My experience with the GammaSpectacular GS-NEUTRON

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.

My experience with the GammaSpectacular GS-NEUTRON

Postby Trent Carter » Sun Jul 16, 2017 8:13 pm

Preface:
===========================
In the complex quest for Fusion one of the most under resourced necessities is Neutron detection. I am a physicist by education, not day job, and have orders of magnitude more experience with radiation and radiation detection than the laymen, and I still grossly underestimated the task of detecting neutrons. I assumed that COTS (consumer off the shelf) solutions existed and could likely be sourced second hand in turn key condition from eBay or the like. While this may be partially true it is rarely the case.

I read the various FAQs from Richard Hull on the topic but was still unconvinced it would be a challenge. But as is usually the case Richard was right. Richard had a slight advantage being years ahead of the great He3 shortage that effectively started in the early 2000's and was able to purchase an HE3 tube. I will note however that nearly owning a HE3 tube does not guarantee success.

Neutrons, why are they so hard to detect?
=====================================
Neutrons are neutral and thus don't interact via charge. Also before moderating down to thermal velocities they are very unlikely to interact with anything making high energy neutrons even harder to detect. By comparison it is easy to detect Alpha (He Nucleus), Beta (Electron), Protons, Xray and Gamma (just Photons). I won't try an cover this topic here, as it is well covered on this forum and wiki.

Terms:
===========================
I am using the term: "generate" neutron to be synonymous with emit or to free a neutron. Not to create a neutron from three quarks.

What is needed for a Neutron Counting System?
==========================================
1. Neutron Counter (usually a Tube filled with 3HE or BF3 and/or lined with Boron : Or some type of scintillation detector with Photomultiplier (PMT))
2. Moderator (HDPE, paraffin WAX, or even Water) *water and HV do not mix
3. Pre-Amp (circuit very close to the tube that has a large resistor in-line with the HV and HV capable cap to decouple the signal, and an OpAmp or two)
4. HV Power supply (varies by tube type; the GS Neutron needs 460 V, but other tubes can need in the 2kV range)
5. Discriminator (this looks at the signal and decides if it is noise or a real Neutron event)
6. Counter (This device usually counts total Neutron events and calculates N-CPM)
7. Neutron source:
a. Known source; most don't have access to this
b. Unknown source above background Neutrons from cosmic ray neutron generation i.e. PO-210 + Beryllium : Measured in my lab at 11.7/hr or PO-210+Be+Cosmic at 19/hr
c. Cosmic background Neutrons (very slow source at about 8-12 pulses / hr (as detected by this unit per spec), measured in my lab at 7.5/hr

HV Supply:
===========================
Most Neutron tubes, even turn key systems like the GS-NEUTRON need a HV supply. GammaSpectacular offers a nice one, but it is costly as most HV supplies are. I recommend it unless you both have a suitable clean supply and/or know what you are doing. HV Kills, Dirty HV just generates noise.

I used my own HV supply as I own countless units, and had a fantastic little Spellman MM1PN1.5/12/S that measures 1.5x1.5x0.75" and is variable 0-1.5kV and <<$100. It can be operated as a positive or negative supply as it is isolated. I got mine from eBay for a song, bought several. The best part about this supply is the fact that the output varied directly with the input and at 5.2 V IN it outputs 460 V out, exactly what the GS-NEUTRON requires. This means I can use a 5V USB supply, which everyone has handy. These types of supplies sometimes need +/- 15V supplies which are more expensive and harder to package.

Emco makes a lot of nice, small, reasonable priced HV supplies similar to this Spellman unit.

Signal Discrimination:
===========================
You need to discriminate Neutron signal from noise. To do this requires some sort of active analog system, or digital system. This is a good time to pitch Carl Willis YouTube video Neutrons_Setup; he uses a small rack with HV, discriminator, and counter in one unit. Its a $ unit, but every handy.

For signal discrimination I used my Rigol DS1054Z Digital Scope and Pulse discrimination (countless methods to discriminate signals including math modules). Here is why I dedicate a digital scope to this task.
a. It has very powerful discriminator software, way more than a dedicated analog rack setup.
b. It has a screen and memory so you can see the signal and save them.
c. It can output a Pulse Signal when a Neutron is found, which can go to a counter, or a dedicated integrated control system like I use.
d. For ~$300 USD it is a tool that can be used for other projects as it is a 4 ch dig scope that can decode I2C, SPI, RS232, etc.

Counter:
===========================
So you have discriminated a Neutron from noise and now you need to count it.

For a counter I initially used my HP 5334A Universal Counter (great lab gear). It does what you might expect, counting and calculating rate. But its rate system is not friendly to low CPMs, but it is a nice tool for <$100
For my production system which has some difficult requirements, like being 100% remote operated from anywhere in the world or beverly hills, I use a compute platform with Interrupt based digital inputs. These inputs just need the pulse in TTL. So I tested with the HP counter, then integrated into my compute system.

Unpacking and Initial Opinion of the GS-NEUTRON:
============================================
One of our fellow Fusor.net members created a Neutron detector in late 2016. I will let him go unnamed as I can't recall the forum rules on the matter, but if you have any time invested to Fusor.net you already know everything on this subject.

The product purchased was the GammaSpectacular GS-NEUTRON. I will also refrain from posting a link or details about pricing but its an easy google.

The product arrived very fast via EMS from Australia to the USA. It was well packaged and arrived intact.

As far as engineering goes it is well designed and well made. It would look just as nice on your office desk or in a glass cabinet, a nice piece visually.

It has an integrated HDPE moderator with a diameter of 90mm
It uses CHM-12 tube with a recommended 450V bias
Its efficiency is 15% (more on that later)
It is designed to be used with a sound card at the data acquisition side, this seems to be a hold-over from the normal Gamma work of this company, but it is a handy interface if it meets your timing (pulse-width) requirements.
It runs off of 1x 9V battery for ~50 hours, I have been getting more like 30 hours, but 9V batteries are cheap. This 9V supply just runs the pre-amp built into the unit. It does not supply the 450 V for the HV section.

Sensitivity:
========================
If you sent 100 neutrons through a 1cm cube it will detect 15 counts. That is the claimed 15% efficiency. It varies tube/tube.

I was also quoted a spec of 4200 C/mr if you were to use the calculator at http://www.gammaspectacular.com/fusion_calculator.html

Neutron Source:
=========================
I have two sources:
1. Cosmic Neutrons. These are generated with high energy cosmic radiation hits the stratosphere and generates free neutrons. Expected rate 8-12 Neutrons/hr
2. PO-210 + Beryllium. The PO-210 is an Alpha (helium nucleus) emitter. The Polonium 210 Alpha strikes the Beryllium and emits a Neutron isotropically. The conversion rate is 93 Neutrons per Million alpha particles. This is interesting because PO-210 is sometimes made by irradiating Bi with Neutrons. Then I use it as a neutron source.
a. I purchased an anti-static PO-210 source with 500 micro curies for ~$60 (google staticmaster) You want the 3" cartridge. make sure its fairly new; the half life is 138 days. These products have a date stamped on them. Don't buy them from eBay, they are no longer a good source due to the fast half life
b. I purchased some Beryllium from eBay for <$100. A small piece is fine. Mine is 1"x1/2",1/8" est.
c. Assembly; Put Beryllium metal directly in front of the gold plated PO-220 antistatic strips. Make sure nothing is in between the two items as Alphas are easily stopped by pretty much anything including air. Some clever Fusor.net members put these in a vacuum to increase the alpha on beryllium count.
d. Usage: Once the Neutron is generated it can not be stopped and its direction is isotropic (in all directions random). Your biggest issue is the inverse square law, so put the source as close to the Neutron tubes outer moderator as possible.

DANGER:
=============
PO-210 is bad, really bad, I mean really really bad. One gram can sicken 20 Million people with 10 Million possibly perishing. The static master encases the PO-210 in a gold electroplating process, thats safe, just don't "free" it. Don't get it in your body. Go ahead and wash your hands, they needed it anyway.

Beryllium: Its nasty as well; killing several great scientists (one of the Curie scientist family and Enrico Fermi) before the toxicity was known. You can handle it in solid, but never cut it as the dust is what kills. Why not wash your hands again?

HV Kills every day. It might also kill friend and family if they reach in to help. always have a large red OFF push-button and educate all observers.

Testing:
============================
Here is data from the test bench with the Neutron sources being Cosmic and PO-210+Beryllium

Neutron Detector: GS-NEUTRON with integrated HDPE 90mm Moderator
HV Bias 460 V
*I did put p-wax behind the PO-210+Be to moderate the Neutrons exiting the wrong direction, but I left said moderator in place for both Cosmic and PO-210 testing.

CPM Source Duration Count
0.15 Cosmic 12min 12
0.27 PO-210 190 52
0.12 Cosmic 60 7
0.41 PO-210 63 26
0.09 Cosmic 190 18
0.35 PO-210 60 21
0.14 Cosmic 73 10
0.22 PO-210 108 24
0.35 PO-210 150 52

Average Cosmic Neutrons: 0.125 CPM
Average PO-210+Be+Cosmic Neutrons: 0.32 CPM *Note that PO-210 Sources also include background Cosmic as it was not shielded
Average PO-220+Be (removed average Cosmic) Neutrons: 0.195 CPM

Test Summary: The PO-210+Be Neutron source generated 156% more Neutrons than cosmic and provided a reliable Neutron every 3 minutes.

Ease of use:
=====================================
The GS-NEUTRON was easy to connect and operate with one simple switch ON/OFF.

The output pulses (see images) are 200uS and ~4v with a nice shape that is quite different from the noise.

Noise in the Cosmic / PO-210+Be testing:
=====================================
The GS-NEUTRON was low noise but a discriminator is still recommended

Noise on a working Fusor with a 25kV Spellman supply:
===============================================
There was noise and if I had been just using the sound card based system the noise could have been counted, this would have been erroneous. This can be filtered as the pulse width for the noise was very small ~10uS. But without filtering; if you blindly sided the sound card output line, you would likely over count by one order of magnitude during a Fusor run. Using the Rigol pulse with discriminator set to a threshold of >100uS and min voltage of 3V will discriminate out almost all of the noise.

I will attach images of noise. The source if the noise has not been investigated, so for now lets just call it noise. The noise did coincide with my Geiger counter going off the charts, so likely x-ray, gamma and/or Electrical.

Gamma Rejection:
==================
Unfortunately I do not have a strong Gamma source as they are generally licensed. But I did use all of my sources and none of them triggered any counts. If anyone has both a GS-NEUTRON and a strong Gamma emitter I would appreciate any test results.

Limitations:
========================
a. The pulse width is 200uS, putting a maximum count rate of 1/200uS = 5000 counts/sec as limited by pulse width. The manufacturer gave clear instructions on how to reduce this pulse width by changing the resistor in the output RC circuit. (I have not completed this mod)
b. The moderator is very difficult to remove. This is only an issue for those trying to get in the "club" and need to show moderated and unmoderated values during one run.

Overall Satisfaction with the GS-NEUTRON:
====================================
I am glad I made the purchase and recommend the product. It is not turn-key, but in its price range is as close to turn key as I could source.

I would like to note that the manufacturer was very responsive to my questions and comments.
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Re: My experience with the GammaSpectacular GS-NEUTRON

Postby Trent Carter » Sun Jul 16, 2017 8:38 pm

GS-Neutron on Mr Fusion Mark 3

IMG_3815.JPG


IMG_4240.JPG


Static master PO-210 + Be up against GS-Neutron

IMG_3821.JPG


Clean Neutron Pulse from Cosmic

IMG_3801.JPG


Neutrons with noise during high Neutron Fusor run. 25kV

IMG_3878.JPG


Static master PO-220 500millicuri 3" reload cartridge with beryllium taped in front of the gold po-210 alpha source

IMG_3831.JPG


IMG_3830.JPG


IMG_3832.JPG


Spellman HV module

IMG_4234.JPG


HP Counter
IMG_4235.JPG


Rigol DS-1054z Scope

IMG_4239.JPG


Fusor System in action
IMG_3882.JPG
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Re: My experience with the GammaSpectacular GS-NEUTRON

Postby Richard Hull » Tue Jul 18, 2017 5:02 am

Needless to say, that the cosmic detection rates quoted here are related to the detector tube volume, its fill pressure, gas types, (most BF3 and 3He tubes a have a small amount of other gases added for stability), and the moderator type and volume. Any natural background, (cosmic) counting with a neutron counting system will vary tremendously, system-to-system based on the above qualifications and parameters. No two different neutron measuring systems will agree on background (cosmic counts). One of them will be more or less efficient than the other.

As an example, my 3He system produces a background, (cosmic count) that is relatively stable over the last 13 years of 6.5 to 8 neutron/cosmic counts per minute. During the last massive CME of the sun, I counted over 14 counts per minute tapering off over 4 days to the more normal counts quoted above. I posted on this at the time.

A large CME can throw off enough high speed protons to increase the Earth-based cosmic count. Neutrons at lower altitudes near sea level are vastly decreased due to moderation and absorption by water vapor and air molecules in the atmosphere intervening between the maximum neutron production region of the upper atmosphere (10-30 miles above sea level).

It is scientifically appropriated to subtract natural background from any fusion producing neutron source count under measurement. In the case of my fusor IV, with a pre-run and post-run background count of 7 neutrons per minute, taken against a peak run count of 30,000 or more counts per minute, such superfluous background subtractions are never made as the best hoped for accuracy in such electronic efforts rarely attains +/- 5% accuracy if absolute count numbers are sought.

As mentioned here many times, run-to-run absolute neutron number comparisons can easily be expected to be proportionaly accurate to +/- 1%.

The upshot is that providing you are absolutely sure you are measuring neutrons, and neutrons only, electronically, your backcount is what it is and cannot be judged against any other system that is not an exact replica of your system.

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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Re: My experience with the GammaSpectacular GS-NEUTRON

Postby Finn Hammer » Tue Jul 18, 2017 7:34 pm

Trent,

Thank you for a comprehensive report on neutron detection, I feel very fortunate to read it first off, instead of (perhaps) stumbling across it in the archives.

You write:
===========================
Signal Discrimination:
===========================
You need to discriminate Neutron signal from noise. To do this requires some sort of active analog system, or digital system. This is a good time to pitch Carl Willis YouTube video Neutrons_Setup; he uses a small rack with HV, discriminator, and counter in one unit. Its a $ unit, but every handy.

For signal discrimination I used my Rigol DS1054Z Digital Scope and Pulse discrimination (countless methods to discriminate signals including math modules). Here is why I dedicate a digital scope to this task.
a. It has very powerful discriminator software, way more than a dedicated analog rack setup.
b. It has a screen and memory so you can see the signal and save them.
c. It can output a Pulse Signal when a Neutron is found, which can go to a counter, or a dedicated integrated control system like I use.
==========================================================================================================================================

I own a Rigol DS1104z scope to, and your reasoning for using it to pull the neutron hit peaks out of the noise prompted me to read the manual once again, to try to dublicate your method, but by substituting the signal from a neutron detector tube with a signal from a manually triggered signal generator.
(I was quite embarrassed by the wealth of functions the scope has, which I have not yet found a use for, or even knew existed)
I assume you discriminate by a combination of triggel level and pulse width trigger function.

DS1104Z_20170718-211543 copy(1).jpg

The screengrab above shows the pulse train from the sig gen in yellow, and the signal out of the "trigger out" in blue, and as you see, there is only one trigger event, and therefore there is only one state change on the trigger output.
My question is therefore, how do you make the scope put out a trigger pulse every time there is a pulse on the (in this case) yellow trace?

Cheers, Finn Hammer
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Re: My experience with the GammaSpectacular GS-NEUTRON

Postby Richard Hull » Wed Jul 19, 2017 8:28 am

In general, A trigger circuit, regardless of what that circuit is or where it is has a RC time constant within a monostable multivibrator circuit. Most good variable trigger circuits can produce an output pulse of any width.

There are two basic types of monostable multivibrators. Non-retriggerable and retriggerable. In most controlled triggering devices, the trigger pulse is supposed to have an "on time" far shorter than the output pulse of the trigger circuit.

In a non-retriggerable monostable, that outputs a 100 usecond pulse, the input pulses might be only 500 nanosecond pulses. Once the monostable outputs its timed 100usec pulse from being triggered by the first short "trigger" pulse arriving at the input of the monostable, a forever fixed 100usec output pulse will be put out, regardless if even 50 more trigger pulses follow in rapid succession while the monostable is outputing its fixed 100usec output pulse.

In a retriggerable monostable the first trigger pulse starts the monostable outputing its 100 usec pulse. However, if any more trigger pulses in a train arrive within that 100usec, the monostable starts a new 100 microsecond interval extending the single output pulse by 100usec following the last triggering input pulse in the train.

Example: We have a pulse train of 10, 500 nanosecond pulses at wierd intervals (noise) and the time from the arrival of that first trigger pulse to the last trigger or noise pulse is 58 microseconds. The output pulse will be not 100 usec but 158 microseconds long.

Monostables are the basis for all electronic triggers and related delay circuits. The ubiquitous LM555 is one of the best for slow work while the SN74121/SN74123 are for rather super fast work. The first is non retriggerable and the second is retriggerable.

Your scope manual should discuss how to set the trigger duration and if the feature is retriggerable. All scopes have a trigger level control which is a form of pulse height discrimination. This can be set so that noise and unwanted pulses below a certain set amplitude will not be displayed.

A good digital storage scope is a must have item if the would-be fusioneer is cobbling up his own electronic neutron detection gear from scratch. Such a scope will let you see what is a real detection pulse and what is just so much garbage.

Pulse height discrimination and "windowing" is basically what the single channel analyzer accomplishes in a NIM module. Carl Willis shows how this is setup in his neutron detection videos.

Richard Hull
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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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Re: My experience with the GammaSpectacular GS-NEUTRON

Postby Trent Carter » Thu Jul 20, 2017 3:16 am

Finn,
Per your Q "My question is therefore, how do you make the scope put out a trigger pulse every time there is a pulse on the (in this case) yellow trace?"
There are two parts to a this exercise:
1. Capturing an event
This depends on the tube and circuit. the GS_NEUTRON outputs a positive pulse of 200uS in duration, so a simple Type=Pulse with a minimum pulse width is sufficient set to a high enough threshold.
For a tube where non-neutron pulses can be larger than neutron pulses and shaped differently; I use a Type=Runt with min+max+threshold. I am not sure your unit has Runt, but your example pulses don't need it.
2. Outputting a Trigger,
This is really what you are asking. Your answer would have been obvious had you grabbed a second scope. I will grab an image to help go along with this text. Basically the trigger pulse is a function of the time/division. If you want a shorter pulse, just reduce the time/div. On some digital scopes the trigger duration is configurable. Not he rigor it might be, maybe I just have not learned that feature. It looks like its 60 * div on the Rigol.

To make the trigger shorter just reduce the window by reducing the time/div. My pulses are 200uS (I will be reducing them to 100uS soon) so my target screen width is 20uS/div. How will that limit my maximum countable freq?
Raw Pulses from GS-NEUTRON: 200uS pulse is a maximum of 5000 counts/sec (aka Hz)
Trigger output from Rigol: @ 200uS/div = 12mS trigger pulses = 83 counts/sec (Hz)
Trigger output from Rigol: @ 20uS/div =1.2mS = 833 counts/sec (Hz) >> Thats a lot of Neutrons @ 50K CPM
For the input 200uS pulse to match the output trigger; set the Rigol to 5uS/div, that outputs 260uS on the Rigol Trigger line.

In the first sample image the Rigol is set to a Horiz time value of 200uS / Div and the Tek is set to 10mS/Div. The pulse is just about 12mS.
rigol_200us_per_div_Horiz_IMG_4269_2.jpg

*Note that the Tek has 10x horiz div where the Rigol has 12.

In the second sample image the Rigol is set to a Horiz time value of 20uS / Div and the Tek is unchanged and still set to 10mS/Div. The pulse is ~ 1.2mS.
rigol_20uS_per_div_horiz_IMG_4271._2jpg.jpg


Note that this is basically what Richard said, just specifically for the Rigol and other scope trigger outputs.

One other Rigol note; my unit has a record function under Utilities where at 200uS I can record the last 26 triggered waveforms. So I can do a run and look back at 26 waveforms [neutron captures] in memory, its a nice feature when your eyes are elsewhere.
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Re: My experience with the GammaSpectacular GS-NEUTRON

Postby Finn Hammer » Thu Jul 20, 2017 6:31 pm

Trent,
Thanks a lot for taking your time to clarify this issue in such great detail.
My Rigol is a DS1054z just like yours, but I took the upgrade to 1104 like most folks seem to do these days, so I have the full 24Mb of memory, and all the fancy triggers too, including runt.
This looks like a good excuse to buy one more scope....

Cheers, Finn Hammer
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