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: Re: Instrumentation - neutrons, etc.
Date: Sep 21, 8:34 am
Poster: Richard Hull
On Sep 21, 8:34 am, Richard Hull wrote:
>Would there be any significant advantage, experimentally, in running a pair of scintillation neutron counters (BC720/PMT)? The rates (background/neutron) could be compared against each other.
The neutron rate in background is absolutely uniform to a high order. It is, however, highly diurnal in rate and also somewhat seasonal, long term and highly sensitive to solar flares and sunspot cycles. Almost all ground based fast neuts are solar in origin.
I have pored over a lot of cosmic ray texts in an attempt ot get a firm handle on the neutron background. Most all neuts at sea level are thermal, or epithermal. There are very, very few fast neuts where we live (thank God). The BC720 is sensitive to fast neuts for the most part.
However, (yet another catch 22), we are bathed in a heavy flux of extremely energetic muons, pions, etc., at ground level. 90% of the "background" counts in BC720 in a well made PMT/BC-720 neut counter are mesons in the bev and gev range!! The BC720 has no discrimination power over the ultra powerful meson matter reactions with its volume. The discriminator level, if set too high, will only count meson events!! You need to set the discriminator so that a 3 roentgen C-60 source at point blank range is not seen by the detector and you can rest assured that all terrestrial, earthbound, ground sources are not seen. This leaves you counting fast neuts, and rare mesonic cosmic ray events within the scintillator.
Yes, that's right, I said mesons in the gev and bevrange earlier. Almost nothing cosmic at ground level is in the mev range. It turns out that there is a band of maximum neutron level in the atmosphere at about 10,000 to 20,000 feet. It falls off rapidly below due to scattering and thermalization. The Earth is really a net neutron emitter!!! Lots of campfire trivia here.
98% of all incoming non-solar cosmic rays are PROTONS!! The very weakest Protons hitting the upper atmosphere are 1-2 gev (Mostly, these weak ones are of solar origin). The average energy proton from outer space is in the 18-20 gev range! Some of the highest are in the tev range. There is zero viable explaination for the origin of all these totally isotropic protons in space.
All of these protons wind up producing massive multiple "stars" of mesons and neutrons for the most part within the atmosphere. most of the non-mesonic particles are gobbled up in other reactions way before they can reach sea level. This is why we are only hit (from above) by a zoo of mesons in our daily lives on the surface. Almost all, just zip through us at the energies mentioned. Oddly, there is virtually zero gamma or X-ray cosmic energy hitting the upper atmosphere (<0.5% in the total cosmic ray spectrum) and none at all making it to sea level.
A background count made immediately before a fusor run of 10-30 minutes will give a clear picture of the background level. Never run a fusor at sundown or sunup as the background can change at that time. The best runs are made between 10am and 4pm. Otherwise, I would get a dual counter system as Jim suggests. Frankly I don't think it would impact the data collection one way or the other with a decent fusor. If you are near the noise floor that is another matter. RH
If they were at different distances, the rates should vary inverse square.
>Or, perhaps, different kinds of scintillation plastic (thermal vs fast neuts)?
>Or two different view angles into the same block of plastic?
Neutrons diffuse through stuff and once in a carbonacious medium can go anywhere as they thermalize. They produce proton recoil in any direction. Once epithermal, in matter, there is absolutely no way to determine source direction by either examining the neutron flux or recoiling protons exiting. Damned neutrons!! ?$%^#& That is why a life's work could be wrapped up in becoming an expert on neutron detection and metrology.
Good thoughts Jim, but a bit expensive on the dual BC-720s. In theory two PMTs stareing into the same hunk of plastic should average out to a similar count. (due to diffusion of the neuts and isotropic proton recoil). There is always good news and bad news.