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: nuclear measurements- handy tips
Date: Mar 15, 4:30 pm
Poster: Richard Hull
On Mar 15, 4:30 pm, Richard Hull wrote:
The key to good measurement and accurate data is related to the taking of good background count data with whatever nuclear instrumentation you have. In our case it is neutron background.
This is one of the more difficult measurments. Trying to mask out all the other stuff which makes counters click in a neutron detector is tough.
I have so shielded and prepared my instrument with discriminators that I can hold a seething hot source of Alpha, Beta, Gamma or X-radiation within inches of the counter's sensitive end and I get "Zippo" out on the counter beyond background. Ideally, the only thing which can trigger my counter is fast neutrons!! Fast Neuts of about 3.45mev or more should be the product of D2 fusion reactions. Some Gamma's can be expected, but according to the Bicron data with my Scintillator, they are much reduced in amplitude and unless multiple coincidences occur, (additive), they are easily removed by pulse height discrimination.
This was found to be correct by me. I removed all dicrimination and looked at an O'scope output. On my system, the radium and Co60 sources caused many pulses in the .5 -1.5 volt range (thousands/minute!). Fast neutrons created huge 6-8 volt pulses. Thus, I could safely set the discriminator for 4 volts and safely be assured of never seeing rather intense gamma ray or X-ray readings.
There is no substitute for clean, long term background readings. I have not really done a gross study of where background fast neuts might come from, but primary cosmic radiation would be unlikely as the neutron only has a 15-20 minute lifetime when out strolling around on its own. most of our fast neuts are probably solar in origin with a few produced by secondary cosmic ray reactions in the upper atmosphere. Also, the counter can be activated by any Baryon or heavy charged particle which can cause proton recoil in the scintillator. All these contribute to a variable background count. This count will vary with discriminator setting as well as diurnally and with changing ionospheric conditions.
With this in mind, A background count is best taken just before and just after a fusor run with the discriminator level FIXED!
I have found that a minimum of 10 minutes is needed to acquire a reasonable background with 20 minutes being preferred. Remember, this is both before and after your run. Likewise, I try to make a fusor run at least 10 minutes long. This is a busy time filled with monitoring conditions and jockeying controls. Time slips away and time endings are missed and one must go to the next minute, or more. I was using a standard stop watch.
To avoid this, I purchased the neatest little toy which will, for $15.00, save a lot of hassle and let you pay attention to your work instead of the clock. It is Radio Shack's "talking timer". It has a great countdown mode where you set a fixed time to countdown (up to 24 hours) Let us say, 20 minutes. The time verbally announces the minutes under ten minutes with a pleasing woman's voice "nine minutes left"..., etc. Under one minute it announces every ten seconds. "forty seconds left"...etc., until it reaches ten seconds when the classic count down from 10 begins on the second and at zero a beeper pulses until you stop it. This allows you to forget the time and attend to business until the last moments when you can be prepared to take data, shut down the fusor or whatever.
Armed with the two neutron backgrounds, I find the average of the two readings over the two twenty minute periods and divide by 60 to get a back ground Count per minute. I then figure the count per minute rate during the fusor run and the difference in the fusor rate per minute and the background rate is what I term the Delta CPM for my equation especially developed for my particular counter efficiency and geometry.
While my count might not be close to yours, I find my particle background is in the 2-4 neut/minute range. Current fusor levels record about 23 neuts/minute. While statistically highly significant, this thing is not clickin' away like a geiger counter. An error of just couple of counts per minute will change the output data by 500 neutrons/second!!!!
Data collection must be scrupulously overseen and shepherded to get meaningful and truthful, results.
Why spend time and money, sweat bullets over dimensions and tolerances with making a fusor if you aren't going to collect data well? It's all part of good science and good sense.