| Difficult | Execution Time | Data Analysis | Radioactive Sources |
|---|---|---|---|
  |
  |
Yes | Yes |
Hardware setup
This experiment guide is referred to the SP5630ENP educational kit. If you don’t have this kit, choose your own from the following list to visualize the related experiment guide: SG6118B – Emulation Kit.
Equipment: SP5630ENP – Environmental Kit Plus
| Model | SP5630ENP | Additional Tool |
|---|---|---|
| Description | Gamma Radiation and Shielding Laboratory | Additional Gamma Radioactive Source  |
Purpose of the experiment
Study the characteristics of the 137Cs spectrum, with special relevance given to the low energy spectrum. The student can learn effects related to the experimental observation of a gamma decay and have basic information about the experimental setup used in gamma spectroscopy. Estimate the energy of the backscatter peak and of the Kα line.
Fundamentals
The Compton effect is linked with experimental issues, since it is caused by the interaction of photons with the electrons instrument that measure the gamma radiation. In a real detector setup, some photons can and will undergo one or potentially more Compton scattering processes (e.g. in the housing material of the radioactive source, in shielding material or material otherwise surrounding the experiment) before entering the detector material. This leads to a peak structure, the so-called backscatter peak.
The basic principle for the backscatter peak formation is the following: gamma-ray sources emit photons isotropically, some photons will undergo a Compton scattering process with a scattering angle close to 180° and some of these photons will subsequently be detected by the detector. The result is an excess of counts in the Compton part of the spectrum, the so-called backscatter peak. This peak has an energy approximately equal to the photopeak energy minus the Compton edge energy.
Carrying out the experiment
Experimental setup.
Place the i-Spector Digital into the base and the ¹³⁷Cs source in the dedicated source holder. Power on the i-Spector Digital and connect the Ethernet cable, then wait until the detector temperature is stable, as shown in the web interface: this can take up to half an hour after power-on.
Check the waveform and, if needed, adjust the trigger threshold and the gate width. Then start the acquisition of the energy spectrum, for example for 5 minutes. In the acquired spectrum, identify the main features of ¹³⁷Cs: the photopeak at 662 keV, the Compton continuum ending at the Compton edge (about 477 keV), the backscatter peak (about 184 keV) and the ¹³⁷Ba X-ray peak at about 32 keV. Finally, select a ROI around the 662 keV photopeak and check the values reported in the ROI fit, such as the peak position, the FWHM, the energy resolution and the net counts.
Results
The user can calibrate the system by using the spectrum itself. The backscatter peak and the Kα line can be identified. After calibrating the spectrum, it is possible to estimate the energy of the two peaks and compare them with theoretical predictions.
¹³⁷Cs spectrum acquired with the i-Spector Digital web interface. The arrows indicate, from left to right, the ¹³⁷Ba Kα X-ray peak (32 keV), the backscatter peak (about 184 keV), the Compton edge (about 477 keV) and the photopeak (662 keV).
