Hi!
There seems to be another option besides the gas tube and specifically "loaded" neutron scintillators which is using ordinary organic scintillators (at least liquid ones) together with a pulse discrimination circuit. The discrimination between neuts and gammas is performed by looking at the duration of the falling edge of the PMT signal (as opposed to traditional puls height discrimination) - neuts (actually the recoiling action they cause) produce a longer decay tail than gammas and can therefore be seperated from the gamma events. The time difference is in the 100ns range and the technology was (Perkin Elmer) already used in the early 70's. Has anyone tried it since it seems to me easier than mixing plastics with scintillating crystals which will optimistically yield 1% neut detection probability compared to the 78%! neut detection probability of an "ordinary" liquid scintillator (NE 213, 2", 1Mev neut).
Theoretically a digital scope connected to a PMT/scintillator should be sufficient to see the different puls shapes of neuts and gammas (possibly with a "window trigger" to ensure that all events have roughly the same amplitude to get a "clearer" picture). Anyone to try?
Sources:
T.W.Crane, M.P.Barker, Chapter "Neutron Counters", - Fig. 13.16 which deals with nuclear assay techniques (sorry don't know the title but the source was posted here sometime ago)
Perkin Elmer "Neutron and gammy discrimination with Stilbene and other liquid scintillators"
Alternative neutron detection method
-
guest
Re: Alternative neutron detection method
Use of plastic (mostly) and liquid scintillators for fast neutron counting has been extensively discussed here in the past. I am currently working on a setup using a plastic scintillator and a 3" PMT tube. Most of my efforts at present are going into a fast preamp to interface between the PMT and counter setup.
In a typical fusor setup, there should be no need for gamma discrimination. Most of the radiation output of the fusor is relatively soft X-rays, which are blocked by the chamber walls (assuming a stainless steel vacuum chamber). A lead shield around the PMT will take care of background radiation except for energetic cosmic rays.
The new fast preamp circuit is up and running, though it needs some value tweaking to center the quiescent current in the second stage. I plan to run it with my PMT sans scintillator to get an idea of the base count level due to the PMT dark current. Next would be a run using the scintillator and a gamma source to get an idea of the average pulse height so that I can properly set the preamp gain. I will share my results when I have them.
In a typical fusor setup, there should be no need for gamma discrimination. Most of the radiation output of the fusor is relatively soft X-rays, which are blocked by the chamber walls (assuming a stainless steel vacuum chamber). A lead shield around the PMT will take care of background radiation except for energetic cosmic rays.
The new fast preamp circuit is up and running, though it needs some value tweaking to center the quiescent current in the second stage. I plan to run it with my PMT sans scintillator to get an idea of the base count level due to the PMT dark current. Next would be a run using the scintillator and a gamma source to get an idea of the average pulse height so that I can properly set the preamp gain. I will share my results when I have them.
-
guest
Re: Alternative neutron detection method
Hi Richard!
You are right, using ordinary plastic scintillators has been discussed extensively earlier. And yes too, 30kV collisions can't produce gamma rays in the fusor - but don't forget the background radiation! Quote from Richard Hull: "I have a lot of BC 400 here and have several detectors made up and the stuff is noisy as hell! Joe was getting thousands of counts per minute - background!! Compare this to a BF3 tube in moderator's 1 count every 1.8 minutes - background"
The difference lies in the discrimination methodology (pulse HEIGHT vs. pulse SHAPE) - as Richard Hull has stated in the above mentioned post using pulse height discrimination dramatically reduces the efficiency because you have to cut off the low energy background "tail" till the compton edge. By doing so you also dramatically loose neut efficiency too because many neut recoils in an "unloaded" scintillator are weak. As result the high neutron efficiency (Richard Hull quotes 38% for BC400) is reduced to about 3% but still with gamma background (the common K40 is at about 1,4MeV) so you still have to search for the neut signature in the garbage. My 2"x2" NaJ detector e.g. gives many hundred counts per minute (that is background only up to 0,5 MeV on my self built MCA!) and this is not caused by the PMT dark current!
So - if the pulse SHAPE method would work you can both boost the efficiency AND reduce the gamma background noise drastically = smaller and cheaper detectors and/or cheaper setups (less stringent voltage, vacuum, etc. requirements).
Jürgen
You are right, using ordinary plastic scintillators has been discussed extensively earlier. And yes too, 30kV collisions can't produce gamma rays in the fusor - but don't forget the background radiation! Quote from Richard Hull: "I have a lot of BC 400 here and have several detectors made up and the stuff is noisy as hell! Joe was getting thousands of counts per minute - background!! Compare this to a BF3 tube in moderator's 1 count every 1.8 minutes - background"
The difference lies in the discrimination methodology (pulse HEIGHT vs. pulse SHAPE) - as Richard Hull has stated in the above mentioned post using pulse height discrimination dramatically reduces the efficiency because you have to cut off the low energy background "tail" till the compton edge. By doing so you also dramatically loose neut efficiency too because many neut recoils in an "unloaded" scintillator are weak. As result the high neutron efficiency (Richard Hull quotes 38% for BC400) is reduced to about 3% but still with gamma background (the common K40 is at about 1,4MeV) so you still have to search for the neut signature in the garbage. My 2"x2" NaJ detector e.g. gives many hundred counts per minute (that is background only up to 0,5 MeV on my self built MCA!) and this is not caused by the PMT dark current!
So - if the pulse SHAPE method would work you can both boost the efficiency AND reduce the gamma background noise drastically = smaller and cheaper detectors and/or cheaper setups (less stringent voltage, vacuum, etc. requirements).
Jürgen
-
guest
Re: Alternative neutron detection method
Hi Richard!
You are right, using ordinary plastic scintillators has been discussed extensively earlier. And yes too, 30kV collisions can't produce gamma rays in the fusor - but don't forget the background radiation! Quote from Richard Hull: "I have a lot of BC 400 here and have several detectors made up and the stuff is noisy as hell! Joe was getting thousands of counts per minute - background!! Compare this to a BF3 tube in moderator's 1 count every 1.8 minutes - background"
The difference lies in the discrimination methodology (pulse HEIGHT vs. pulse SHAPE) - as Richard Hull has stated in the above mentioned post using pulse height discrimination dramatically reduces the efficiency because you have to cut off the low energy background "tail" till the compton edge. By doing so you also dramatically loose neut efficiency too because many neut recoils in an "unloaded" scintillator are weak. As result the high neutron efficiency (Richard Hull quotes 38% for BC400) is reduced to about 3% but still with gamma background (the common K40 is at about 1,4MeV) so you still have to search for the neut signature in the garbage. My 2"x2" NaJ detector e.g. gives many hundred counts per minute (that is background only up to 0,5 MeV on my self built MCA!) and this is not caused by the PMT dark current!
I'm not sure about the effects of lead shielding which will surely reduce the low energy gammas but will be far more "transparent" to the higher energy gammas.
So - if the pulse SHAPE method would work you can both boost the efficiency AND reduce the gamma background noise drastically = smaller and cheaper detectors and/or cheaper setups (less stringent voltage, vacuum, etc. requirements).
Jürgen
You are right, using ordinary plastic scintillators has been discussed extensively earlier. And yes too, 30kV collisions can't produce gamma rays in the fusor - but don't forget the background radiation! Quote from Richard Hull: "I have a lot of BC 400 here and have several detectors made up and the stuff is noisy as hell! Joe was getting thousands of counts per minute - background!! Compare this to a BF3 tube in moderator's 1 count every 1.8 minutes - background"
The difference lies in the discrimination methodology (pulse HEIGHT vs. pulse SHAPE) - as Richard Hull has stated in the above mentioned post using pulse height discrimination dramatically reduces the efficiency because you have to cut off the low energy background "tail" till the compton edge. By doing so you also dramatically loose neut efficiency too because many neut recoils in an "unloaded" scintillator are weak. As result the high neutron efficiency (Richard Hull quotes 38% for BC400) is reduced to about 3% but still with gamma background (the common K40 is at about 1,4MeV) so you still have to search for the neut signature in the garbage. My 2"x2" NaJ detector e.g. gives many hundred counts per minute (that is background only up to 0,5 MeV on my self built MCA!) and this is not caused by the PMT dark current!
I'm not sure about the effects of lead shielding which will surely reduce the low energy gammas but will be far more "transparent" to the higher energy gammas.
So - if the pulse SHAPE method would work you can both boost the efficiency AND reduce the gamma background noise drastically = smaller and cheaper detectors and/or cheaper setups (less stringent voltage, vacuum, etc. requirements).
Jürgen
-
Richard Hull
- Moderator
- Posts: 15037
- Joined: Fri Jun 15, 2001 9:44 am
- Real name: Richard Hull
Re: Alternative neutron detection method
I am very interested in any method of neutron detection which reduces costs or complexity of systems in place now.
The liquid scintialltors are covered in the "Fast Neutron Physics".
The discrimination methodology can be developed much more fully today than in the 50's when the above reference was written, and I look forward the reports on its success and methodology here in this forum. The real pity is the lack of good, known, steady sources of neutrons. What a bitch!
The fusor is too sporatic and touchy to operate in amateur hands to be used as a calibrated source. It is, however the only source we have as a group. Individuals may luck onto a pal or friend who can get them "inside" an institution with a neutron source. The pity here is that there is so much red tape, and an outsider exposed to neutrons with their source on the sly while farting around with an unknown instrument is a heart stopping thought for any tight-assed administrator within the institution.
Nowadays, every time a lid is removed from a source it has to be logged (who, what, when and why). In most places, there are now alarms in place or wall mounted dosimeters which will catch the unlogged entries. Heads will roll.
Neuts are bad news, but a rare exposure for instrumentation purposes is tolerable.....Just not on my watch.
Richard Hull
The liquid scintialltors are covered in the "Fast Neutron Physics".
The discrimination methodology can be developed much more fully today than in the 50's when the above reference was written, and I look forward the reports on its success and methodology here in this forum. The real pity is the lack of good, known, steady sources of neutrons. What a bitch!
The fusor is too sporatic and touchy to operate in amateur hands to be used as a calibrated source. It is, however the only source we have as a group. Individuals may luck onto a pal or friend who can get them "inside" an institution with a neutron source. The pity here is that there is so much red tape, and an outsider exposed to neutrons with their source on the sly while farting around with an unknown instrument is a heart stopping thought for any tight-assed administrator within the institution.
Nowadays, every time a lid is removed from a source it has to be logged (who, what, when and why). In most places, there are now alarms in place or wall mounted dosimeters which will catch the unlogged entries. Heads will roll.
Neuts are bad news, but a rare exposure for instrumentation purposes is tolerable.....Just not on my watch.
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
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
-
guest
Re: Alternative neutron detection method
In the fusor environment, a lot of the low level "grass" from PMT-based intstrumentation is probably due to the dark current from the PMT itself. A decent bialkali photocathode will have a dark current of several thousand photoelectrons /sec-cm^2 at room temperature. Lower work function photocathodes are far worse. These pulses are low amplitude, and can be easily rejected with a decent discriminator or SCA (or the input level adjust on your counter). They do complicate calibration of any PMT based radiation detector. This is especially true in the case of a proton recoil detector, where the energy spectrum of the recoil protons is pretty much a continuum from zero to the max energy of the incoming neutrons. This means that a certain percentage of the recoil events must needs be rejected by the discriminator, reducing the effective efficiency of the detector. This can be accounted for with a proper calibration with a neutron source. As Richard Hull found out a few months ago, Ludlum will do a neutron calibration run for $70 - a real bargain.
A properly shielded plastic scintillator will detect very little in the way of background radiation. Joe Zambelli reported using a calibrated plastic sciltillator some months back (in the old forum). Background count rates, if I remember correctly, were on the order of several tens of events/min with a discriminator.
When I am able to do the measurements, I will post results for backgound pulse rate/avg amplitude of a naked 3" PMT sans scintillator, the same PMT with scintillator and with/without radiation shielding, monitoring the natural background. I have found that at least with a NaI scintillator, most of the big events are cosmic radiation, with a frequency of several tens of events/minute. My preamp is almost ready, so I should be able to post at least preliminary findings some time this month.
A properly shielded plastic scintillator will detect very little in the way of background radiation. Joe Zambelli reported using a calibrated plastic sciltillator some months back (in the old forum). Background count rates, if I remember correctly, were on the order of several tens of events/min with a discriminator.
When I am able to do the measurements, I will post results for backgound pulse rate/avg amplitude of a naked 3" PMT sans scintillator, the same PMT with scintillator and with/without radiation shielding, monitoring the natural background. I have found that at least with a NaI scintillator, most of the big events are cosmic radiation, with a frequency of several tens of events/minute. My preamp is almost ready, so I should be able to post at least preliminary findings some time this month.
-
Richard Hull
- Moderator
- Posts: 15037
- Joined: Fri Jun 15, 2001 9:44 am
- Real name: Richard Hull
Re: Alternative neutron detection method
Richard Hester seems determined to beat th' system and is always ready to skin th' cat a different way. I like that. I hope to listen and learn from his efforts as they progress.
I have to be a devil's advocate lest the wild eyed spend themselves hopelessly upon the rocks rather than advance steadily along conventional lines. Reading and experimenting often save a lot of wasted effort by not going up blind alley's which have already been tried. Nonetheless it is often the bold spirit which finds new avenues of approach.
Richard has worked at the idea of using more modern techniques applied to an old problem found wanting or which had no available solution with the technology of the time.
This is a good approach. A lot of older failed ideas might be able to be dusted off and reworked in light of newer technological advancements. This effort does require a lot of reading.
It is obvious that Richard Hester and I have been reading a lot. In my case, it has been so much so, that lately, that is about all I am doing. Now that I have done fusion and "secured the good", it is imperative that I stop and become smarter through study before going off "in search of the better". In my heart, as an empiricist, I lust to just slap fusor IV on the pumps, but I resist the temptation.
Richrd Hull
I have to be a devil's advocate lest the wild eyed spend themselves hopelessly upon the rocks rather than advance steadily along conventional lines. Reading and experimenting often save a lot of wasted effort by not going up blind alley's which have already been tried. Nonetheless it is often the bold spirit which finds new avenues of approach.
Richard has worked at the idea of using more modern techniques applied to an old problem found wanting or which had no available solution with the technology of the time.
This is a good approach. A lot of older failed ideas might be able to be dusted off and reworked in light of newer technological advancements. This effort does require a lot of reading.
It is obvious that Richard Hester and I have been reading a lot. In my case, it has been so much so, that lately, that is about all I am doing. Now that I have done fusion and "secured the good", it is imperative that I stop and become smarter through study before going off "in search of the better". In my heart, as an empiricist, I lust to just slap fusor IV on the pumps, but I resist the temptation.
Richrd 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
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
-
guest
Re: Alternative neutron detection method
I think I've found the before mentioned Zambelli post on the old board:
"...detector in a thick stainless steel housing and lined it with 0.25" of lead... help a little bit in reducing background. ... For reference, my SCA allows me to select a window of energies to monitor anywhere within its 0-10 volt range. I set my lower level discriminator at 0.5V and and a window width larger enough to count everything above this. With my 2" diameter by 2" deep detector I get a background count of about 240 cpm. ...(neut) detection efficiency of over 30%".
240 cpm background is in the same range as what I would expect the background to be once you remove the low end "grass" and add (little) shielding. This figure sounds still high to me even for a 30%+ neut detection probability to detect e.g. a x 10^3/sec isotropic neut fluxe with certainty (assuming you wan't to keep the fusor and HV as basic as possible). Therefore my question whether anyone has found either theoretical or practical signs of the "pulse shaping gamma/neut discrimination" approach? Perkin Elmer even built a (dedicated for this purpose?) 552 puls shape analyzer NIM module.
Having a cheap & sensitive neut detection could convince me (and possibly others) to go from the demo stage to a neutron producing version since it (maybe I'm wrong) would dramatically reduce the effort in the fusor construction. Am I asking for too much?
Jürgen
P.S. The pulse shape discrimination has a (at least from my point of view) hurdle in such that the 100ns (that's about 100% difference between a gamma and neut deceay) occours at 1/100th of the peak amplitude thus requiring a possibly large dynamic range (signal processing shouldn't be too difficult since fast (0,5 - 2,5GHz) logarithmic converters are cheaply available by Analog Devices).
"...detector in a thick stainless steel housing and lined it with 0.25" of lead... help a little bit in reducing background. ... For reference, my SCA allows me to select a window of energies to monitor anywhere within its 0-10 volt range. I set my lower level discriminator at 0.5V and and a window width larger enough to count everything above this. With my 2" diameter by 2" deep detector I get a background count of about 240 cpm. ...(neut) detection efficiency of over 30%".
240 cpm background is in the same range as what I would expect the background to be once you remove the low end "grass" and add (little) shielding. This figure sounds still high to me even for a 30%+ neut detection probability to detect e.g. a x 10^3/sec isotropic neut fluxe with certainty (assuming you wan't to keep the fusor and HV as basic as possible). Therefore my question whether anyone has found either theoretical or practical signs of the "pulse shaping gamma/neut discrimination" approach? Perkin Elmer even built a (dedicated for this purpose?) 552 puls shape analyzer NIM module.
Having a cheap & sensitive neut detection could convince me (and possibly others) to go from the demo stage to a neutron producing version since it (maybe I'm wrong) would dramatically reduce the effort in the fusor construction. Am I asking for too much?
Jürgen
P.S. The pulse shape discrimination has a (at least from my point of view) hurdle in such that the 100ns (that's about 100% difference between a gamma and neut deceay) occours at 1/100th of the peak amplitude thus requiring a possibly large dynamic range (signal processing shouldn't be too difficult since fast (0,5 - 2,5GHz) logarithmic converters are cheaply available by Analog Devices).
-
Richard Hull
- Moderator
- Posts: 15037
- Joined: Fri Jun 15, 2001 9:44 am
- Real name: Richard Hull
Re: Alternative neutron detection method
Jurgen is correct in his surmise that an easy neutron counter would make the second stage (neutron producing fusor) a lot more attractive to aspirants and would-be fusioneers.
I call it the neutron detection wall. It is no fun making neutrons and claiming fusion unless you can count the stuff. Neutrons are so hard to count that there is little mistaking neutrons in a neutron counter once you have shown that no other particles are extant which can trigger the counter. (that is why I just love the heck out of the BF3 system and the BC 720 scintillator scheme.) These systems are, by nature, discriminatory at the first order, albeit rather costly to buy or build with amateur hands. Most cheaper systems and claimed higher efficiency systems demand a bit of either complex signal manipulation or possible sensitivity to often uncontrolable external sources of counts.
Still, as I have said, the effort should repay the assiduous experimenter and, if proved successful on even a limited scale, make the plight of a newcomer bewildered by the many requirements to make a fusion device,a bit easier.
Richard Hull
I call it the neutron detection wall. It is no fun making neutrons and claiming fusion unless you can count the stuff. Neutrons are so hard to count that there is little mistaking neutrons in a neutron counter once you have shown that no other particles are extant which can trigger the counter. (that is why I just love the heck out of the BF3 system and the BC 720 scintillator scheme.) These systems are, by nature, discriminatory at the first order, albeit rather costly to buy or build with amateur hands. Most cheaper systems and claimed higher efficiency systems demand a bit of either complex signal manipulation or possible sensitivity to often uncontrolable external sources of counts.
Still, as I have said, the effort should repay the assiduous experimenter and, if proved successful on even a limited scale, make the plight of a newcomer bewildered by the many requirements to make a fusion device,a bit easier.
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
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
-
Tom Dressel
- Posts: 287
- Joined: Fri Jun 22, 2001 4:44 pm
- Real name:
Re: Alternative neutron detection method
Just as an aside, I hade lunch today with one of the neurologists at Abbott Northwestern Hospital who does monitoring during spinal surgery. It is high risk surgery to be straightening the spine or decompressing the spinal cord (it is REALLY BAD to have the patient wake up paraplegig or worse yet quadraplegic) They use what they call evoked potentials. They screw an electrode into the scalp over the motor strip (the part of the brain that control voluntary movement), then send a train of 200 volt pulses (yes 200 volts) into the scalp, and look for the electrical activity of muscle twitches in the foot. If they loose the evoked potentials while the surgeons are cranking down on the straightening hardware they back off until the potentials return.
They are looking for an appropriately timed microsecond long, milivolt high, pulse in a sea of grass on the scope. They use a lot of familiar sounding techniques, like signal averaging, integrating and gating. Some of the newer instrumentation use software algorithms to eliminate the guesswork.
Tom Dressel
They are looking for an appropriately timed microsecond long, milivolt high, pulse in a sea of grass on the scope. They use a lot of familiar sounding techniques, like signal averaging, integrating and gating. Some of the newer instrumentation use software algorithms to eliminate the guesswork.
Tom Dressel
Re: Alternative neutron detection method
After looking into this a bit, I concur that this method would work, and very possibly be better than the discrimination based on amplitude that most amateurs are familiar with. Gammas and neutrons produce different pulse shapes, which can be used to discriminate between them. In fact, there are NIM modules used specifically for this type of discrimination, called Pulse Shape Analyzers, etc. It looks like this is actually fairly frequently done; Ortec's web site (for their 552 NIM module) http://www.ortec-online.com/electronics/sca/552.htm actually mentions neutron/gamma discrimination as the most frequent application of pulse shape analyzer modules!
Also, as the neutrons interacting with the scintillator may produce pulses of many amplitudes, discriminating on shape rather than amplitude could be used to increase sensitivity on scintillator systems, (maybe make BC-400 a much better contender?)
On the other hand, BF3 and He3 proportional counters produce a monoenergetic pulse for all neutrons, from the charged particle that is emitted by the neutron interaction. Here, amplitude discrimination is definitely the way to go.
This is how I summarize my understanding at this point, between the discussion here and a few days of research on this idea on my own.
-Ryan Edgar
Also, as the neutrons interacting with the scintillator may produce pulses of many amplitudes, discriminating on shape rather than amplitude could be used to increase sensitivity on scintillator systems, (maybe make BC-400 a much better contender?)
On the other hand, BF3 and He3 proportional counters produce a monoenergetic pulse for all neutrons, from the charged particle that is emitted by the neutron interaction. Here, amplitude discrimination is definitely the way to go.
This is how I summarize my understanding at this point, between the discussion here and a few days of research on this idea on my own.
-Ryan Edgar