Gamma Ray Spec. Final Report

!1 Identifying Unknown Radioactive Sources via Gamma-Ray Spectroscopy

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Jake Cohen

I. PHYSICAL OVERVIEW

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Gamma ray spectroscopy is the technique of utilizing a radioactive element’s gamma ray emissions to determine the relevant properties of that element. To name a few, these properties include the half-life, amount of element present in a given sample, and the absolute activity (total amount of gamma emissions). Through gamma ray spectroscopy, these various attributes can only be calculated after energy (or energies) of the prime gamma ray(s) has been determined. When the nucleus of an unstable atom undergoes an energy change and becomes more stable, it emits ionizing radiation. This radiation can be in the form of alpha particles, beta particles, and gamma rays. By the photoelectric effect, the gamma rays emitted by a radioactive material can transfer their energy to an electron in a crystal, whereby the electron will travel a short distance while colliding with many atoms in the crystal. The electron’s energy is converted into photons when it collides with the atoms, and the number of photons released is proportional to the original energy of the gamma ray emitted by the radioactive material. In this lab, a photomultiplier tube (PMT) converts these photons into a small current, which is then amplified and measured by an Analog to Digital Conversion (ADC) process. After proper calibration, a computer is used to show and thus verify the original energy of the gamma rays.

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A. Identifying unknown sample

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If a layer of material, is put in between the radioactive sample and the crystal, the number of

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gamma rays detected is decreased according to the equation (1) N(x) = N o e− µ x

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where No is the number of gammas detected without any material in between, x is the thickness of the material, and µ is the attenuation value - a constant unique to each radioactive material. Manipulating this equation for µ gives:

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µ=

ln(

N(x) ) No . x

By finding µ, the element in question can be identified. In practice, one finds the Half Value Layer (HVL) of element. This is when the the number of gamma rays detected has been reduced to half the original number, whereby

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HVL =

0.693 µ .

(3)

In this specific case, the HVL of an unknown sample carrying two elements was determined using the aforementioned methods. The sample was put under stacks of lead with a total of ten different thicknesses. After graphing µ and finding the slope of best fit, the elements identified in the unknown sample were 137Cs and 65Zn.

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II. EXPERIMENTAL PROCEDURE

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(2)

For a standard gamma ray spectroscopy setup, a high voltage supply is connected to the PMT. The scintillating crystal and the PMT are one piece, and sit on a platform above a stack of

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horizontal slots. The scintillation counter then connects a multichannel amplifier and analyzer which is connected (via USB) to a computer (Figure 1).

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B. Procedure

The radioactive sample was put on a square piece of plastic that fits into the slots, thereby providing the ability to move the sample at discrete distances from the crystal. In all instances of collecting data, the sample was put in the second-highest slot. This kept the sample close enough to the PMT while still providing enough space to put in multiple layers of lead. The computer software (Spectrum Techniques UCS10) was calibrated using a known source - 137CS. The following settings were set after calibrating: High Voltage - 550V; Course gain - 4; Fine Gain - 1.5; Counts full scale - logarithmic; Conversion Gain 1,024. Once calibrated, the high voltage was turned on and the data was collected. For each thickness, the data was collected for ten minutes. After ten minutes, another layer of lead was placed in between the sample and the PMT. In order to ascertain an accurate number of gammas that were detected at the photopeak, a Region Of Interested (ROI) from 1,084 keV 1,135 keV was taken about the photopeak peak. The number net counts for the ROI was recorded as the number of counts for the photopeak

associated with the given lead thickness. The data was then put into the software IGOR for providing the line of best-fit.

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C. Observations and Results

From the initial collection of energy levels for the unknown sample, it was immediately recognized that there were two radioactive elements present. One of those elements was identifiable as 137Cs, by comparing it to the data collected from the known sample of 137Cs used for calibration. The second element was narrowed down to three possibilities by the location of its photo peak in the region of 1115keV. As such, the method of determining the HVL of the element was utilized to determine the unknown element in the sample. The data for the ten different thicknesses of lead was tabulated and then put into IGOR for analysis. The raw data can be seen in Table 1 and the analyzed data from IGOR is shown in Figure 2. From the data collected and the analysis in IGOR, it was found that

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µ = 0.0653 , and therefore, by using equation (3),

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HVL =

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0.693 = 10.6mm 0.0653

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! The HVL’s of radioactive sources with a photo peak in the 1100-1130 keV regime were looked up and it was found that the only possibility for the unknown element in the sample was 65Zn.

ln(N(x)/No) (counts)

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!Figure 2: A line

Thickness (mm)

fit to the data points for ten different thicknesses of lead used with the unknown source. It represents the exponential decay of the number of counts as a function of the thickness of lead in between the source and the photomultiplier-tube. The slope of the line provides the attenuation value µ for this unknown source.

Table 1: Raw data collected from eleven different layers of lead, plus one run with no lead, No. III. INSIGHT AND IMPLICATIONS

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A. Pros and cons of the HVL method The generally accepted HVL for 65Zn is 14mm. This implies that the experimentally determined value is twenty four percent off from the accepted value. As such, 65Zn was determined by a slight process of elimination. This alone demonstrates that the approach taken was not ideal. At the same time, however, other methods were utilized with an aim to identify the unknown source, and the HVL method proved the most promising. First, the HVL is a commonly used

value to describe the properties of a radioactive source. Second, the method involved taking a best fit from over ten different values (when No is included in the collected values), rather than simply comparing two values (e.g. comparing the absolute activities of the two sources present in the unknown sample. The HVL method is not without its flaws, though. It was determined that, to attain reliable data, counts needed to be collected for (at least) ten minutes for each different thickness of lead. This became a limiting factor since to test the method each time took nearly two hours. The time with the equipment was limited to five hours at a time, and thus only two full executions of the experiment could be completed within one day of work - thereby limiting the impacts of changes in external factors.

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B. External factors The HVL method was fully executed twice,

!4 and both times delivered the same result of 10.6mm. After the experiment had been run, it was realized that other external factors could have contributed to the error in the experimentally determined value. Background radiation from other radioactive sources near the detector, for instance, could have contributed to slightly skewed values. In the end, however, the complete reason(s) for the disparity between the experimentally determined value and the universally accepted value were inconclusive. The HVL method is not a commonly used approach for determining an unknown radioactive source, and it evidently presented its benefits and its shortcomings. IV. Conclusion

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Because the HVL is not a common method for determining a radioactive sources element(s), there were few-to-no previously recorded techniques to compare our approach with. Nonetheless, we came to a conclusive result and were able to state with decent certainty that our unknown radioactive source included the two elements 137Cs and 65Zn. If the task were to be presented again, other known elements would be used with the same method, and then compared to their known HVL values. As with many laboratory tasks, this endeavor was a prime example that, especially when under a time constraint, as many possible methods and external factors should be considered before moving forward with a specific approach. V. ACKNOWLEDGEMENTS

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The work on this project was done in part with Charles Coombs. Advisory insight and oversight was provided by Giulia Collura and Deborah Fyngenson.