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Equipment/Supplies Pg 3 |
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EQUIPMENT / SUPPLIES (Pg 4) |
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NEUTRON DETECTION |
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Bubble Dosimeter |
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The bubble dosimeter, from Bubble Technology Industries, seemed to be the ideal neutron detector. It is one of the few devices that reacts to fast neutrons, and therefore requires no moderator. It can't be fooled by gamma or other radiation. Unfortunately, the manufacturer has a $300 minumum order, so the amateur would have to buy 3 of the units. (And of course that is before shipping.)
The one serious shortcoming of the Bubble Dosimeter is that there is no immediate indication of the rate of production of neutrons. Rapid changes in the rate of neutron production are not immediately apparent. |
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Bubble Dosimeters from Bubble Technology Industries |
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After careful consideration, we decided against purchasing a bubble dosimeter. |
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Helium 3 Detector |
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The Helium 3 detector's main drawback is cost. These detectors are made by several manufacturers, but the one which we obtained is a Reuter Stokes unit, available at a very reasonable price (relatively speaking!) from Don Orie, of Oetechnologies (oetech.com). It reacts only to very low energy neutrons, therefore it must be used with several inches of a moderator such as water or wax in order to detect the neutrons produced by a fusor.
Its chief advantage over other detectors is its amazing sensitivity. It requires an operating voltage of around 1700 volts, and a charge-sensitive preamplifier to detect the very low amplitude pulse caused by a neutron passing through the tube.
See Equipment/Supplies Pg 6 for a picture of the detector installed in its new paraffin moderator. |
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Ludlum Model 2000 Scaler |
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In some ways a Helium 3 neutron detector tube is similar to the geiger-meuler tube of a geiger counter. However, the pulse from the He3 tube is very short, and has a very small energy level. Therefore a special type of preamp, called a charge-sensitive preamp has to be used, before the normal amplifier and counting or rate-measuring circuits. The pulse, in the millivolt range, has to be separated from the 1200-2000 volt operating voltage which is normally applied by the same device that contains the preamp, amp and counting circuits.
This is a tall order, and can be very expensive.
Fortunately eBay once again came through, when a Ludlum model 2000 scaler became available for a very reasonable price. A surplus unit, it still bears the property tag from its earlier life with the U.S. Department of Energy. It had been calibrated for use with a gamma detector, but the voltage and the threshold are easy to re-adjust for the He3 tube. Adjustment procedures can be found on the neutron detection forum at fusor.net |
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Property tag is evidence of the Scaler's earlier role in life, working for the U.S. Department of Energy |
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Our Ludlum Model 2000 Scaler |
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Home Page |
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Equipment/Supplies Pg 5 |
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