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18 May 2013

Answer to Question #3504 Submitted to "Ask the Experts"

Category: Radiation Basics — Neutrons

The following question was answered by an expert in the appropriate field:

I am using an AmBe source to calibrate a BF₃ tube for use with a neutron generator. The source has a calibration certificate dated June 1997, at 370 MBq and 100 mCi. I would like to know what the effective half-life of the AmBe source is and how many neutrons I can expect per Ci.

Technical information about some of the more widely used neutron generators can be found in IAEA Technical Report Series No. 252 (Gibson 1985). The answer to your question is in Table 3.1 (page 22) of this publication. The half-life of a ²⁴¹AmBe neutron generator is 432 y. The specific source strength (total neutron emission rate per unit activity) is 6.6×10^-5 s^-1 Bq^-1.

The alpha decay of ²⁴¹Am produces ²³⁷Np, also an alpha emitter, with a half-life of 2.14×10⁶ y. The contribution of ²³⁷Np will not appreciably affect the neutron production rate because of this long half-life and the correspondingly low specific activity.

James S. Bogard, PhD, CHP

Reference
Gibson JAB, Piesch E. Neutron monitoring for radiological protection. Technical Reports Series No. 252, International Atomic Energy Agency, Vienna; 1985

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