In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.Subject: More on Neutron counter techniques #1
Date: Oct 08, 2:12 pm
Poster: Richard HullOn Oct 08, 2:12 pm, Richard Hull wrote:
All,
I will limit my discussions to the proton recoil system I use - (Bicron BC-720 scintillator attached to a PMT). I will also focus on its use with the fusor system.
Proton recoil can occur when fast neutrons enter any hydrogenous material. In this case, it is the styrene plastic of the scintillator. These charged heavy particles can then hit the silver activated zinc sulfide inclusions in the special scintillator. This produces a flash of light (scintillation) which transmits through the clear plastic into the PMT causing an electron avalanche and electron multiplication. This, inturn, puts a pulse of current in the anode circuit which is amplified further for display and counting.
Assuming we have a functional counter system made up from the above, it is instructive to first just hook it up to a digital storage scope and observe the pulses evolved from back ground for several hours.
I first take a Co60 of Cs137 source and tape it to the thin metal window at the end of the detector just .5 cm from the scintillator. Bicron professed a virtual gamma ray insensitivity for the scintillator and they are correct. The gamma rays are at least a full order of magnitude in amplitude below even the weakest neutron/baryon/meson signals found in the major background components. Likewise, they have a pulse signature (appearance) which also sets them appart from the proton recoil event. In short, they are easily discriminated by pulse height right out of the recording scenario.
Now we have a counter that is 100% proof against locally produced natural radioactive materials and eminations (alpha, beta, gamma).
It is part of the Bicron Literature to note that the counter can't be used as a neutron spectrometer, per se. Proton recoil energy is a function of incident particle angle as well as energy. Thus, no reducable data is carried in the photon amplitude! What we see are a vast range of background events caused entirely by "cosmic rays". The term is nebulous and should be as the mix is rather profuse. We may be assured that only those more massive items, secondary neutrons, mesons and other items in the mix of really large kinetic energies will be involved in making a pulse appear on the scope. The number of pulses seen per unit time will depend on our pulse height discrimination level.
Naturally, it is assumed that you are not co-jointly fiddling with diffeent PMT high voltage settings and your preamp is hooked to regulated supplies. These are usually "doped" out ahead of time before the terrestrial pulse height discrimination process mentioned above. get your PMT data sheet and never ever go too near the max allowed anode voltage. Most of the time you won't hit even half the maximum. I adjust my HV until i start to get noticable floor noise (zero line hash) on the scope and then back off a notch.
If we just ace out the Co60 and Cs137 sources mentioned above, we can often count 10 or more pusles per minute. A really long term trial over many different times of the day and night over a couple of weeks will reveal stunning variations which seem diurnal in nature. They are!
The chance of solar neutrons entering our counter are vanishingly small. The short lifetimes of neutrons, coupled with the required velocity to survive the trip plus with the attenuation factor of our moisture laden atmosphere and the fact that we will see only very fast neutrons anyway, militates against such events.
All the real neutrons counted in background will be secondary radiations created in and around earth's atmosphere mostly by incredibly energetic solar protons. A whole gang of ultra hot mesons and other nuclear debris will also trigger the counter.
An occassional count will be internal to the counter. Some electron avalanches will be triggered by radiations from the materials within the tube materials themselves. All PMT's are carefully constructed and materials selected to eliminate this effect, but there is always some radioactive material around even in the best constructed tubes.
You will be stunned by the amplitude of some pulses being again a full order of magnitude larger than 90% of all the other pulses. Normal fast neutrons, (2-5 mev), will fall about in the middle of your pulse systems detection pulse heights.
With a lot of looking and background data collecting under your belt, You may now start to implement the actual man made neutron data collection process.
I leave a scope attatched, always! To just trust in an electronic counter or scaler is a fool's paradise. Yes, I have a nice HP counter attached and set its level trigger discriminator mostly by the scope! A scope will show noise and other false signals which would otherwise be taken for a count by the neophyte. In most simple work involving small voltages in d-d fusors, the count rate will be just above background. Hopefully, it will be satistically meaningful. This is where a full understanding of your local background level and rigid pre-run and post-run background counts will payoff.
A quick lesson in solid angle interception and counter efficiency is in order. That, in the next installment
Richard Hull