FAQ - Neutron background counts and counting.

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Richard Hull
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FAQ - Neutron background counts and counting.

Post by Richard Hull » Wed Nov 26, 2003 3:52 pm


This is interestingly pointed out in an old posting "neutron counting experience" 1/16/2003 by Carl Willis et al.

Carl ultimately figures he has a high background tube. This may or may not have been the case.

Neutron counters are a true crap shoot, especially BF3 types. There are different tubes, different geometries and different volumes. There are different fill gases, all at different fill pressures, operating at different voltages. In virtually every situation there is a different moderator. In surplus counters, all these different things also exist with different and unknowable histories. This doesn't even speak to different locales of use at differing elevations.

Is it any wonder there is such a vast range reported by amateurs in their local background neutron counter rates?

Still there are some obvious blank statements. "Your neutron counter should never detect a gamma source, no matter what."

"You neutron counter should, hopefully, count under one count per minute"

Let's break this down... Understanding what kind of background is reasonable is important in figuring out whether your counter is functional, barring having a know neutron source at hand.

First test......

Does the counter respond with at least one count every ten minutes?

If yes, it just might be OK.

If you read Carl's post, as mentioned above, the discriminator can be set incorrectly and the counter might not count a roaring hot neutron source at all, but still pop off a count every 6 minutes. (intense cosmic probably).

So you should ultimately send your counter off to Ludlum for calibration. When it returns, put finger nail polish over the discriminator pot shaft and never touch it!!! For now it is set correctly and whatever your background is, it is now correct for your counter and tube regardless of age or history.

Consistent high background counts are found in good, new, large volume tubes or in very old low volume tubes that have a history of high count rates, or in tubes near the end of their service life.

Most of those of us who have BF3 neutron counters have LOW VOLUME TUBES. These tubes are run at pressures just under atmospheric pressure with B10 enriched BF3. This is natural boron that has been isotopically reworked or enriched with boron 10, which is the atom we want for detection purposes. Normal Boron only has about 20% B10 in it, the rest is B11)

If there is a standard neutron survey instrument tube, it would be a short, Nancy Woods, 60CM pressure filled tube operated at about 1400 volts. These tubes are normally about 0.3% efficient at detecting slow neutrons. (see data on this found in the reference "Fast Neutron Physics", Marion and Fowler, volume one. - Books and refs forum).

He3 tubes are rarer, usually pressurized above atmosphere and are just slightly more efficient in a good moderator than a BF3.

Most Survey instruments have just enough moderator to give them a flat lined REM count rate out to about 2mev neutron energies. This means that they are too thin to thermalize all of the fasts and too thick to count all of the thermals. For thermals, the tube is often easily removed with a tug where the tube enters the moderator. (not always the case).

*** Note *** REM Roentgen Equivalent Man A hand waving equivalence for absorbed dose in man for varying levels of neutron energies. This smoothes out the count rate for varying energies and RBE values which is nice for health physics, but terrible for absolute counting. What we seek is absolute counting of our fusion 2.45 mev fusion neutrons. Thus, we must find a conversion factor for each of our individual moderators to get back to actual, effective neutron fluence. For most simple encountered surplus neutron counters, this is an 8 to 1 or 10 to 1 ratio.

NORM, Naturally Occuring Radioactive Material contributes just about zero to the background count of a fresh, moderated neutron counter tube's background. Almost the entire background count in any tube is related to cosmic radiation either interacting with the counter tube walls or gas or blasting parts of the moderator nuclei or particles into the tube. Thus, the neutron counter can be seen to be sensitive to solar flares and CMEs (Coronal Mass Ejections).

The upshot is that in any neutron counter that is properly setup, a backgorund count between 1cpm and 0.1cpm can be considered in the "normal" range. This can vary wildly between different counters when compared side by side and in a single counter day to day. (Sometimes hour to hour!)


BF3 and HE3 tubes can be rather temperature sensitive. It is not so important to have a tube at a specific temperature, but to KEEP IT AT THE SAME TEMPERATURE throughout all tests on a specific day where tests are run. That is, both background and neutron counts should be done at the same ambient temperature. Likewise, You should allow at least a 20-minute warmup period on the counter as the background can sometimes appear artificially high in some instances following turn on.

Why all the whoopla and wind over the deck about background counting?! This very question mentioned in metrologically adroit company bespeaks ignorance on behalf of the person asking it.

A statictically accurate background with well defined mode, median and standard deviation determines the "noise". It determines the boarder between useless scud and usable data. It is the point of origin of neutron reality for any neutron metrologist.

Neutron counting is an art form, as the insturments it uses are all secondary and tertiary indicators with terrible efficiency. So even with every thing operating at the limit of the scientific art, statistics is the only thing that helps separate fact from fantasy. This is never more important than when working with neutron sources that are weak or intermittent. (beginner fusors) Unfortunately, beginners are the most likely to not have the skills or instrumentation needed to do quality work. This allows the suitably "annointed" to poke fun at and dismiss much of the amateur effort.

It is no small matter to note how terribly poor was the neutron measurement in the early CF business (1989) as electrochemists pulled neutron counters from other departments and tried to make heads or tails out of their results. This faux pas lead to bitter recriminations, scientist-to-scientist! Imagine the demeanor scientist-to-basement fusioneer/fusor dabbler! Only good satistics with good base-line noise level data can convince the skeptical.

It is well known that any good neutron metrologist can use any good neutron counter to give relative readings (one count to the next - time to time) to accuracies approaching 0.5%. However, it is also understood that in a cold count looking for absolute neutron numbers, a 10-20% accuracy is the order of the day.

To sum up, we are attempting to count to absolute values a neutron fluence from a relatively weak source using counters designed for health physics that are 0.3 % efficient, near background levels, often just outside of the noise band. Everything physical is working against good measurement. Only good statistics can attempt to pull the rabbit out of the hat and not have the audience laugh us off stage.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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