LiI & NeutronRAE sensitivity vs. energy

[Rich Feldman]

Good, it's not too late to edit my post under FIles and put the discussion here.

I just posted under FIles a user's manual for a product much discussed in the last few days.
Refer to Specifications on page 32.

raespec.GIF (9.54 KiB) Viewed 7282 times

With some personal experience setting specifications for expensive instruments,
I don't see where the neutron sensitivity value is claimed to apply across the entire detectable energy range.
Some customers might think that connection is implicit, but not me.
Kind of like the scaling of gamma cps to sieverts: accurate to +/- 20% for a Cs 137 source, and unspecified for other energies.
"Gamma" channel sensitivity to X-rays below 15 keV has been reported, and comes as no surprise. But that's not covered by the datasheet, and is of dubious value for determining personal exposure.

Given what Jim and Nick posted about neutron absorbing cross sections.
I can think of 2 unlikely ways RAE might get a flat neutron count sensitivity from thermal to 14 Mev.
1. Use special lithium 6.
2. make a spectrometer that quantifies each pulse from photodiode. THen scale the reported cps values to compensate for the probability of undetected neutrons at that energy.

As a reminder: with the exception of those using only activation of stable isotopes, all neutron club members got there using commercial devices expressly designed and sold for detecting neutrons.

[Nick Peskosky]

Rich,

To further increase our depth of knowledge on Li-6 based detectors and to put this question to rest, I have contacted engineering support from RAE Systems to see if they will provide insight on the neutron energy response characteristics (and possible embedded compensation) of the LiI(Eu) scintillation detector utilized within the PRD. I'll update this post when they provide a response to the RFI I forwarded to their technical/product support division.

[Rich Feldman]

Thanks, Nick. My breath is bated. Here are a couple of other fronts for investigation.

1. Long discussion on fusor.net from 2 years ago, about Scionix neutron detectors with LiI(Eu) and PMT's.
viewtopic.php?f=13&t=6321&p=41975
Rich Normand and I got some. Chris Bradley mentioned:
"If these are from the 'polimaster' type pager dosimeters, then it may be worth noting the rated sensitivity of those is 1.3 counts cm^2/thermal neutron, and 0.05 to a Pu[alpha]-Be spectrum neutron. "

2. An ebay search found only one item, 331433542391, which closed on 1/6/15 and may soon be inaccessible.
SAIC Neutron Radiation Detector Probe Scintillation Crystal Lithium Iodide RARE!
“Cleanly salvaged part from neutron dosimeter, The crystal is epoxy sealed with a plastic PMT light ”
"This is a small 12mm diameter lithium iodide scintillation crystal. This crystal is from a SAIC neutron detector. It was originally coupled to a small PMT. It responds to FAST neutrons very well, so it may be of interest to fusor builders. Even though it is designed for neutrons, it still detects gamma rays too, just turn up the detector gain. The only difference is the spike of light with a neutron capure event is far brighter than any gamma event. This allows you to discriminate the energy by lowering the detector gain so normal gamma background is negligible. All you need to make your own detector head is a small PMT tube or large area PIN photodiode detector. They fit the R-7400 bare can style PMT tubes exactly, but a bigger PMT will give better resolution. I have the suitable PIN photodiode in another listing with a couple high gain amplifier boards. If you need this, here is your chance. This item is pretty hard to find I have lots of lasers and other unique stuff for sale this week."

I sure hope we have not discouraged Bern from trying a moderator to get larger count values.

[Richard Hull]

Good going guys! We look forward to being educated on this issue a bit more. I agree that software can be written for compensation over large energy ranges in some cases. I have just done it for my Arduino GM counter dose estimate software. The fly in the oinment is the slop you allow in such generalizations.... +/- 20% is fine except when some level of precision is needed at very low levels.

Those in the know here already understand that absolute neutron count measurement is a real bear even at higher levels and can be a study in statistics are extremely low levels.

Richard Hull

[Jim Kovalchick]

Clearly the neutronrae was not designed for experimentation, but neither were some of the survey meters used for some past fusor projects. As I said in a couple different ways already, no need to obtain other instrumentation if a moderation test shows that neutronrae is seeing neutrons. I am not understanding what the big deal is.

I have a Neutronrae LiI sensor I pulled out of a scrap unit. I know how to get a spectrum out of it because I already did it with the similarly configured CsI detector out of the same Neutronrae. If my son wasn't busting his tail in a classroom in Ann Arbor right now, I would get him to fire up his fusor and get a spectrum from the sensor and put this to bed. That will have to wait until he comes home for the summer or when I find another ready source of neutrons.

In the mean time, Bern should get a block of parafin from the grocery store and go to work. If that doesn't work, then think about investing in bubble detectors or pushing voltage higher.

[Rich Feldman]

To update Chris's data from the Scionix thread, I looked up Polimaster PRD's. http://www.polimaster.com/products/pers ... /pm1703gn/

poli_img.PNG (107.65 KiB) Viewed 7231 times

Some are amazingly similar to the NeutronRAE II,including Bluetooth, but with twice as much LiI(Eu).
From user's manual at http://www.polimaster.com/files/downloa ... al_eng.pdf

Polimaster is much less ambiguous than RAE about the neutron spectral sensitivity. Thanks to Chris Bradley for the tip.

Bern, I hope you're following this. One or both of us may learn something about consumer research.
Moderators are cheap.

[Bern Bareis]

Hi Jim and Rich,

My concerns about applying a moderator are as follows:

1) The NeutronRAE II neutron sensitivity, like other LiL(Eu) detectors is more sensitive at the lower energy end than at the higher end (say 2.5MeV vs 14MeV). Since it detects all the way to up to 14MeV within its published rating for sensitivity (1 to 2 cps per 2.5 neutrons/second/cm^2), I am concerned that at the low end (e.g. 2.5MeV and lower), the detection level will be closer to the higher end of the rated sensitivity, in other words closer to 2 counts per second for every 2.5 neutrons. If you put in the moderator, the NeutronRAE II may well detect a 2.5MeV neutron as well as a thermal neutron and would not result in a significant amount of extra neutrons detected. If an exact sensitivity curve is available from RAE Systems, that will help and may be able to be applied to sort out any differences, if applying a moderator.

2) If the differences when applying a moderator are not significant, it may not mean that the NeutronRAE II is not detecting neutrons, it may simply mean that it detects as well at 2.5MeV from the D-D fusion as well as it does with thermal neutrons exiting the moderator.

If a Lil (Eu) detector like the one in the NeutronRAE II is found to be acceptable for use by fusor.net for detecting neutrons at some point, then if the sensitivity is as good as the specifications provide for, this will certainly help in detecting neutrons with few neutrons being generated by the fusor. Consider that its neutron sensor can be placed within approximately 3 inches of the center of the cathode grid of a six inch diameter fusor. If the sensitivity for 2.5 MeV neutrons is say 2 cps per 2.5 neutrons/second/cm^2, then you could detect very low amounts of neutrons being generated (which is what I was planning on).

I do plan on turning up the voltage a bit more during future testing; however, I would appreciate some insight from the group regarding whether or not measurements that have been discussed at say -38V and 12mA (assuming a 6 inch diameter fusor) are measured at the fusor or at the power supply with a ballast resistor in between. If pushing the voltage higher will allow for a consensus that neutrons have been detected with the NeutronRAE II, I am glad to do that prior to moving forward with the purchasing of the BTI PND 33. As I mentioned earlier, I have a power supply capable of up to -50kV at 20mA; however, the HV feedthrough is only rated to -40kV, so I won't be running it more than about -37kV, but I can certainly run it to say -20kV or -25kV. The detector of the NeutronRAE II is currently positioned exactly 15.6 cm from the center of the cathode grid. Based on the fusor configuration, sensitivity of the NeutronRAE II, and other information provided, if there is a an amount of neutrons per minute detected that will be acceptable let me know. Earlier posts have indicated at least 35 cpm would be acceptable and I believe at least that can be accomplished with the current configuration. The NeutronRAE II can be set with a cpm window for data collection. Let me know your thoughts on this. Thank you.

Best Regards,

Bern

[Rich Feldman]

Bern,
I think you are clinging to an interpretation that's unreasonably favorable to your instrument. Maybe you have skimmed over the previous posts in this thread. Do you or someone you know work for RAE?

Are we looking at different specs? For the one copied in OP above, I bet the interpretation should be:
"The sensitivity will be between 1 and 2 cps per 2.5 neutrons/s/cm^2 for our standard spectral distribution." Probably that of the neutron sources RAE uses to characterize and demonstrate detector modules. Note the standard Cs137 source for gammas. What is the 2.5 doing in there?
I will further wager that the sensitivity to thermal neutrons is at least 10 times greater than that for 2.5 MeV fast neutrons. To me that does not contradict the spec, interpreted broadly.

Let's wait and see if Nick gets a response from RAE to his questions about this detail.
Does their technology provide a flatter response than Polimaster, discussed above? (Note 1)

We want you in the neutron club. Does someone have to make and send you a moderator? Are you exploring the deep-pockets-but-no-spare-time branch of the fusor road?

Note 1. Online investigation of Pu-Be sources led me to this interesting chem lab project: http://www.csun.edu/~hcchm003/481/481lmflx.pdf

[Bern Bareis]

Hi Rich:

Jim and Richard have mentioned turning up the voltage as a potential means of qualifying the NeutronRAE II neutron detection. If, for example, I increase the voltage a little and place the NeutronRAE II in a grounded, totally enclosed metal chassis to act as a Faraday shield, then what else would the NeutronRAE II neutron detector be detecting except neutrons when exposed to the operating chamber? If that is not a viable option, then I am planning to just move on to a BTI PND 33 bubble dosimeter as that appears to be a path that will be readily accepted.

Nick and Jim:

Thank you for your insight and efforts.

Hi Richard:

To get an idea of the fusor operation you described in your earlier post, I would appreciate it, if you can let me know if your 38kV 12 mA measurement was taken on a fusor system applying a ballast resistor or not and additionally, where the voltage measurement was taken (e.g. at the fusor or at the power supply, if a ballast resistor was used). Also, what is the diameter of the vacuum chamber (if spherical) and what was the deuterium vacuum pressure level at the time? Thanks again.

Best Regards,

Bern

[Nick Peskosky]

I've received a response from RAE Systems regarding the detector. While it's not as technically descriptive as we would like, it seems to confirm me and Rich's suspicion that the scintillator material has a higher response rate to N's in the thermal energy range. I'm trying to get in touch with the actual product manager who provided the response or a product engineer so that I can obtain some datasheets specific to the detector in question [although RAE makes their own LiI(Eu) units and this information may be proprietary].

"Please see our Product manager's response below:

The response characteristic of neutron sensor has not been measured. However, to place a suitable H-loaded moderator around the detector is good to obtain better count rates.

Abi

RAE Systems by Honeywell Technical Support
3775 North First Street.
San Jose, CA 95134
Technical Support - 888-723-4800 option 2 Repair Processing - 888-723-4800 option 1 Repair Processing Fax Number 408-952-8486 rae-callcenter@honeywell.com http://www.raesystems.com"

[Bern Bareis]

Hi Nick,

Abi is likely considering neutrons from multiple sources across the energy range of thermal to above 14MeV. Given that, of course a moderator (as long as it is not completely blocking neutrons) is going to cause an increase given such a broad range of neutron energies, but the question has been how much of an increase will there be when considering D-D reactions? Lots? Some? ... and whether or not that would be enough to be a difference that would be considered by those questioning the neutron detection capability of the NeutronRAE II as significant enough to accept that it is indeed detecting neutrons sufficiently?

Hence, my choice of an alternate method for neutron detection.

Best Regards,

Bern

[Rich Feldman]

the question has been how much of an increase will there be when considering D-D reactions? Lots? Some?

I bet the answer is Lots. Polimaster uses LiI(Eu) just like RAE. Their sensitivity specs are explicit about the neutron spectra: one number for Pu-Be sources and another for thermals. The latter numbers are higher by factors of 20 or more. Pu-Be spectra are strongly weighted toward the D-D-fusion end of the fast neutron range. http://www.geneseo.edu/nuclear/nuetron-pube-spectra [sic]

... and whether or not that would be enough to be a difference that would be considered by those questioning the neutron detection capability of the NeutronRAE II as significant enough to accept that it is indeed detecting neutrons sufficiently?

Hats off to you, Bern, for staying focused on your fusor's apparent purpose: getting onto the Neutron Club roll.

With that out of the way, you could now give something back to the forum. You have a known-good fusor and a LiI(Eu) based neutron ratemeter small enough to fit inside a modest block of moderator. Let's see the numbers! If you enjoy bolting parts together more than fabricating them, I know someone who could send you a ready-to-use moderator in exchange for you using it in an experiment. One where any outcome that yields knowledge counts as a success.

Best regards,
Rich

[Nick Peskosky]

Given that, of course a moderator (as long as it is not completely blocking neutrons) is going to cause an increase given such a broad range of neutron energies, but the question has been how much of an increase will there be when considering D-D reactions? Lots? Some?

As I stated in your Neutron Club petition thread, it's probably a statistically significant increase. Capture cross-section for Li-6 at thermal/epi-thermal neutron energies is almost two orders of magnitude higher than it is at the 2.45 MeV (fast neutron) energy spectra peak of a D-D reaction. As Rich suggests, it should be a simple/inexpensive experiment to perform. All you need is 2-3" of hydrogen loaded moderator (water, LDPE, HDPE, paraffin) between the detector and Poissor to test this hypothesis. If you have the time, give it a shot and report the results! I know many of us our interested in the real world performance of newer He-3 replacement scintillation materials.

[Rich Feldman]

All you need is 2-3" of hydrogen loaded moderator ... between the detector and Poissor to test this hypothesis.

In my un-practiced understanding, we expect additional increases in count rate from moderator placed on the far side of the detector. And on the sides of the detector. Even behind the source. Let go of the straight-ray model.

I found this educational reference by our old buddy George Dowell: http://qsl.net/k/k0ff/Manipulating%20Ne ... Rad%20Lab/
Near the bottom, his experiments show a reduced count rate when boron is nearby, even outside the direct path. Your experimental checklist might include scintillation-detector runs after removing that big green piece of borated plastic.

[Nick Peskosky]

Rich you are correct, my paradigm was assuming a cylindrical proportional gas detector with cylindrical moderator... which the RAE dosimeter is not. Good catch.

[Bern Bareis]

Hi Nick,

What it sounds like is that results using a moderator will be considered as valid even though they are relative to former direct output baseline results from the NeutronRAE II that were met with skepticism. If there was no question that the NeutronRAE II was detecting neutrons, then neutron emissions would have been confirmed earlier. Perhaps you can see why it makes sense to me to just apply a different solution to confirm neutron emissions.

Kind Regards,

Bern

[Richard Hull]

Again run a count with you old counting system get X neutron counts. May or may not be neutrons for any number of reasons we gave, but really, who cares....We didn't.
Run the same system with all things identical except the counter system is in a proper moderator and get X+Y counts and we know you are making neutrons. No arguments, just instant membership in neutron club. That would have been all that it took. (assuming you got more counts in the moderator than out in the open.)

Richard Hull