FAQ - Activation Materials - Indium

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Richard Hull
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FAQ - Activation Materials - Indium

Post by Richard Hull »

Indium is a very special material. For those looking to activate one of the easiest of all elements, Indium is a true gem.

Indium consists of two naturally occurring isotopes. In113 (4.3%) and In115 (95.7%)
Indium113 is the only stable isotope of indium. In115, the bulk of all natural indium is radioactive! The decay rate is very slow taking 600 trillion years to beta decay (~.48 mev max) to half its mass. Such decay rates are un-measurable by the amateur.

Indium113 has a relatively smallish thermal cross section of only 4 barns. It activates to In114 that has but a 72 second half-life. In114 is, for our purposes, a beta emitter. The beta energy is nearly 2 mev (max). However, due to the paucity of this stable isotope in natural indium and its lower cross section, this activation, while real, is only a minor radiator from activated indium. There is a long lived, (50 day), metastable isotope of In114, but it is of no consequence, radiologically speaking, in amateur hands.

Indium 115 activation to In116 m1 is the animal we are looking for. Indium 115 activates to three products, In116, In 116m1 and In 116 m2. The thermal cross section of In115 to In116 is 45 barns, to In116m1 is 154 barns and to In116m2 is 4 barns.

Indium116 has a half life of about 13 seconds and emits an extremely strong 3.3 mev beta (max) A bit of a short half life, but fully activated in under one minute exposure.

Indium116m1 is the ultimate activation product! This is due to its longish half-life of 54 minutes and In115’s rather monstrous cross section for this particular reaction. Full activation would require 4 or more hours of exposure. This is a bit impractical for most fusor runs, but a 30 minute exposure would suffice to really make this superb beta and gamma emitter show up with fusors capable of more than 500,000 n/s isotropic emission. In116m1 has a max beta energy of about 1 mev and any number of gamma rays of which the two most significant occur at 1 mev and 1.3mev. Another, less prominent gamma ray, is found at about 417kev.

Indium116m2 is a non-starter due to a short 2-second half life and a very low cross section for this reaction. It is mostly a low energy gamma emitter. (.164mev)

Summary

Indium activation is, thus, all about what can be retrieved from the activation of its predominant, already unstable, isotope In115. Only the products In116 and 116m1 are important with the In116m1 being the obvious winner and darling activation product.

Indium requires a longer activation time than the #1 amateur activation material, silver. However, for the effort, one obtains a significant beta emitter and a source of really interesting gamma rays suitable of the amateur armed with a gamma ray spectrometer.

Activation procedure:

For those with a gamma spectrometer, a thicker piece of indium is to be sought up to 0.1 inches thickness. For Betas, the foil is preferred. For gamma work, a very large area piece of indium is better as the average sodium iodide detector can see the strong gammas from any number of angles. The soft, activated metal can be draped over a larger area of the entire crystal.

Indium’s cross-section for neutrons of less than thermal energies climbs by the 1/v rule to some amazingly large cross sections. Feel free to use a large moderator, but place at least 1.5"-2" of moderator between the indium and the fusor shell. Indium is blessesed with wide resonance bands as found in silver. As such, indium can stand less moderator infront of it, allowing placement of the material a bit closer to the fusor.

Activate for as long and as hard as you dare. There is little rush, as in the case of silver, for you have a 54-minute half-life material in In116m1. To test with both a GM counter and a gamma-ray spectrometer. I would make a 2 minute count with the GM for betas and then, smartly move the material over to a gamma-spec. for a long count run on gammas.

Checkout Jon Rosentstiel's earliest Indium activation effort at
viewtopic.php?f=13&t=5263&hilit=indium#p33707

I include an ENDF plot of the two indium isotope cross-sections.

Richard Hull

References:

Table of Isotopes, 6th edition, Lederer, Hollander, Perlman, 1965, John Wesley & Son.
ENDF VII 300K library (on line)

EDIT May 2010**************************

When considering a moderator, the plus ultra data and now to be considered the final word, is given in a fabulous exposition by Carl Willis. All here should consult
viewtopic.php?f=13&t=6104#p40456

This being an ancillary to the above FAQ

R.H.
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