Alpha Spectrometry Report

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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Jon Rosenstiel
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Alpha Spectrometry Report

Post by Jon Rosenstiel » Mon Sep 25, 2006 9:52 pm

A couple of months ago I bought a used Canberra 7401 alpha spectrometer from fusor list member Geo. What follows is a report on my experience of getting it up and running.

The unit came minus detector, so I bit the bullet and ordered a new, 300 mm^2, 19 keV warranted resolution, PIPS detector from Canberra. Cost me $630.00, which, considering its performance, didn’t seem too outrageous.

The unit also came minus its original front panel pump/off/vent vacuum selector valve, so I constructed a replacement using some valves I had on hand. I mounted the valves on a narrow 19” rack panel just above my mca. (See photo)

Built into the 7401’s double width nim module are: Detector bias supply, preamp, shaping amp, calibration pulser, counter and discriminator, so no other nim equipment is really required. Just connect the energy output jack to your mca’s ADC in jack and you’re set to go.

Geo was kind enough to lend me a Th-230 alpha source disc to use in the testing of the spectrometer.

Time for photos… The upper image shows the complete setup mounted in a 19-inch rack cabinet. Above the mca are the 7401’s chamber vacuum control valves. (More on the valves later) Below the mca is a nim rack containing the 7401 (far right). In the center is a Tennelec TC-800 precision pulser, (It’s not mounted in the nim rack, it just happens to fit in there nicely). On the left, but not being used, is an Ortec spectroscopy amplifier. Below the nim rack is an MKS capacitance manometer readout that is indicating the pressure in the 7401’s chamber. By the way, the mca is displaying a horizontally expanded spectrum from Geo’s Th-230 source.

Second image: Th-230 spectra, live time of 1200 seconds. Labeled are its 4620.5 keV (23.4 % intensity), and 4687 keV (73.9 % intensity) alphas.

Third image: Th-230 spectra with expanded vertical scale. Un-labeled and just to the left of the 4500 keV marker is the 4479.8 keV (0.12 % intensity) alpha of Th-230.
Other labeled peaks are, (I believe):
4784.5: The 4784 keV alpha of Ra-226.
5493.5: The 5490 keV alpha of Rn-222.
6001.7: The 6002 keV alpha of Po-218.
7690.0: The 7687 keV alpha of Po-214.

I’m fairly certain about the identities of the above alpha peaks, as a gamma spectrum (using hpGe detector) of the Th-230 source revealed Goldilocks and the three bears (Ra-226 & Pb-214) plus the usual score of Bi-214 gammas. Evidently Geo’s Th-230 source is old enough to have generated some daughter products.

More about the vacuum control valves…
The right-hand valve is the main shutoff valve. (Isolates pump)
Second valve from the right is the chamber vent valve.
Third valve from the right is the vacuum bleed shutoff valve
Left-hand valve (needle valve) is the vacuum bleed valve. (Used to adjust the 7401’s chamber pressure)


When I first got the spectrometer up and running I figured that the deeper the chamber vacuum, the better. Then I noticed that I was getting excessive background counts, mainly from Po-218 and Po-214, for SEVERAL days after I spec’d the Th-230. Hmmm… A little Googling turned up the fact that recoil contamination is a big problem for alpha spectrometers. The recoils are attracted to and implanted into the detector’s surface, and cannot be cleaned off. Canberra has a couple of good papers about this (and alpha spectroscopy in general) located here:

From the above paper: “To reduce (up to 1000 x) the amount of recoil contamination two things must be done. One, place an absorber (air works well) between the source and the detector that will allow the high-energy alphas through while stopping the more massive daughter nuclides”, and two: “Apply a small negative bias, as little as 2 V, to the source plate to attract the daughter nuclei back to where they originated”.

For the biased sample tray I turned to this paper from Ortec: ... ination%22

I pretty much copied their design. A chunk of printed circuit board with four-3 volt coin cells providing –12 V bias to the sample tray. With the biased sample tray and a chamber pressure of 4 Torr (with a source to detector distance of 2 cm) recoil contamination was greatly reduced if not completely eliminated.

Last image is my version of the biased sample plate.

Jon Rosenstiel
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Alpha spec 020.jpg
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Re: Alpha Spectrometry Report

Post by richnormand » Mon Sep 25, 2006 10:35 pm

Very nice work.... as always Jon.
How effective is the recoil mitigation setup with bias. I have been thinking of doing that too but up to now I have been using a bad vacuum (in a sewer pipe configuration) using air to control things. Is a few volt bias that much more effective?

Jon Rosenstiel
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Location: Southern California

Re: Alpha Spectrometry Report

Post by Jon Rosenstiel » Tue Sep 26, 2006 1:49 am

According to the Canberra paper, one needs to have both the correct air density and the biased sample plate to be effective.
The charged detector attracts recoils, air or no air, according to Canberra. The negatively biased sample plate makes sure the recoils are attracted back to the sample plate, away from the detector. I didn’t care to do any more recoil contamination tests with my brand new detector, so I did as the Canberra paper suggested.

Jon Rosenstiel

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Re: Alpha Spectrometry Report

Post by wayne » Tue Sep 26, 2006 11:14 am

Jon- Nice work.

Did you tune the pressure?
Would other gasses give a different performance?


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Richard Hull
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Re: Alpha Spectrometry Report

Post by Richard Hull » Tue Sep 26, 2006 4:05 pm

I also bought one of the 7401s and have the manual that comes with it. Now I will add all your data including your post to complete my operations book on Alpha spectro work.

To tell the truth, I just haven't got around to the effort yet. My big event is next week and my work office is moving next week to large quarters also with its last 15 years of clutter to sort through.

I already have a couple of canberra PIPS detectors of the 300 mm^2 size and should be good to go. It is sad that the original user had removed the supplied 7401 valving system, but they worked off an external manifold so the supplied valve was superfluous and got yanked.

Thanks for setting off all the trip wire mines in this effort. You should make my journey a bit easier.

Great work as always.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Jon Rosenstiel
Posts: 1405
Joined: Thu Jun 28, 2001 5:30 am
Real name: Jon Rosenstiel
Location: Southern California

Re: Alpha Spectrometry Report

Post by Jon Rosenstiel » Tue Sep 26, 2006 4:31 pm

According to the Canberra paper best results were obtained with an air thickness of 12 to 16 μg/cm^2 between the source and detector. The Canberra paper has a handy chart that plots the relationship between source to detector distance and pressure. (For air)

Take for example my setup in which the source to detector distance is 2.0 cm. According to Canberra’s distance vs. pressure chart 3.8 mm Hg would give me an air thickness of 12 μg/cm^2, and 5 mm Hg would give me an air thickness of 16 μg/cm^2. I adjust my air bleed needle valve for a pressure of 4 mm Hg. A nice, round number.

My understanding is that different gasses (or liquids, or solids) would not make any difference in performance. As long as one ends up with an absorber that is equal to an air thickness of 12 to 16 μg/cm^2.

Jon Rosenstiel

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