Another look at plain old scintillaing plastic.

[Richard Hull]

As neutron detectors such as BF3 and 3He tubes are getting harder to locate, I took another look at what regular scintillaion plastic can do.

It turns out that scintillation plastic is as sensitive to fast neutrons as the 4 atm 3He tube is to thermals! Both are about 77 percent efficient!

There is always that gotcha' related to gammas though and therein lay the rub.

Scintillation plastic is a superb gamma detector. The fusor has a lot of x-rays near the shell which for the amateur fusioneer means that a weak fusor will light up the plastic with both gammas and fast neutrons.

According to the data I read, for 1mev neutrons, scintillation plastic of 5cm thickness is about 77 percent efficient!

I think that since a fusor making neutron would have 30, 40, or even 50kv x-rays poking throught the shell, a well done x-ray shield could absorb the bulk of the xrays and instrumentation might be able to "upper level detect-out" what remains with a moderately secure neutron signature remaining. It is worth a try.
5cm is a pretty thick chunk of scintillation plastic.

A 3-inch by 3-inch cube attached to a 3" PMT would make a fairly decent neutron detector I bet with a 1/8" to 1/4" thick lead plate in front of it. I will try it out at some point and report back.

While the grand BF3 and 3He tubes do need moderation to turn our fast neutrons into slow neutrons, they are virtually gamma insensitive and in high fields, the gammas can easily be level detected out of the count.

Richard Hull

[Chris Bradley]

I had plans for, and acquired materials for, trying out a real-time activation detector. As I understood it, these are, or at least were, used for various nuclear-industry purposes, so it is not top-of-hat stuff.

As some scint plastics are extremely active to betas, I was planning to create a sandwich of layers of indium and beta sensitive scint. Jon Rosenstiel did likewise with a beta tube, in fact, reported here. The 'very' short lived In activation products (ms) appear to produce a ready supply of near-immediate detectable betas, and, of course, you also get the benefit of a second 'confirmation' of real neutrons by monitoring the half-life of the longer lived products (minutes) once everything is switched off and the x-rays stop.

[Carl Willis]

The downside of the plastic scintillator is its sensitivity to gammas, per Richard's remarks. However, this is not just a case of sensitivity, but selectivity: pulses made by low-LET particles (betas and Compton electrons, chiefly) of a given energy are much larger than those made by high-LET particles like the recoil protons from fast neutron scattering. So organic scintillators used for neutron detection often require pulse-shape discrimination or careful shielding. I recall Joe Zambelli's apparatus from almost 15 years ago was a simple plastic scintillator. Simple in principle, but beginner beware when it comes to making a convincing case for neutron counts!

Chris mentioned the activation sandwich counter. A ready-to-go plan for such is shown in Glenn Knoll's Radiation Detection and Measurement, Figure 19.25 in the third edition. It shows a stack of 31 silver foils alternating with 32 plates of NE110 scintillation plastic 3.2 mm thick. The plates are coupled to a common PMT at one end. The original citation for this particular design is Slaughter and Pickles, NIM 160 p.87 (1979).

-Carl

[Richard Hull]

Pulse shape analysis is not a simple amateur issue and as Carl notes nuclear measurement neophytes beware if you attempt this method. The silver sandwich or any activation method tends to demand a minimal neutron flux. This is something that first pass amateur fusioneers rarely can muster based on limited supply voltage levels and beginning operational issues. We may be making neutrons in our earliest stages and first pass attempts but we have to prove it to everyone's satisfaction. (mostly our own).

If your detection method is marginal due to low flux and low detector efficiency, you may need to work from a statistical standpoint. This can be tough as longer run times are demanded to lift the proof of neutrons out of the scud. First pass systems are often not operable for extended periods at or near their maximum potential.

Richard Hull

[George Schmermund]

As long as we're back to the application of scintillation detectors I thought it might be worthwhile to recapitulate the virtues of Ag activation in a liquid scintillator: viewtopic.php?f=13&t=6245

[Richard Hull]

If you've got the flux, silver activation in any hydrogenous media is a surefire success. A lot of Newbs, unfortunately, have near background flux levels though on their first pass when they are desparate to join the Neutron club.

Richard Hull

[Philipp W]

Hi,

I've recently purchased a BC412 crystal off ebay, also including the PMT and dynode chain (http://www.ebay.com/itm/251358936976).

As per the crystal's data sheet (http://hardhack.org.au/files/BC412.pdf) it is also sensitive to fast neutrons, so it could probably be used as a neutron detector (I don't yet have a Fusor, but I'll check once I get my system finished). The energy range for gammas is listed from 100keV to 5MeV, but I wonder if it is also sensitive to lower-energy radiation.
If not, could one just get rid of the lead-shield since the crystal wouldn't see the gamma rays produced by a Fusor anyway (assuming voltage levels below 100kV)?

Regards,
Philipp

[Richard Hull]

I would keep a lead shield over the scintillator plastic. Neutrons will zip right through and it will extremely limit any x-rays that hit it from the fusor. Background counting is very important here as such plastics typically have a high background. A simple plastic scintillator can give false positives as Carl notes above. It is often a matter of pulse shape analysis with simple scintillators.

Richard Hull

[Philipp W]

Sure, a lead-shield will certainly make things much cleaner and less noisy, no doubt.

I'm looking forward to doing some tests with the scintillator once it gets here; currently I'm working on the HV-Supply for the PMT and the readout electronics (I don't have an off-the-shelf ratemeter to use with the crystal, so I try making most of the electronics myself).

Philipp

[Peter Schmelcher]

Phillip some digital oscilloscopes you may have used can be configured as a pulse shape discriminator and counter. For example, the TDS3000 series has a telecom test mask option that you can enable with a software-hardware mod. This option will test a waveform for 8 different shape error violations, however, you can modify the test template and use it with any signal. Basically the template is 8 straight line segments and if the signal crosses a segment it gets counted.

Another very good approach uses a sound card and free software developed by Marek Dolleiser at Sidney University. Search for PRA pulse rate analyzer. The only minor drawback is the sound card digitizing speed so the PMT output needs some signal conditioning. Simplistically AC coupling, integration for pulse stretching, and low pass filtering. Richard Hester, Doug Coulter and Steven Sesselmann have applicable posts.

Have fun
-Peter

[Philipp W]

Hi Peter!

Nice to hear from you again - how's your Fusor coming along?

Thanks for the pointers: I'm afraid I can't get my hands on a DSO very easily, but I'll check out the Software you mentioned!

But I don't expect do be doing spectroscopy that soon, since I have to get the low-level stuff working first (and I have to spend some time at University too )...

Philipp