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Farakh Aleem NE-11 MSNE
Abstract
In this experiment, beta (β-1) spectra of H^3 and C^14 were studied using Liquid Scintillator counting system LSC 2 after making the efficiency calibration of the system using the same two sources, since their activity was known. The samples of the two sources used were dissolved in liquid scintillator 2,5- diphenyloxazole (PPO) solution. Using the same setup, half life of K^40 was also determined. Source of K^40 was 500mg of KCl dissolved in the scintillator solution.
Introduction
A scintillator emits a brief flash of fluorescent light after interacting with radiations. Liquid scintillators are primarily used for low energy beta emitting radioactive nuclides. Liquid scintillation counting is done by dissolving a suitable amount of scintillator in a solvent. The radioactive substance is added to this solution. The mixture is sealed in a glass or plastic vial. The vial is then placed in a light tight counting system where commonly two photomultiplier tubes are used to detect the scintillation light.
The solvent absorbs most of the energy of incident radiations and transfers it to the scintillator molecules by emitting high velocity electrons. Commonly used solvents are toluene, dioxane and xylene. The scintillator absorbs the energy from solvent and emits light photons in far violet or ultraviolet region. P-terphenyl and 2,5-Diphenyloxazole (PPO) are common primary scintillators. The “wave shifter” is added to the solution for matching the wavelength at which the response of the PMT is good. It absorbs the emissions of primary solute and emits photons of longer wavelength. It also improves the transparency of the scintillator solution. Bis-2-(5-Phenyloxazolyl)-benzene (POPOP) is a common wave shifter. Certain additives and solubilizers are added to liquid scintillator to improve the efficiency of energy transfer from solvent to primary solute and to improve the dissolution of certain samples in liquid scintillator.
In case of liquid scintillators, maximum counting efficiencies are achievable because radioactive substance and detector material are thoroughly mixed so that there is no window between source and detector and also the 4π-geometry is available. The problems of self absorption of radiation in the sample and backscattering from the window of the detector are not present in liquid scintillation counting.
The liquid scintillator cannot be used for energetic X and gamma rays detection due to its low atomic number. One important problem in liquid scintillators is “quenching”. It refers to all radiationless transitions and represents loss of information in counting measurements. This is mostly caused by unwanted substances present in the solvent e.g. Oxygen. They tend to absorb energy from solvent but do not radiate it. Therefore most of oxygen is purged from solution while sealing in the vial.
Farakh Aleem NE-11 MSNE
Experimental setup and results
HV
Experimental setup was arranged as shown above in figure 1 and H^3 standard source of known activity mixed in scintillation solution in a plastic vial was inserted between the PMTs. After setting the threshold of the working channel A at 20 to reduce noise and window level at maximum, the voltage was increased to the value where counter started giving some counts then voltage was increased up to 1200 with the steps of 100 V and corresponding values of counts/10 seconds were taken. The voltage value with maximum counts was then set and value of counts per minute (cpm) was noted. From the known activity (dpm) of the source, its value on experiment day was calculated using the following equation:
( )
Where t was in days. Then efficiency of LSC system for H^3 source was calculated as:
Figure 1. Experimental Setup
Farakh Aleem NE-11 MSNE
0
50
100
150
200
0 500 1000 1500
Counts/20 sec
Threshold Level
H-3 Beta Spectrum
Opt. V = 800 V cpm = 26869 dpm = 30600 On 1/5/ t = 10235 days T1/2= 5730 yrs = 2091450 days = 3.31E-07 d-^1 dpm = 30496 On 16/5/ η = 88.00 %
Then window level of working channel A was set at 20. By increasing threshold level from zero to maximum with steps of 20 or 30 and collecting respective values of counts/20 seconds, differential pulse height Beta spectra of both the sources were obtained by setting their corresponding operating voltages. Observations and results are shown below:
Table 3. H^3 Differential pulse height spectrum
Threshold Counts/20 sec 0 10 20 29 40 42 60 66 80 76 100 105 120 96 140 141 160 139 180 101 200 142 220 137 250 165 280 167 310 160 340 171 370 156 400 161 430 169
Figure 2. Differential Beta Spectrum of H^3
Farakh Aleem NE-11 MSNE
0
100
200
300
400
500
600
700
0 200 400 600 800 1000 1200
Counts/20 sec
Threshold Level
C-14 Beta Spectrum
Table 4. C^14 Differential Pulse Height Spectrum
Threshold Counts/20 sec 0 239 20 372 40 450 60 550 80 520 100 554 120 612 140 561 160 576 180 605 200 522 220 526 240 539 260 524 280 503 300 500 320 539
Figure 3. Differential Beta Spectrum of C^14
Farakh Aleem NE-11 MSNE
Vial diamtere =2.3 cm
Vial thickness = 0.2 cm
Inner diameter = 1.9 cm
Height of liquid = 3.3 cm
Volume of liquid = π*(1.9)^2 *3.3/4 = 9.36 cm^3
Volume of equivalent sphere = (4/3)πx^3 =9.36 cm^3
x = 1.3 cm
f = 1- exp(-0.06*1.3) = 0.
𝜂 = 89% + 0.075*11% = 89.82%
dpm= 386/𝜂 = 430 = dN/dt
Number of K^40 atoms in the sample; N = (W)(NA)(A)/(M)
where
W = Weight of sample in grams
NA = Avogadro Number
A = Natural Abundance of K^40 = 0.0117%
M = Formula Weight of K^40 = 74.
N = (0.5)(6.0210^23 )(0.0117%)/(74.555) = 4.7210^17
𝜆 = (dN/dt)/N = 9.11*10-16^ min-
T1/2 = 0.693/𝜆 =7.610^14 min = 1.4510^9 years
Discussion
The efficiency calibration of liquid scintillation counting system was made after operating voltage optimization for the PMTs in the system in order to maximize the system efficiency. This was done for two different beta sources H^3 and C^14. The system efficiency for H^3 was much lower than that for C^14. Its reason can be that βs from H^3 are more energetic than those from C^14 due to relatively much shorter half life of H^3 so that comparatively larger fraction of H^3 beta particles may not be detected in the scintillator. It was observed that for same operating voltage the count rate for C^14 is greater than that for H^3. This can be due to the fact that carbon is solid and hydrogen is gas so C^14 has higher atom density so
Farakh Aleem NE-11 MSNE
as to give greater count rate. For both sources efficiency was observed to be less than 100%. There can be two reasons responsible for this:
Escape of some fraction of beta particles and Quenching effect of unwanted solutes in the scintillator solution e.g. Oxygen.
Since beta particles have continuous energy spectrum, a skewed bell shaped continuum was obtained in differential pulse height spectra in case of both H^3 and C^14. The end point energy or Emax of beta particles is obtained where the continuum cuts the voltage axis or energy axis in some cases.
Finally, the half life of K^40 was calculated to be 1.45Gyrs whose actual value given in literature is 1.28Gyrs. The percent error is 13.28%. The calculated value of half life is greater than the actual value because count rate (Activity) obtained from the experimental setup is lower than the actual count rate. This is due to the low efficiency of the counting system. Also, it was assumed that the entire amount of KCl was concentrated at the centre of the equivalent sphere. But actually, this was not the situation because radioactive material was thoroughly mixed with scintillator so as not to give the 100% detection efficiency for βs. The assumption for the cylindrical vial to be a sphere may also contribute to the error in calculation of half life. Despite all these sources of error, the calculated value of half life is in good agreement with the actual one. So the assumptions used are found to lead towards good approximation.
Conclusion
It was found that the liquid scintillator counting system has different counting efficiencies for different radioactive sources. This was done by making efficiency calibration of the system for two different sources. For the two sources, beta spectra were also obtained by using SCA designated by channel A. Both the spectra had continuous behavior as already has been experimentally proved. The results could be improved by analyzing the spectra using an MCA. The half life of K^40 was also determined using the liquid scintillator counting system after optimizing the operating voltage for the system. The half life was found to be 1.4510^9 yrs. From its good agreement with the actual value 1.2810^9 yrs, it was concluded that the method used in calculation yields good results despite all the corresponding assumptions.
