At 04:06 PM 5/8/2010, Jones Beene wrote:
-----Original Message-----
From: Abd ul-Rahman Lomax
> This is what I expect to see: I'll be lucky if I register on the film
one neutron track per several minutes, maybe ten minutes, average....The key
to the film is getting the film very close to the source, so
neutron levels on the level of one per minute can be accumulated.
Now, if I have a gamma detector, how do I know that the gammas came
from the source? Couldn't they come from cosmic rays? Consider how
much background there is on an ordinary Geiger counter!
Yes good point - the differential rate may be too low - i.e. to show
anything significant above background. But we should be able to guesstimate
that in advance.
There have been lots of efforts to detect neutrons. The best efforts,
with high technology, came up with results of low significance. But
I'd sure like to be able to monitor neutrons in real time. I'm
hoping, though, to find some other metric. After all, neutrons are
not at all the primary signature of the reaction, they are rare
byproducts, apparently.
The variable to consider as to whether the effort would add anything - is
the effective capture rate of the film vs. the meter. The film would have a
known cross-section for capture, whereas a thin Gd plate of the same area
size would capture essentially 100% of neutrons. It could even be placed
behind the film to pick up those that passed through if the film capture
rate was low.
Of course. The film capture rate will be low. It's basically how many
neutrons will be diverted by passing through 0.065 inch of acyrlic.
There is another 0.004 inch of polyester, which is the LR-115.
In general terms, if the background rate (due to cosmic rays etc) were say
10 counts per minute, and we estimate the cell rate at 5 counts, then 15
would show on the meter when the cell was operating, and that might be
considered statically significant. If there were 5 counts/min coming from
the experiment and only one was captured by the film, then the addition of
the Gd + GM meter might be worth the efforts since it adds a confirmation
- plus, it is all probably predictable in advance.
Well, if that kind of increase takes place across many experiments,
it could be enough. But I'm suspecting that 5 counts per minute is optimistic.
To predict the number of counts, one could determine an estimated
detection and diversion rate for the acrylyic and the film (more will
be diverted than is detected, but my sense is that most neutrons will
sail through the acrylic and film without significant interaction).
I have to remember that I need to keep it simple, very simple.
