FAQ: Neutron Sources
Posted: Mon Aug 09, 2004 12:32 pm
This FAQ is a defensive FAQ rather than the usual informative and HOW-TO type of FAQ.
First of all, you can probably never have, or own, an effective neutron source based on a radio-isotope.
You should certainly never entertain "rolling your own" source.
1. it will fail
2. it is illegal.
3. it is dangerous
Here is the low down on why you shouldn't even attempt this effort.
Most normalized sources for neutrons based on radioactives are of the Alpha-Be reaction or the Photo-neutron type. The absolute worst is the photo-neutron type. In this source an intense gamma source is sealed up with antimony and the photons shear off neutrons. These sources are lethal to even be near and require a minimum of a 1 Curie source. The reaction is a very inefficient one and such sources are rarely used or assembled anymore.
The classic neutron source is the Ra-Be source based on the action of alpha particles with beryllium. The alpha-Be source is just about the best there is. Still, even in the best of these sources, it is a given that a minimum of 10,000 alphas must impact the Be at full energy (~5mev) to yield one single neutron.
Let's do some math. Question: What would be a minimum alpha activity to produce a viable source that would yield a 2 neutron per minute count into a standard moderated BF3 counter? (This count is easily detectable and far above background enough to pick out immediately with no statistics.)
This is going the wrong way round in normal calculations, so we work backwards.
A BF3 tube at a range of 25cm (about the minimum distance from our counter to source), would represent a 12cm sq moderated tube at 1% efficiency.
The sphere would be 4XPiX25^2 = ~7900cm sq. of which the tube is but a 12/7900 part and so for every 660 neutrons isotropically emitted only one would enter the tube. We would need a hundred entering the tube to detect one. So 66,000 neutrons would need to be emitted every minute to get one CPM or 132,000/min to get 2 counts. this works out to about 2,200 n/sec from the source to get 2 cpm in the counter.
2,200 X10,000 = 22 million alphas per second demanded of the source. provided it is flawlessly intermixed (radium dust to Be dust) No home-made source can have this and thus most are target systems. This means a one Pi effective emitter so we will need double the activity and thus a 44 million alpha per sec source.
With 37,000 alphas/second/uCi we would need a 44,000,000/37,000 uCi source. This is about a 1.2 millicurie source!!! In terms of smoke detectors you would need 1200/0.9 or over 1330 smoke alarms.
This is for two lousy counts every minute!
DOES EVERYONE GET THE PICTURE HERE!
To continue on.... The modern N sources are all Am241-Be systems as the old Ra-Be systems were horrible gamma emitters which made even a two minute encounter with one in the open at 20 inches give you about a life time of normal exposure. Very nasty.
The Am-Be sources are still gamma emitters but of only the weakish 59kev gamma that is easily shielded either by the source container, itself, or light shielding around it.
So you can see that so far as rolling your own is concerned, it is out of the realm of possibility. Far too much isotope needs to be on hand to make an effective neutron source.
I can't control what each of you do, but I would ask that you would refrain from posting, here, any attempts at assembling a neutron source. This is mainly due to the current climate in the US and that you are fostering an illegal activity.
The success of a working fusor will supply your counters with enough activity to check them out. Beyond this, you can send your counter to Ludlum for calibrtion and check out if you are unsure of its functionality.
The best neutron source is a working fusor for it can be turned on and off. There are no storage issues and no contamination or nuclide handling problems. About as clean and sweet as it gets.
Richard Hull
First of all, you can probably never have, or own, an effective neutron source based on a radio-isotope.
You should certainly never entertain "rolling your own" source.
1. it will fail
2. it is illegal.
3. it is dangerous
Here is the low down on why you shouldn't even attempt this effort.
Most normalized sources for neutrons based on radioactives are of the Alpha-Be reaction or the Photo-neutron type. The absolute worst is the photo-neutron type. In this source an intense gamma source is sealed up with antimony and the photons shear off neutrons. These sources are lethal to even be near and require a minimum of a 1 Curie source. The reaction is a very inefficient one and such sources are rarely used or assembled anymore.
The classic neutron source is the Ra-Be source based on the action of alpha particles with beryllium. The alpha-Be source is just about the best there is. Still, even in the best of these sources, it is a given that a minimum of 10,000 alphas must impact the Be at full energy (~5mev) to yield one single neutron.
Let's do some math. Question: What would be a minimum alpha activity to produce a viable source that would yield a 2 neutron per minute count into a standard moderated BF3 counter? (This count is easily detectable and far above background enough to pick out immediately with no statistics.)
This is going the wrong way round in normal calculations, so we work backwards.
A BF3 tube at a range of 25cm (about the minimum distance from our counter to source), would represent a 12cm sq moderated tube at 1% efficiency.
The sphere would be 4XPiX25^2 = ~7900cm sq. of which the tube is but a 12/7900 part and so for every 660 neutrons isotropically emitted only one would enter the tube. We would need a hundred entering the tube to detect one. So 66,000 neutrons would need to be emitted every minute to get one CPM or 132,000/min to get 2 counts. this works out to about 2,200 n/sec from the source to get 2 cpm in the counter.
2,200 X10,000 = 22 million alphas per second demanded of the source. provided it is flawlessly intermixed (radium dust to Be dust) No home-made source can have this and thus most are target systems. This means a one Pi effective emitter so we will need double the activity and thus a 44 million alpha per sec source.
With 37,000 alphas/second/uCi we would need a 44,000,000/37,000 uCi source. This is about a 1.2 millicurie source!!! In terms of smoke detectors you would need 1200/0.9 or over 1330 smoke alarms.
This is for two lousy counts every minute!
DOES EVERYONE GET THE PICTURE HERE!
To continue on.... The modern N sources are all Am241-Be systems as the old Ra-Be systems were horrible gamma emitters which made even a two minute encounter with one in the open at 20 inches give you about a life time of normal exposure. Very nasty.
The Am-Be sources are still gamma emitters but of only the weakish 59kev gamma that is easily shielded either by the source container, itself, or light shielding around it.
So you can see that so far as rolling your own is concerned, it is out of the realm of possibility. Far too much isotope needs to be on hand to make an effective neutron source.
I can't control what each of you do, but I would ask that you would refrain from posting, here, any attempts at assembling a neutron source. This is mainly due to the current climate in the US and that you are fostering an illegal activity.
The success of a working fusor will supply your counters with enough activity to check them out. Beyond this, you can send your counter to Ludlum for calibrtion and check out if you are unsure of its functionality.
The best neutron source is a working fusor for it can be turned on and off. There are no storage issues and no contamination or nuclide handling problems. About as clean and sweet as it gets.
Richard Hull