Answer to Question #6434 Submitted to "Ask the Experts"

What gamma-ray spectra are produced by ²⁴¹AmBe and ²³⁹PuBe neutron sources? The source in either case is in 10-mm Plexiglas, 45-gallon tank, filled with water and surrounded with bricks. You access the source from the top. In the inelastic scatter reaction, the nucleus receives some internal energy as well as kinetic energy. This slows the neutron, but leaves the nucleus in an excited state. When the nucleus decays to its original energy level, it normally emits a gamma ray. Is the photon produced measurable? If yes, can I use a survey meter or a GM detector (probe: HP-270, or a low-energy gamma scintillator with an energy window of 15 mg cm^-2)? Or do I need a neutron detector?

You describe the source as being contained within a 45-gallon Plexiglas tank. If you attempt to make any photon measurements while the source is in the tank, I expect that your measurements will be weighted most heavily by the 2.2 MeV gamma rays that result from capture of neutrons that have slowed down in the water and been captured by hydrogen (¹H(n,γ)²H reaction).

There may also be gamma rays that result from some inelastic scatter events, as you describe, with heavier elements in the configuration, but these normally will be much less abundant than the capture gamma rays from hydrogen.

In fact, for the tank size you describe, most (but not all) of the neutrons emitted by the source will slow down and be captured ultimately in the hydrogen of the water and will result in the 2.2 MeV capture gamma rays. At locations close to the tank you should be able to measure these with various detectors.

A high-energy (at least 2" thick) NaI(Tl) scintillator detector with a multichannel or single-channel analyzer is probably the most informative system to use since you will be able to specifically identify the 2.2 MeV photons. A low-energy NaI(Tl) scintillator, perhaps of the type that you mention, will probably respond but will be quite inefficient at this energy and, if used with a single or multichannel analyzer, will not be very helpful in energy definition because of the very low photopeak efficiency.

In principle, the HP-270 GM probe that you mention should also be sensitive to these gamma rays, but the reading will depend on how great a source strength you have, and the lack of energy discrimination makes it difficult to interpret what you are measuring.

Applicability of other types of survey meters depends on their specific design characteristics. Some air ionization chambers are suitable, although their sensitivities may not be adequate, depending on the source strength. In general, plastic detectors, such as plastic scintillators, will often respond to the neutrons (through n, γ reactions) along with the gamma radiation and may be useful if you simply want to get a response, regardless of whether it comes from incident gamma rays or neutrons.

There are some other gamma rays and x rays emitted during the decay processes for ²⁴¹Am and ²³⁹Pu. ²⁴¹Am, in its decay, emits some relatively low-energy x rays and gamma rays, the most significant being a 60 keV gamma ray emitted in about 36 percent of the decay events.

The Am:Be sources of the type you are likely using commonly employ a double stainless steel encapsulation fabrication process that attenuates a considerable fraction of these photons, although you will expectedly see some of the 60 keV photons if you look for them with an appropriate detector.

The ²³⁹Pu decay produces some low-energy x rays, less than 20 keV, with a combined yield of about 4 percent, but these are pretty much all attenuated in the typical double stainless steel encapsulation. There are also a variety of very low-yield higher-energy gamma rays emitted in the decay process, but their yields are so low (combined yield of roughly 0.05 percent) that you will not likely see them.

If you are interested in measuring the 60 keV photons from ²⁴¹Am, you will likely be more successful if you remove the source from its shield container and make measurements directly in air. Naturally you must take appropriate precautions to minimize your neutron exposure if you attempt this, and you should get approval from your radiation safety officer if your use of the source is normally restricted to the water tank configuration. The low-energy scintillation probe that you mention would be suitable for short-duration in-air measurements, although some direct inelastic scatter events of neutrons in the detector itself will produce some higher-energy inelastic scatter gamma rays that will add to the response. Prolonged exposure of the scintillation detector to the neutrons can damage the detector.

Good luck in your measurements.

George Chabot, PhD, CHP