Fusion Message Board

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Subject: Re: Homemade neutron detector? (Am-Be source)
Date: Jan 26, 7:18 pm
Poster: Richard Hull

On Jan 26, 7:18 pm, Richard Hull wrote:

>> I have four nice very old industrial smoke detector Am241 strips of 15uc each! I put all four together and then covered them with finely powered elemental Be .9999 pure. RH
>I believe the commercial sources use a very (!) thin foil of Be. JL


Not really, I have three company's data for these sources all use an intimately mixed and sintered, SS, double encased, cylindrical mass of Americium oxide and Beryllium dust. This is for max efficiency.

As you note, the cross section is close to worthless, but it works provided you obtain a stunningly lethal amount of alpha producing rad spooey and mix it with the Be. RH
>Interesting, too.. I haven't done the "plausibility analysis" of the radioactive boyscout story, but as I recall, this guy tried using the Am241 from 100 scrap (presumably consumer) smoke detectors.. So he had 100 uC. He claimed he was getting neutron activation (I sort of doubt it based on the above calculations).. Once he got some radium, though... How many curies (i.e. grams) of radium are on a typical clock dial (or, more to the point, how much radium was in the paint)? milligrams? micrograms? I wouldn't think it would need much, and, of course, the Ra is somewhat expensive, so they wouldn't have been too profligate with the stuff.

Remember the kid is now a man and the 1uc Am241 found in modern smoke alarms was not always the limit. I have a number of older (not that old) First Alerts with 5uc in each. They keep lowering the rad spooey levels as the electrometer chips plunge in price and rise in input impedance. I have a recent sample child of analog devices with a .2 fempto amp realizable input bias current with an input impedance of .1 petaohms. This sucker will detect bad breath at 100 meters! They have approached the limit of even special housings for the chip itself, much less what the real industrial user can achieve in application to utilize their input impedances. I play with electrometers a whole lot in my work with ultra subtle electrosatics sensors.

As regards the radium in clocks and watches it is on the order of 5-20 uc between wristwatches and westclox "Big Bens". Though I have a couple of WW II special aircraft instruments that have over 250uc Ra activities. They really slathered the stuff on WWI and WWII aircraft instrumentation (nothing too good for "our boys") consumer stuff was always skimpy, but still works to this day. All, that is 100% of the WWII stuff, will never glow again. The Ag activation in the ZnS was long ago burned up by the intense radiation found in these instruments.

Richard Hull