background neutron counting and moderator

[AllenWallace]

For those of you who have neutron detectors, what is your typicial background rate? also, if you remove the rmoderator, does the count change much? Your elevation above sea level?

Our typical rate is about 30 cps and we are at 4700 feet in elevation. adding a moderator only slightly decreases the background count.

[Richard Hull]

Something is wrong with your counter. It could be as simple as too low a discriminator setting. There are effectively zero neutrons in the background even at 4700 feet. Most neutron background counts are actually cosmic rays and at or near sea level this amounts to about 1 count every minute or two!! 30 cps is outrageously high.

For fusor neutrons, you have to have a moderator in place at all times. If you remove the moderator your count should be virtually background, even with a fusor roaring away just inches from the detector.

Richard Hull

[AllenWallace]

Actually, the detector is brand new 22 inch long Helium-3 tube with an
integrated electronics package. I rather suspect that the counting
efficiency is *much* better than a BF3 detector. Also, my elevation
might dramatically increase the background rate... I’m not sure. In any
case, the manufacture told us to expect this kind of background rate.

Most of the background neutron counts comes from cosmic rays. For
fun, search the web for sites with keywords relating to cosmic rays,
neutrons and space weather.

[Richard Hull]

He3 is about double the efficiency of BF3 and that means about 1 cpm here in Richmond. I have an 18" Reuter and Stokes He3 tube that is at about 2 atmospheres and have used it in background tests here Barely got 1 cpm once properly discriminated against hot rad sources. The neutron maximum in cosmic secondaries peaks at 12 miles altitude and backs off very rapidly to virtually zero at ground level. Current estimates, (little real data here), puts sea level neuts due to cosmics at 5 neutrons per square meter per HOUR!! Other cosmics can give a neutron counter real counts like the many ultra hot muons and pions left over from showers. Cosmic ray wise at the ground level that is about all that is left.

The bottom line is that is is not really important for our purposes what sets off a neutron counter in background counts as long as they are stable and we subtract them from any real reading.

If a background count before and after an experiment differs by more than 10% then there is a problem with the electronics or the detector tube, itself!

Richard Hull