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Californium-252, AmBe Substitute?

The only current supplier of new AmBe neutron well logging sealed sources is QSA Global Inc. (successor to Amersham) using Americium of Russian origin.  The USDOE government labs are no longer selling Americium to sealed source manufacturers apparently because none is currently being produced.  Claims the Russian Americium is inferior to the material previously supplied by the US government are apparently false (QSA Global says the material is purer than the USDOE product ever was at greater than 99% purity), but whatever else can be said, there can be no doubt that the exclusive reliance on imported Russian Americium to support the US oil industry is a serious threat to energy independence.  Due to demand, lead time for delivery of a new AmBe source has been in the neighborhood of two years recently, and of course the cost for new AmBe sources has soared due to scarcity and lack of competition.

There are a number of ways to generate neutrons for use in well logging operations (see table below).  The Alpha Reaction wherein neutrons are produced when alpha particles impinge on low atomic weight isotopes such as beryllium, has been used by the well logging industry since the commercialization of neutron logging in 1941.  Radium-Beryllium was originally used, but Americium-Beryllium eventually replaced the older Radium based neutron sources due to less "gamma ray trash" emissions.  AmBe sources contain an intimate mixture of americium oxide and beryllium metal powders compacted at high pressure.  The ratio of beryllium to americium oxide is in the range of 2:1 to 20:1 depending on the design and desired neutron output.  It is also possible to make a chemical neutron source from Plutonium-Beryllium, Polonium-Beryllium, and other combinations, but none of these options offer any relief in the present Americium shortage situation due to regulatory or other concerns.

Sealed Tube Neutron Generators, technically small accelerators, are used in well logging tools for certain relatively exotic logging procedures.  Their use as a replacement for AmBe chemical neutron sources has been discussed, but thus far the economics are prohibitive.  These devices are expensive initially, and maintenance on them is also expensive.  Well logging neutron generators produce a mono-enegetic 14 MeV neutron* as compared to the average 4 MeV (11 MeV maximum) energy level of an AmBe neutron.  These high energy neutrons are useful for logging procedures where activation of wellbore material is required.  (*Well logging neutron generators use the D-T process which yields 14 MeV neutrons;  the D-D process yields a neutron spectrum more similar to Californium-252, but said process is harder to implement in a downhole generator.)

Spontaneous Fission sealed sources may be our best replacement for AmBe sources.  Californium-252 spontaneous fission well logging neutron sources are presently available at much less cost and much quicker delivery lead times than AmBe sources.  Californium was the sixth transuranic element to be discovered and was first produced in a cyclotron at the University of California in 1950.  Only about 3% of the decays of Californium-252 are by spontaneous fission, but even so, Californium-252 is over three orders of magnitude more efficient at neutron production than an AmBe source of the same activity (this has some desirable regulatory implications as discussed below).  Californium-252 produces an average 2.3 MeV (6 MeV maximum) neutron as compared to the average 4 MeV (11 MeV maximum) AmBe neutron.  Research seems to indicate this neutron energy level difference presents no serious problems for conventional neutron well logging, and in some circumstances may even yield a superior log.

Unfortunately, Californium-252 has a relatively short half-life of 2.645 years as compared to 458.1 years for an AmBe sealed source.  This major drawback is offset by ready availability, dramatically lower cost, and much lower source activity for the same or greater neutron yield than an AmBe source.  Since Californium-252 sources can be quite compact, it is possible to prepare the source with extra capacity initially to help compensate the relatively rapid decline in neutron yield, and it is possible to place multiple "pills" in a source holder rotating said pills to adjust neutron yield.  AmBe yields 2.2x106 neutrons per second per Curie while Californium-252 yields an impressive 4.4x109 neutrons per second per Curie.  A mere microgram (g) of Californium-252 yields 2.3x106 neutrons per second!  Consequently, Californium-252 well logging neutron sealed sources can be fabricated with neutron yields equivalent to large multi-Curie AmBe sources, yet the activity level will be below 50 milliCurie (mCi).  This greatly simplifies life in the oil patch when it comes to shipping these sources, especially internationally.  It also simplifies other regulatory issues like storage security, placarding, and more.

Given the upside, we may need to get used to the idea that our neutron well logging sealed sources will have a shorter half-life.  Besides, booms in the oil patch have a half-life much closer to Californium-252 than to AmBe.  Contact us if we can be of assistance.

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07-09-07
Last 10-20-10