FAQ - Neutron Detectors

If you have a question about this topic, the answer is probably in here!
Post Reply
User avatar
Richard Hull
Moderator
Posts: 14991
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

FAQ - Neutron Detectors

Post by Richard Hull »

A generalized account for neutron counters and counting was given in an earlier FAQ.

This FAQ will deal more with the "head" or actual detector. Much of this data has been covered before but in a scattered fashion of the last 6 years here.

Neutron detection:

All neutron detection is a secondary detection. The neutron is never detected, itself, but something the neutron caused to happen is detected. ( alpha particle creation- ionization detection)

No neutron detector will detect all neutrons entering the detector. Thus, the efficiency of all neutron detectors is very low and for many, it is pretty darned poor, at best.

The best detectors - The Gas Proportionals ***************

The best detectors are forms of gas proportional counters with some gas amplification taking place in them, albeit limited compared to the geiger tube by comparison. As presented, these tubes are relatively gamma immune, that is, they can be immersed in high gamma fields and not count this radiation.

Typical counters of this type are the He3 and the BF3 detectors.
The action of these tubes varies based on cross section of the gas within, fill pressure and isotopic concentration. Let's look at each in turn.

The coveted He3 detector *****************

The best of the best is the He3 detector (helium 3). This is a very rare isotope of helium and in this detector, there is always a pure or 100% isotopic filling. This gas has the highest cross section of all fill gases and, at any given pressure, will outperform all other contenders. Virually all He3 detectors are filled to at least atmospheric pressure 760mm or 76cm. Many are filled to 2,3 or even 4 atmospheres. The He3 detector can be over 10% efficient for thermalized neutrons in high pressure tubes.

The extreme expense of such tubes is strictly due to the price of the fill gas alone. Almost all He3 is obtained via the decay of millions of curies of tritium (reactor product). This expense keeps these tubes out of the hands of the casual neutron measurement scenario. They are almost exclusively used in low level neutron detectors, for obvious reasons. To use one in a high flux environment would just be throwing money away.

It is not uncommon for a detector tube of only medium size (1" x 7") and 100cm fill pressure to cost over $1000.00, new.

Helium 3 tubes also demand the highest of voltages for bias in a detector amplifier. This can be as high as 3500 volts!

The venerable old BF3 tube *****************

The boron trifluoride neutron detector tube is the workhorse of the industry since the 1930s and can measure relatively well at both high and low levels of neutron radiation. The prices vary from relatively cheap to very expensive. (more later on this)

The cross section for boron 10 is very high for thermal neutrons and it is an ideal fill gas. The isotopic concentration of B10 in normal boron is low. Regular boron-triflouride is thus not heavily loaded with the detection molecule. This is the fill gas in very cheap BF3 tubes ($200.00-$300.00). The detection effciency in a tube filled to just under atmosphere with this gas, can be as low as 0.2%. B10 Enriched BF3 tubes filled to 1.5 atmospheres can approach 2% efficiency, but cost over $600.00 in medium sized tubes. The classic Nancy Woods tube so common to most modern BF3 counters is a small tube filled with enriched BF3 but at a subatmospheric pressure. The normal efficiency here might be 1% for thermal neutrons, at best, but allows the tube to operate at only 1400 volts bias which can be maintained in portable instruments well enough. Due to its small size, ~3"X1", and low pressure fill, it can be had for ~$400.00.

Like the higher pressure filled He3 tube, high pressure BF3 tubes require voltage up to 3000 volts.

Due to a combination of reasons, the BF3 has stood the test of time where a million technological advancements happening all about it has not unseated it or impacted its continued "most favored" status. It is certainly the number one detector of choice by the amateur fusioneer, mainly due to the vast abundance of older surplus units in the market place compared to any other form of used neutron counter.

The "also ran" Scintillator-organic detector. ********

This form of neutron detector has always been a rather poor step-child in the neutron detection arena. If is a relatively inexpensive solution compared to the gas counters, but rarely,even in its best iterations approaches 0.35% effeciency and, as such, is not good for ultra low level neutron detection. It is not gamma immune, inherently, but gamma signals are easily discriminated out of the picture electronically. So it does represent a "quiet" neutron detector, in a working embodiment, much as the gas proportional counters do. Thus, it has a low background allowing low neutron numbers to be read with some confidence providing you have the time and work the statistics to ferret data out of the background.

The best organic detectors are plastic with zinc sulfide scintillator embedment such as the BC720 by Bicron. These are not cheap and $300.00 is the norm for a 2" diameter organic scintillator.

A lot of the early discussions of this group was built up around this element, for if you could afford the Bicron piece and had a working photomultiplier setup, you could almost assemble a rough counter for that price alone.

Over the ensuing years, the BF3 is now the defacto standard among this group due to a 15 fold improvement in efficiency which pays off in low level measurment scenarios so common to fusor startups.

The nearly forgotten Boron lined chamber: *****************

Another good counter tube is the boron lined proportional chamber. This is ,effectively, a geiger counter with a thin enriched layer of boron 10 on the inside walls. The efficiency can rival the BF3, but the noise immunity is terrible as gamma's, cosmics, and even high energy beta particles will make this baby count away. The gas amplification can rival the GM tube and discrimination is just not possible except will special versions of the tube where the efficiency is compromised a bit.

The expense of this tube is the key factor keeping this tube out of the hands of most amateurs. Once very popular (40's - 50's), this device is not used much in new installations. However, such tubes are still manufactured mostly as replacements for exisiting setups.

Fission chambers - Right....Try and get one........**********

Fission chambers tend to use a directly fissionable isotope that responds to thermal neutrons. Most such chamber use plutonium or U235 in a thin deposit on plates inside the tube. This is often in the near gram or multigram quantity level. This makes all such tubes licensed by the NRC and a controlled item.
(read, "amateurs need no apply")

These tubes are fairly efficient, (1%-4%), and totally immune to other radiation for the most part. In spite of the license requirements, they are also tremendously expensive.

Other fission tubes are less expensive and are encountered surplus. These use the fertile U238 and are a no license required item. These detect fast neutrons directly! These are totally immune to other radiation! It would seem that this is a dream tube, but the efficiency is terrible often being under .05%. Such tubes are reserved for a rather large fast flux.

I know of no current source for new fission tubes.

Bringing up the rear - activation systems *******************

For steady, large flux levels of mixed neutrons, an activation system is often used. These can be assembled very cheaply and are usually just a G-M tube stuffed in a moderating medium with indium or silver foil rolled or wraped around it. These are fairly slow readers (based on activation equilibrium time) and find the most use as "area monitors" most of the time around reactors. Think of them as a sort of integrating, neutron, dosimeter system. I have never heard of one being used in a fusor based operation. Certainly, they would demand a fusor be in top form and really "putting out" to read much of anything.

Bubble detectors - look Ma no batteries **************

*************** UPDATE November 2004 ********

The bubble detector just became the best fast neutron detector on earth!!
It is also the cheapest and most reliable of all fast neutron detectors.

You must read my Bubble posting for the full story

viewtopic.php?f=13&t=5522#p33966

Note* the He3 detector in larger sizes and high pressure fills remains the most sensitive neutron detector.

Solid State Detectors - Forget 'em ********************

The solid state neutron detectors are all poor, at best, and require a bit of flux to even wiggle the needle. The small active areas of such detectors limits there use where we are concerned. They are also very expensive in the larger versions, most of which are still experimental in nature. Perhaps in time, a good, low cost, solid state device will evolve, but don't hold your breath.

***********************************************************

There you have it. If I have any more comments, they will be added as needed.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
henryhallam
Posts: 73
Joined: Wed Feb 04, 2004 1:11 pm
Real name:
Contact:

Re: FAQ - Neutron Detectors

Post by henryhallam »

Thanks Richard - very informative.
Do you know how easy it is to use, e.g. a Ludlum model 12-4 counter with a different type of tube? (He3/a different BF3). Is there a standard connection or must a whole new controller be used?
Also, how often must they be sent off for calibration? Is it really necessary at all since you're lucky to get within 20% of the true value? The counter I've bought was calibrated in 1997 and I'm being offered recalibration now before it's sent out, at $130. Worth it?

Thanks

Henry
Jon Rosenstiel
Posts: 1494
Joined: Thu Jun 28, 2001 1:30 am
Real name: Jon Rosenstiel
Location: Southern California

Re: FAQ - Neutron Detectors

Post by Jon Rosenstiel »

Richard,

I think you're selling the activation detector short. I did a few tests earlier this year comparing the sensitivity of my Ludlum 12-4 BF3 detector to my home made indium activation detector.

Please see:

viewtopic.php?f=13&t=5485#p33929

A quick rundown of my experiment….

Fusor output = 3.9E+04 n/s
BF3 detector count rate = 40cpm
Activation detector count rate = 141cpm (background subtracted)

The bottom line is that at a fusor output of 3.9E+04 n/s my indium activation detector is producing 3.5 times the count rate of my BF3 detector.

Jon Rosenstiel
User avatar
Richard Hull
Moderator
Posts: 14991
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: FAQ - Neutron Detectors

Post by Richard Hull »

Jon, I am stunned that 39 kilocounts would activate anything!!!

I just hit 35,000 counts the other night with Fusor IV! (see the activation post datum) I'll have to try that activation with some indium here. Activation systems are just not mentioned in the texts for viable, low level neutron measurement. Indium may be a special case due to its cross section. Silver is the most used with GM wraps and relys on the narrow epithermal high cross section region. (not good).

I must check this out with the indium. Meantime, see my post I am putting up today regarding last night;s He3 experiments.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Post Reply

Return to “FAQs: Neutron - Radiation Detection”