License exempt Alpha sources

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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Richard Hull
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License exempt Alpha sources

Post by Richard Hull » Thu Oct 16, 2003 8:40 pm

These are rare birds.

First of all, the main reason for small license free radiation sources to exist at all are for energy calibration purposes, mainly in nuclear energy spectrometers.

Alpha emitters are traditionally high "Z" actinides or trans-uranics. They have long half lives, for the most part, and are thus biologically dangerous if ingested even in nano-gram quantities. Alpha particles have a high RBE rating.

The reason that license free beta and gamma sources can contain some of the higher activities, (>370,000 particles/second emission rates), and contain some very biologically nasty isotopes is that they can be sealed up real nice in a very secure container. The containers are designed to survive, intact, even in landfills, well beyond several half-life periods of the isotope. (Typically, over 100 years)

Such is not the case with Alpha sources. Alpha rays of even the most potent energies (>7mev) can't penetrate more a 30-50 microns thickness of matter. Therefore, a biologically and mechanically safe sealing cover just can't be had if the energy spectrum is to be preserved in the real world. The source can be mounted and contained totally on three sides, but must be free to blast its particles to the outside world for the scientific mission at hand. The most normal method of containment on the open side is a mono or dual layer vapor deposition of pure gold. This will lock down the alpha emitting material from casual dispersion and contamination, yet let over 99% of the particles pass un-molested. Other schemes are lack luster efforts at best with micro thin physical covers such as mica or mylar. Needless to say, these are easily punched through by accident or intent.

Due to the above, the most common alpha source that is issued, as such, for license free calibration, is a Polonium 210 source. Its short half life of about 138 days makes it a low threat item against ingestion as what little may ultimately lodge in the body will decay before major damage can be done as would be the case with the ingestion of, say, Radium or Americium.

Most alpha spectrometers are operated under a poor or "low" vacuum situation which will vastly increase particle range. Normal open air ranges are no more than 3-5cm. The vacuum will allow the particle to suffer less attenuation on its way to the detector. For this in vacuuo work, no window at all is allowed due to pressure gradients rupturing such contrivances.

So we are back to the typical short half-life, gold deposition, naked source. Typically, such sources are expensive and must be replaced yearly.

Richard Hull
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Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: License exempt Alpha sources

Post by ChrisSmolinski » Fri Oct 17, 2003 11:36 am

"StaticMaster" polonium based static removal sources are rather inexpensive, I recently obtained a replacement cartridge (500 uCi) for $16. The cartridge is constructed such that alphas are emitted from open slots, no disassembly or other tampering of the device is required. Of course an air gap is introduced resulting in reducting in alpha energy.

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Richard Hull
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Re: License exempt Alpha sources

Post by Richard Hull » Fri Oct 17, 2003 2:42 pm

Are you certain of that activity! That is half a millicurie! OUCH! That's gotta' hurt. 18.5 million beq.!

Richard Hull
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Fusion is the energy of the future....and it always will be
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Re: License exempt Alpha sources

Post by Carl Willis » Sat Oct 18, 2003 9:22 pm

I've bought StaticMaster units from NRD before. They're hot, as Chris says, and contain two 250 microCi source strips. You can put one up against the inside of an old CRT face (this was before I got some P4 from you, Richard) and see a bright glow with shadows cast by the little metal protective grill.

I actually took a picture of this situation with my regular 1.3 megapixel digital camera, in which the glow is very obvious.

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Re: License exempt Alpha sources

Post by davidtrimmell » Sun Oct 19, 2003 2:31 am

Carl, what allotrope of P? Red or white?

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Re: License exempt Alpha sources

Post by Richard Hester » Sun Oct 19, 2003 4:44 am

That's P4, an activated sulfide or silicate based phosphor, not phosphorous.

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Re: License exempt Alpha sources

Post by Carl Willis » Sun Oct 19, 2003 6:42 am

Sorry for the confusion. (Yeah...I could use some white phosphorous also...)

The ZnS:Ag powder phosphor sold by Bicron has a trade name of P4. Richard Hull made a bulk buy a while back and has been bagging it in smaller quantities for resale, but I think the supply is now depleted. Great stuff.

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Re: License exempt Alpha sources

Post by Frank Sanns » Sun Oct 19, 2003 7:42 pm

If I did my math right, it would take 9 half lifes to get down to the activity of 1uc of Am 241. That is 3.4 years if the half life is 138 days!

Do you think 4 of these fresh Polonium sources would be enough to provide significant ionization within a fusor?

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Re: License exempt Alpha sources

Post by 3l » Mon Oct 20, 2003 12:43 am

Hi Frank:

I believe that Americium 241 has a half life of 432.7 years.
Relatively a quick decay compared to Pu 240 which was a standard at SAC for alpha detectors at 93,000 years.
Nobody but the Government has acess to those sources.

Happy Fusoring!
Larry Leins
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Re: License exempt Alpha sources

Post by Richard Hull » Mon Oct 20, 2003 3:42 pm

Some corrections and clarification.

No one has a Pu source or wants one it was not discussed. No one has or wants and Am source.

What is in hand is a Po source (138d) half life. Frank is correct in his computations it would take a while for the Po source to die back to a the strength of a smoke detector source.

NO! The Po source is not strong enough to start a fusor ionizing to the power level desired as 37,000 X 250 or 9.3 million alphas per second are only a 18.6 million charge unit current. This is only an ALPHA ionizing current of 18.6x10e6 / .62x10e19 or 3 pico amps!! Even here, the mean free path of the alphas at fusor pressures would warrant that you did not ionize the D2 well at all. More likely the alphas would just spend themselves into the walls of the chamber.

Now those alphas are in the 5 mev range and if you could turn every one of their kinetic energy into ionization energy for deuterium, (13.6 ev), then you would have 5x10e6 X 18.6x10e6 / 13.6 = 93 trillion ions! That would be 93x10e12/.62x10e19 or 15 microamps of deuteron ion current. Still too whimpy.

If you really need about 10 ma of deuteron current for fusion and did it at STP you could get away with using about
250 CURIES more or less to do the job. (absolutely lethal and unobtainable) In vacuuo, however, you would need much more than this.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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