X-Ray and Neutron Detection


“Geiger Counter”     We wanted something which can detect relatively low energy x-rays. We settled on a CDV-700 “geiger counter” as modified by Geoelectronics, and known as a “Leni”. We also purchased a pancake type geiger-meuler probe with a thin mica window from the same source.

It is capable of reacting to alpha, beta and gamma radiation, and does a good job of detecting the low-energy x-ray radiation produced by a fusor. We were happy with the standard of workmanship shown in the Leni modifications, and had a good all-round experience in dealing with Geoelectronics. Their sales are all done through eBay.Unfortunately, more complex equipment is required to detect neutrons!



Bubble Dosimeters from Bubble Technology Industries

Bubble Dosimeter   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 minimum 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.

After careful consideration, we initially decided against purchasing a bubble dosimeter.

Bubble Dosimeter Update   In August, 2007, a group purchase made it possible to obtain a BTI PND 28 bubble neutron dosimeter at a reasonable cost. It was first used on Sept. 14, 2007.

A picture of the results can be seen at Beyond the Demonstration Fusor.

Helium 3 Detector  

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.

Below is a picture of the detector installed in its new paraffin moderator.


The Completed Moderator shown with the Helium 3 neutron detector installed. The moderator is made from a 22 1/2 inch section of 6 inch diameter thin-wall PVC tubing. The bottom was closed with a disk of 1/4 inch plywood, sealed with Varathane and then epoxied into place. The center tube is 1 inch I.D. copper. Between the tubes is almost 20 pounds of paraffin wax.

For this application, the cable length shown may be  excessive.

The top of the moderator, with copper inner tube and paraffin visible. The space between the tubes was first filled with small chunks of solid paraffin, and then melted paraffin was added in several consecutive pours.

The bottom of the moderator, showing the sealed plywood disk epoxied into place

Ludlum Model 2000 Scaler

In some ways a Helium 3 neutron detector tube is similar to the geiger-mueller 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

Our Ludlum Model 2000 Scaler

Property tag is evidence of the Scaler’s earlier role in life, working for the U.S. Department of Energy


A photograph of our homemade pulser with the cover removed. It is used as a source of test pulses for the Scaler.

The large capacitor is for DC blocking, as the scaler’s input also provides the operating voltage (up to 2000 volts) for the detector. Even with the blocking capacitor, care must be taken not to connect or disconnect the pulser with the Scaler turned on.


Once the neutron output of the fusor is sufficiently high, the neutrons can be slowed by a moderator, and then elements such as silver or indium can be exposed to them. The result is that some of the atoms of the exposed element will capture thermal neutrons and be changed to a different isotope or element, usually one that is radioactive. The identity of the element or isotope created can often be identified by measuring its half-life. Isoptopes with very short half-lives (a matter of minutes) lend themselves particularly well to this method. Silver is a good example, and is the element that we have chosen for our first attempt at activation.In order to measure half-life, some method must be employed to continuously record the radiation level of the activated sample over time, so that its rate of decrease can  be analyzed. This usually means interfacing the radiation detection device (geiger counter) to a computer which can continuously record the radiation level. One of the least expensive ways of doing this is to utilize a program sold by Black Cat Systems,  after interfacing the geiger counter to the computer serial port.

Black Cat Systems provides a schematic for a very simple interface which seems to work flawlessly, yet is easy to fabricate. Below is the schematic and pictures of the device which we assembled using point to point wiring on a prototype board. It seems to work perfectly. The serial connector was soldered directly to the prototype board. A short patch cable connects the miniature phone jack on the interface board to the earphone jack of the geiger counter.

The completed interface circuit is shown connecting the earphone output of our CDV-700 geiger counter to the serial port of a laptop. The laptop can then record the pulses from the geiger counter.The data can subsequently be graphed or otherwise analysed to discover the half-life of the material being measured.

Above are two views of the circuit, point-to-point wired on a small prototyping board.The serial connector is mounted by soldering the pins directly to the pads on the board.

The main radioisotopes detected after neutron activation of silver are AG-110 (with a half-life of 24 seconds) and AG-108 (with a half-life of 2.37 minutes).

Both decay mostly by Beta decay, and little gamma radiation is emitted.Fortunately our pancake detector is capable of detecting Beta as well as Gamma.