded at 1733 and 1333 KeV respectively. Additionally, a coincidence peak recorded at 2506KeV was also noted in the file calibration spectrum. The coincidence peak was from protons as a result of 60Co decay being absorbed simultaneously in the detector. Finally, the three peaks were used to calibrate MCA using the calibration function.

half-life of 128I

Calculation of the half-life of 128I was done from separate files. This part was not performed in the maestro system. To calculate this,  files that had precalculated values were downloaded online. The values in each file were obtained after the NaI crystal was activated through exposure to neutrons for 45 minutes. Every single file had counts of ROI readings recorded every two minutes after subjection to about 200 KeV wide centered around 500 KeV. Data in each file was recorded in a table in table .1 below. The recorded data were then used to calculate the average count rate in each time interval (N/T)and the natural log of average count rate each time ln (N/T).

Endpoint of energy     

In calculating the endpoint energy, a file named finlaspect had data of the decay spectrum of 128I in the NaI crystal acquired for one hour. A vertical scale was expanded by extrapolating the curve, and data further analyzed by ASCII software.

Finally, a recalibration was done to determine the damage caused by neutron activation. And the resulting effect on scintillation crystal output. To do this, a final calibration file that had spectra from 137Cs and 60Co was to be analyzed to determine if it shows the peaks at the correct energy. And to improve the accuracy, the MCA was recalibrated to obtain the second estimate of the beta spectrum endpoint.

Results and errors

Fig. 1, the calibration file spectrum.

Time (s)	Change in time  ( )	Gross count (S)	Change in counts N	N/	Ln (N/)
118.5	118.5	63467	63467	353.59	6.28
237.1	118.6	122827	59360	500.51	6.22
355.4	118.3	177842	55015	465.05	6.14
473.9	118.5	229724	51882	437.82	6.08
592.7	118.8	278703	48979	412.28	6.02
711.2	118.5	324811	46108	389.10	5.96
830.1	118.9	368373	43562	366.38	5.90
949.0	118.9	409089	40716	342.44	5.84
1067.9	118.9	447941	41852	351.99	5.86
1186.7	118.8	484307	36366	306.11	5.72
1305.7	119	518802	34495	289.87	5.67
1424.8	119.1	551618	32816	275.53	5.62
1544.0	119.2	582320	30702	257.57	5.55
1663.2	119.2	611308	28988	243.19	5.49
1782.4	119.2	638713	27405	229.91	5.44
1901.6	119.2	664721	26008	218.19	5.49
2021.0	119.4	689093	24372	204.12	5.32
2140.3	119.3	712577	23464	197.18	5.28
2259.6	119.3	734592	22015	184.53	5.22
2379.0	119.4	755418	20826	174.42	5.16
2498.5	119.5	775349	19931	166.65	5.12
2618.0	119.5	794165	18816	157.46	5.06
2737.5	119.5	811616	17459	146.10	4.98
2857.0	119.5	828405	16789	140.49	4.95
2976.6	119.6	844108	15703	139.30	4.88
3096.1	119.5	859026	14918	124.84	4.83
3215.6	119.5	873269	14243	119.19	4.78
3335.3	119.7	886592	13323	111.30	4.71
3454.8	119.5	899426	12834	107.40	4.68
3574.5	119.7	911481	12055	100.71	4.61

Table 1. The recorded ROI at intervals, and calculation of the change in counts and ln of change in count.

A graph of ln () vs Time (s).

Figure .2 final spect

Figure 3. Final calibration file

Discussion

Determination of beta particle energy can be achieved through charged particles detectors. However, other more sensitive detectors such as sodium iodide (NaI) scintillation detector can be used to measure low energy particles. In the case of this experiment, NaI (TI) crystal detector was used to determine the energy of emitted beta particles when 127I decayed to 128I. 127I is a stable isotope. Neutron irradiation activates it to a radioactive 128I isotope. Unstable 128I isotope decays to a more stable 128Xe isotope, and releases 𝛽-particles in the same process. Below is an illustration of the process.

127I(n,γ) 128I(β−,γ) 128Xe. (Gilmour et al., 2016, p. 80)

The graph of Time (s) vs Ln (N/) can be used to calculate the decaying 127I nucleus. The activity, which is the number of decays per unit time, contributes to the number counts (N) recorded in the decay file (detector).

Thus activity (A) = which is the same as =

Activity (A) = t therefore, Ln (N/) = k-t

From the graph plotted, gradient = t . from the gradient, the half life of the 128I can be calculated as shown below.

Gradient = . From the graph, gradient =.

Thus, the gradient of the graph = ==-1/2384

t = -1/2384 = 0.693 /T1/2= 0.0006053 s

The energy released during beta decay is shared between antineutrino and electrons, and it is released continuously from the source. Endpoint energy is the maximum energy produced by the beta particles. It is the mass difference between the parent and daughter nuclei. Endpoint energy can be obtained from the decay spectrum by extrapolating the curve obtained when the vertical scale was extended.

Conclusion

The ability to measure low energy beta particles requires sensitive detectors that can accurately measure and give the exact reflection of what is expected. This experiment proved that NaI scintillation is one of the detectors that are suitable for this activity. By closely analysing the endpoint energy from the beta decay spectrum, it clearly shows how beta particles have low energy. The half-life calculated from the graph did not give a clear or accurate value due to some errors that might have occurred in the calculation process. However, the half-life 128I is 25 minutes (Lin & Chao, 2009, p. 180), and based on the mass difference calculation, the expected energy is 0.002278 amu, which is equal to 2.12 MeV.

References

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CHOPPIN, G. R. E. G. O. R. Y. R., LILJENZIN, J. A. N.-O. L. O. V., & RYDBERG, J. A. N. (2002). Absorption of Nuclear Radiation. Radiochemistry and Nuclear Chemistry, 123–165. https://doi.org/10.1016/b978-075067463-8/50006-6

Gilmour, J. D., Holland, G., Verchovsky, A. B., Fisenko, A. V., Crowther, S. A., & Turner, G. (2016). Xenon and iodine reveal multiple distinct exotic xenon components in Efremovka “nanodiamonds.” Geochimica et Cosmochimica Acta, 177, 78–93. https://doi.org/10.1016/j.gca.2015.12.028

Kang, H., Min, S., Seo, B., Roh, C., Hong, S., & Cheong, J. H. (2020). Low Energy Beta Emitter Measurement: A Review. Chemosensors, 8(4), 106. https://doi.org/10.3390/chemosensors8040106

L’Annunziata, M. F. (2007). Beta Radiation. Radioactivity, 119–140. https://doi.org/10.1016/b978-044452715-8.50005-0

Lin, C. C., & Chao, J. H. (2009). Radiochemistry of Iodine. Comprehensive Handbook of Iodine, 171–182. https://doi.org/10.1016/b978-0-12-374135-6.00017-0