Sciencemadness Discussion Board

Be-Bi neutron generator

LiD - 12-5-2013 at 07:08

Hi there,

This is my first post here, so sorry if I write something what I shouldn't.

I have red the Los Alamos reports and also a few other articles, but I can't figure out, that why do they do not use Be-Bi(Po) neutron generators.

As we know these elements have a really small neutron cross section:
0.0092 σa/barns Beryllium
0.034 σa/barns Bismuth
0.03 σa/barns Polonium

And we also know that Bi is made up from one isotope Be209 with a half life 1.9×1019 y and an alpha decay and Po210 is produced from Bi209 with a neutron adsorption.

Another important info is that Be is a alpha to neutron "converter" 9Be + 4He → 12C + n and also a neurton multiplier: 9Be + n → 2(4He) + 2n

So the main question would be, that would it be possible to make a neutron source from a Be-Bi alloy after initiating it with a neutron flux?
If a small percent of the Bi turns to Po, than the cycle would start again.

I know that the alpha to neutron reaction is only working for 1 particle out from a million but I was just wondering that could this work or not.

[Edited on 12-5-2013 by LiD]

unionised - 12-5-2013 at 08:59

Quote: Originally posted by LiD  
Hi there,

This is my first post here, so sorry if I write something what I shouldn't.

I have red the Los Alamos reports and also a few other articles, but I can't figure out, that why do they do not use Be-Bi(Po) neutron generators.

As we know these elements have a really small neutron cross section:
0.0092 σa/barns Beryllium
0.034 σa/barns Bismuth
0.03 σa/barns Polonium

And we also know that Be is made up from one isotope Be209 with a half life 1.9×1019 y and an alpha decay and Po is produced from Be209 with a neutron adsorption.

Another important info is that Be is a alpha to neutron "converter" 9Be + 4He → 12C + n and also a neurton multiplier: 9Be + n → 2(4He) + 2n

So the main question would be, that would it be possible to make a neutron source from a Be-Bi alloy after initiating it with a neutron flux?
If a small percent of the Be turns to Po, than the cycle would start again.

I know that the alpha to neutron reaction is only working for 1 particle out from a million but I was just wondering that could this work or not.


"And we also know that Be is made up from one isotope Be209 with a half life 1.9×1019 y"
No it isn't.

There's are two reasons why they don't use Bi as a source of alphas to knock neutrons out of Be.
The first is that the alphas produced don't have enough energy.
The second is that Bi is very nearly stable. It has a very low radioactivity (when I was a student it was thought to be stable because nobody had been able to detect any radiation from it).
So it doesn't make many alphas and these in turn would give rise to not many neutrons.

phlogiston - 12-5-2013 at 12:24

Try to not to mix up your Be's and Bi's. It's confusing.
Quote: Originally posted by LiD  

And we also know that Be is made up from one isotope Be209 with a half life 1.9×1019 y and an alpha decay and Po is produced from Be209 with a neutron adsorption.


you probably meant:
"naturally occuring Bi is made up from one isotope: 209Bi."

and

"Po is produced from 209Bi with a neutron absorption"
Quote:
If a small percent of the Be turns to Po, than the cycle would start again.


The "Be" should also probably have been "Bi"

LiD - 12-5-2013 at 15:10

Sorry a lot, just missed that, edited.

But You knew what have I meant. So at all, it's not a good idea and it won't work, right?

On paper it looked really nice.... Maybe with a small microcurie amount of Am as "initiator"?

phlogiston - 12-5-2013 at 16:22

I can't think of any good reason why it should not work to some extent, but given the low efficiency of generating neutrons via alpha-irradiation of Be (as you mentioned yourself), combined with the low efficiency of generating 210Po from 209Bi by neutron irradation, I suspect the resulting neutron source would probably be rather weak.

Even neutron sources with purified alpha-emitters (Ra / Pu / etc)+Be sources are not particularly intense.