Fusion Message Board

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: Geiger and X-ray stuff
Date: Mar 07, 10:35 pm
Poster: Richard Hull

On Mar 07, 10:35 pm, Richard Hull wrote:


Scott Little was kind enough to work out the math and figure the attenuation of my fusor to X-radiation. I have copied the body of his data reduction in the paragraph which follows......

Regarding your x-ray shielding, I calculate the following attenuations for
> your 0.090" SS walls:
> Energy (keV) Fraction transmitted
> 20 10^-20
> 30 10^-7
> 40 10^-3
> 50 10^-2
> 60 10^-1
> Of course, this doesn't really tell you what the exposure rate will be but
> it gives you some indication. Also, you need to remember that, at 40kV for
> example, only a very few of the x-rays produced have 40 keV of
> energy....most are about half that in energy.

Thank you Scott for this information!! As most of you can see I should be OK up to 50KV even though the machine will undoubtedly fail for insulation problems before 30KV.

Geiger Counter Efficiency and the Geiger Region.

I have just finished a bit of research around the geiger counter and its physics, construction and manufacture. The key reference is the well known 1976 report to the defense department by Dr. Serge Korff of the University of New York. Korff was "the expert" in this field at the time.

The report deals strictly with the realistic and practical manufacture and assembly of geiger tubes and all of the engineering specifics to affect the same.

When converting my fusor to a geiger counter in a recent post, I noted that the pressure was just a few torr after back filling inorder to get enough gas to get a geiger plateau. I also noted that the plateau was extremely narrow. I further noted that the count was about 3 times normal background

It turns out all of these items were touched on in Korff's superb effort.

Geiger plateaus are very narrow and steep with gas pressures under 40 torr.

The proportional region is that region where a counter gives a response only proportional to the energy of the particle producing the ionization. If the voltage across the tube is now further increased. Any particle causing ionization gives a maximum response via gas amplification. This is the classic geiger region or plateau and is where I want to be. In normal geiger tubes the region can be almost 100 volts wide! The key..... most geiger tubes are pressurized to hundreds of torr!

My fusor, at 3-5 torr, had a microscopic region of geiger action which immediately fell to arc discharge within a 5-10 volt plateau!

Next, The efficiency of a counter is a function of pressure in atmospheres (p), the specific ionization of the fill gas (z) and the distance from the wall to the anode wire in cm. (s).

Efficiency is equal to 1-e(natural log base) raised to the -szp power.

Plugging in the values above indicates that my fusor was only 10% efficient as a geiger counter! That makes that 3X background count now a 30X background count!

In order to improve the efficiency and broaden the plateau I am planning to allow for a dry argon back fill to 150 torr following a fusor run in future.

Lots to learn, lots to learn.

Richard Hull