COMPTON SCATTERING

Table of Contents

• Discussion
  
• Object
  
• Procedure
  
• Report

Discussion

Compton scattering of gamma rays is discussed in the companion notes, "Compton Scattering". Please refer to that document for background material on the phenomenon under investigation. A review of the notes followed by a simple calculation leads to some conclusions about the scattered gamma rays which will be measured when conducting an experiment to verify the model presented. When conducting the experiment in close quarters which present considerable opportunity for gamma ray scattering:

They will be at significantly lower energy than the source energy, they will be many orders of magnitude lower in intensity than sourcegammas, and there will be a competing field of gamma rays due to:

• gamma interactions in the source shield and collimator,
• gamma scattering from the beam impinging on other materials, and
• multiple interactions of the target-scattered gamma rays.

These effects must be considered if meaningful data is to be obtained.

Object

The object of this experiment is to measure the energy and relative scatter interaction rate as a function of scatter angle for a beam of monoenergetic gamma rays impinging upon a target of known material.

Procedure

1. Install the source and detector shields on the benchtop goniometer as shown, with the detector shield on the moveable arm.

A goniometer is a mechanical device used to measure (and vary) the angle between components of an experiment, as shown in this drawing.

2. Install and align the experiment components.
3. Check the operation of, and calibrate, a MCA system with scintillation crystal.
4. Set full scale slightly higher than source energy.
5. Check that the anticipated lowest energy scattered gamma will be recorded.
6. Install the detector in the detector shield and fit a suitable collimator whose aperture has been measured.
7. Select a thin, single-element target (approximately 2 cm diam and 2 mm thick) and mount it on a support adapter to match the pivot baseplate configuration.

The angle between the normal to the target surface and the source beam should be half the scatter angle to the detector. (i.e. when the scatter angle is 90^o, the target is 45^o to the beam), oriented as shown in the sketch.

8. Move the detector to a small scatter angle, so that the area projected by target to the beam is maximized.
9. Identify the source to be used and its holder.

The sketch shows a typical sealed source rod, but reactor-produced sources such as a gold foil may be used.

10. Determine the actual source location in the source shield.
11. With the source out, fit the collimator and align the source components and target.

This may be done by putting a flashlight bulb at the source location and holding a sheet of paper behind the target. The beam should be slightly wider than the target at that location.

NOTE: if the detector collimator is deep, the same process should be used to align it and the target.

12. Install the source and survey the area.
13. Restrict access (rope off plus signs) to the beam beyond the experimental set-up.

At this point all physical set-up data should have been recorded and checked, along with source and target information. It may prove useful to also record the light beam diameter at the target during the alignment step.

14. Record the scatter data
15. Position the detector at the scatter angle of interest.
16. Record data for a suitable live time (3 hours or more). Good statistics are vital!
17. Remove the target (leave the support in place).
18. Using the MCA subtract mode, record data for the same live time.
19. Repeat for other scattering angles (in the range 30^o to 90^o).

Report

• Plot both MCA spectra for a typical scatter angle (and any other of particular interest).
• Identify and discuss all peaks of interest.
• Strip the no-target spectrum from the target spectrum for all scatter angles.
• Plot and identify all peaks of interest.
• Plot the theoretical values of scatter gamma ray energy versus scatter angle.
• Include the measured data.
• Plot the theoretical values of relative differential cross-section versus scatter angle.
• Calculate expected count rate in the peak (for the scattered gamma-ray ).
• Compare with the measured data.
• Discuss and present conclusions.