NEUTRON FLUX MEASUREMENT USING GOLD FOILS

Table of Contents

• Discussion
  
• Techniques
  
• Procedure
  
• Report

Discussion

When investigating the distribution of neutrons in moderating and diffusing media, gold is often used as the detecting material for the following reasons:

• Naturally occurring gold is 100% Au-197.
• There are no competing species in a pure gold foil.
• Au-197 has an appreciable thermal neutron absorption cross-section, 98.7 barns.
• It is a 1/v absorber.
• The metallurgy of gold is well known, and it is chemically stable.
• The decay of the activation product, Au-198 is straightforward...
• Au-198 decays to the ground state of mercury, Hg-198.
• The decay scheme is also not complicated...
• For 99% of the disintegrations, a 0.961 MeV (max) beta is emitted followed by a 0.411 MeV gamma.
• Other emanations are small, and often neglected.

1% 0.29 MeV beta 0.2% 1.09 MeV gamma, and 0.8% 0.676 MeV gamma.

The half-life of the Au-198 (2.69 d) is convenient. In combination with the cross-section, it provides reasonable activation with short irradiation times. For counting times under 10 minutes, a decay correction is usually not required. Also, extremely "hot" foils can "cool" overnight before counting. Finally, the foil will be usable (no residual activity) in roughly a month's time.

The simultaneous emission of a Beta and a Gamma makes coincidence counting possible. In addition, gold foils may be used with cadmium covers to identify thermal and fast components of the neutron flux. The absorption cross-sections of gold and cadmium vary with energy of the incident neutron as shown:

If we place the gold foil in a neutron spectrum, the activation of that foil will be due to the entire neutron population, which includes thermal and fast components. Covering a foil with 20 or 30 mils of cadmium will effectively block out all the neutrons below the "cadmium cutoff energy" of 0.4 eV. The gold foil covered in this fashion will be activated by the fast component of the flux. The activation due to the thermal flux can then be found:

A_thermal = A_bare - A _covered

Techniques

There are some obvious, but very important considerations which must be addressed when using foils:

• Since decay times to counting are important, results should be normalized to the end of the irradiation.
• Foils are not of equal mass: therefore, results should be normalized (per gram of foil).
• If different exposures are made, calibration foils should be employed to normalize data run-to-run.
• There is a flux depression around a strong absorber, therefore do not place a bare foil on or next to a Cd-covered foil...
• Leave 3"-4" spacing.
• Flux can be determined by accurate evaluation of the absolute activity of the foil...
• Which requires careful calibration of the detector, or use of coincidence counting techniques.

Procedure

Since polyethelyne/plexiglass provides moderator properties similar to water, foil carriers made of these materials will be used.

1. Under the direction of the reactor staff, place bare foils in a carrier and place it in a selected location(s) in or adjacent to the reactor core.
2. After a suitable irradiation at a constant power level, remove the carrier and the reference foil, NOTING TIMES THROUGHOUT THE PROCESS.
3. Repeat, with covered foils at the same location(s).

Note that two runs are made, one with a bare foil at the location(s), and another with Cd-covered foils at the same locations. This process eliminates the effects of flux depression (caused by the Cadmium) on the bare foil. In this procedure, reference foils are used to normalize the data between the two runs. Count the foils using the solid-state MCA which should be calibrated during the preparatory phase of the experiment.

Don't forget background!

Report

1. For each location, determine the normalized count rate at the time of removal from the reactor.
2. Calculate the thermal and fast components of the flux, and include the standard deviation for the measurement and calculated values.
3. Discuss the variation of the fast/slow ratio for the core positions used.
4. If the experiment is conducted using a series of foils mounted on a stringer, analyze the spatial variation of the fluxes.
5. Plot the bare foil normalized count rate versus distance and construct a smooth curve through that data.
6. Do the same for the Cd-covered foil locations, using the same graph. By generating curves to fit the data, it is then possible to subtract one curve from the other even though paired data sets are not available at each location.
7. Plot the activity due to the thermal component on the same graph.
8. Using the activation equation, calculate the fast and thermal neutron fluxes as a function of position.
9. Plot the flux variation on one graph.
10. Discuss your results.