Climbing the Compton edge

Wenjun Li, Douglas Richmond
Department of Physics, University of Wisconsin-Parkside
900 Wood Road, Kenosha, Wisconsin 53141 USA

February 14, 2006

Abstract

Incident photons on certain materials will cause Compton scattering events to occur. The energies and momentum transference that get shuffled around when these events occur can and do free electrons from their atomic captivity.

1 Introduction

By measuring the energy level of the Compton scattering events that happen when γ-rays are emitted from various radioactive sources, we will determine the kinetic energy of the electrons, and calculate the Compton edge energy.

2 Procedure

2.1 Instruments and materials

1. 137-Cs sample
2. 60-Co sample
3. 22-Na sample
4. 133-Ba sample
5. Nal Scintillation Detector
6. Multichannel Analyzer

2.2 The Experiment

We read the gamma decay of each of the elements until characteristic photopeaks developed. At that point the channel number of each peak along with the half-height channel number was recorded. The results are below:

Elementch1 ch1(1, 2) ch2 ch2(1, 2) chedge
22Na 143134, 151363350, 377 92
60Co 335319, 348382367, 394 194
133Ba 98 90, 104 13 11, 15 66
137Cs 186177, 195 - - 130
This data is used for the calibration of the instrument and also as data for determining the Compton Edge and electron energies.

3 Analysis and conclusions

3.1 Calibration

By using 60Co as our calibration data source, we were able to determine the calibration constants a and b,

ch = a + bE γ,n γ,n
we found
a = - 11.097, b = 0.295

3.2 Photopeak and Compton energy determination

Using the caliberation constants along with (3.1) we found the photopeak energy for each element. The photopeak energy,

E γ = hf
mc2 = 2-hf-(hf---Ee)- Ee

Below are the results of our calculations of the energies of the photopeaks and Compton edge.

ElementEγ, 1 (KeV )Eγ, 2 (KeV )Ece,exp (KeV )mc2 (KeV )
22Na 522.36 1268.13 349.48 516.812
60Co 1173.21 1332.47 695.244 1613.12*
133Ba 81.685 369.82 261.346 306.994
137Cs 668.125 - 478.295 530.343

* The channel and count information for 60Co was indeterminable. The photopeak occluded the Compton edge, making it difficult to determine with the procedure that we were implementing. This we believe accounts for the divergence of the calculated value of mc2 of 60Co.