Low Level Gamma Radiation (2023 Update)

For any serious use, I'd budget for a replacement PMT. They age out after several years, which is probably why these assemblies were replaced. They also wear out on account of photocurrent--the usual rule is that you can get an integrated anode charge of about 1000 C/cm**2 of photocathode area before the gain drops by ~half.

If you're going to use an ionic scintillator in contact with an end-on PMT (one where the photocathode is deposited directly on the faceplate) you'll need to keep the cathode near ground to avoid ions migrating through the glass and corroding the PC. That means running the anode at high voltage and coupling the pulses out with a cap or transformer or something.

Using an air gap between scintillator and tube allows you to keep the anode near ground, which is much more convenient, but costs you about half your light. (The collection efficiency goes like the etendue, which has a factor of n**2 in it.)

Cheers

Phil Hobbs

Good information. Thanks.

I'm not planning on heavy use. The idea is to compare the scintillator againt the Radiacode, and try to measure any Radon gas in the basement. Then it will probably go into storage.

If you have radon, try running a CRT television for a few hours and then wipe down the screen with a damp piece of tissue that has been folded to about the size of your geiger tube (or its end window if it has one). I find the dust on my CRT screen is quite radioactive but it has a short half-life. I'm guessing it is some polonium isotopes. The dust that collects on the screen doesn't end up in my lungs, so perhaps CRTs are healthier to have in the house than flatscreens.

Not as healthy as having no TV. That crap bypasses the lungs and goes straight to the brain

On a sunny day (Tue, 28 Jun 2022 13:31:26 +1000) it happened Chris Jones snipped-for-privacy@spam.yahoo.com> wrote in <eCuuK.190184$ snipped-for-privacy@fx01.ams:

Some CRTs from sets that came in for repair had collected tar from smokers.. nicotine. Wiping clean with alcohol increased brightness considerably :-) So yes, functioned as air filter ;-)

On a sunny day (Tue, 28 Jun 2022 17:03:04 +1000) it happened Clifford Heath <no snipped-for-privacy@please.net wrote in <16fcb6d40dd74681$1$391142$ snipped-for-privacy@news.thecubenet.com>:

There are some informative science related programs on (sat) TV Astra1 and there is (at least on the continent) tetelext / ceefax / videotext Caesar: give the people bread and TeeFee or was it games? :-)

Its a shame UK dropped Ceefax. no way to see the program schedule other then internet, talk about a DANGEROUS medium ? Just read this group!

Radon detectors and test kits are 14 USD upwards on Amazon...

I was looking for a sold state PMT diode .. for that tomshardware link I gave, delivery November, about 56 Euro 1 piece

I will hang on to my good old PMTs for now, have also many plastic scintillators. Big PMT will likely last to WW3 (2024??). After that all lights up in the dark anyways

BTW I have a nice scintillation screen too.

It ought to work fine for light-duty use like that, though you'll need the PMT supply and bias string. If you know the tube P/N, I can probably tell you what to use.

Cheers

Phil Hobbs

I don't think the PMT P/N is available. I also assume the bias string is buried inside the assembly. I'll probably have to find the operating voltage by experimenting, a time-honored tradition among hackers.

Turns out the p/n is (more or less) in the listing title: it's some flavour of R7400, whose datasheet is here, courtesy of Jeroen & Co:

. CsI(Tl) emits in the visible (the peak is around 550 nm), so the tube is probably an R7400-02 or -20.

It's an 8-stage tube that likes about 75-100V per stage, and is interestingly fast at 800 ps FWHM. That's fast like an APD, and much faster than a SiPM/MPPC. I ordered a few of them to try out, probably without the scintillator.

For scintillation counting, you don't need brilliant linearity, so a resistor string is probably fine for the dynode bias. I'd start with a current-limited -1 kV variable supply and some 1M resistors, with a

2.2-nF cap across each of the bottom two or three resistors. That way, at 800V you'll be dissipating about 80 mW.

Cheers

Phil Hobbs

[...]

Very valuable information. Thanks.

Go ahead and send your address to me at john (at) miles.io if you'd still like one, and I'll throw one in a padded envelope next time I go to the post office. My GMail account is almost unusable due to people confusing their email address with mine and signing me up for all kinds of junk mail. Right now there are 217,959 unread messages and I don't see yours anywhere. :(

I powered one up just now and took a video:

Hopefully I'm looking at a lot of dark counts or other PMT artifacts, because jeez, that sure seems like a lot of pulses.

-- john, KE5FX

Hi John,

Very nice page !

That is a very nice offer. Thanks.

Various sources say the PMT voltage is critical and must be adjusted individually for each PMT. One way to do this is to find a known radioactive source and generate a spectrum, then adjust the PMT voltage so the source lines up with the known energies in KeV or Mev.

Getting samples of thorium has turned out to be impossible, except for traces in welding rods. However, radium is readily available in the form of watch hands painted with radium. These are for sale extremely cheap on Ebay.

The watch hands no longer glow in the dark since the phosphor will have worn out. However, the radium will still be active since it has a half-life of 1600 years. Below is the radioactive series of radium-226. You can see the first step is radon, which releases alpha particles and gamma radiation:

https://www.ld- didactic.de/software/524221en/Content/Appendix/Ra226Series.htm

Radon gas is extremely dangerous. There is a small amount in every basement. You breath it into your lungs, where it emits alpha particles, which are the nucleus of helium-4. This causes lung cancer and you die.

We need to be able to monitor the amount of radon in our basements and be certain it doesn't increase, such as during the winter when most ventilation is shut off.

So it pays to learn a bit about gamma spectrometers to be able to protect yourself and your family.

Regular radon detectors are very expensive, but gamma spectrometers can be quite modest in cost. You can get the Radiacode, which is a very nice unit, or build the Hamamatsu R7400U PMT version as a backup and verification.

The photomultiplier voltage just adjusts the gain of the photomultiplier tube - photons hitting the photocathode produce electrons in the cathode space, and the voltage drop across the dynode chain just adjusts the number of electrons that each electron hitting the first dynode ends up delivering to the anode.

Getting the energy of individual particles coming out of the radiation source takes rather more than fiddling with the gain of the electron multiplication stages inside the photomultiplier tube. Because you've got a lot of stages of multiplication - anything from 10 to 14 - you can vary that gain a lot.

If your house is built above granite rocks, or any other kind of rock that contains uranium, you can get radon gas leaking into the basement. If it is, put in an extractor fan to move it out before it can drift up to diffuse through the floor into the rest of the house.

Helium four won't do any damage at all, but energetic alpha particles can cause mutations in any cell that they hit, and some mutations can make cell cancerous

If there's uranium - or the like - in the rocks under your basement it makes sense to monitor for radon. If there is, it makes sense to ventilate that space, even in winter.

Learning about geology is cheaper.

Radon itself decays by emitting a 4.6 Mev alpha particle. This isn't any kind of gamma ray, but it is energetic enough that if it hits an adjacent atom it may generate a gamma ray (an energetic photon) which is likely to have a longer range.

The Radiacode 101 monitor uses a scintillation detector - the chunk of thallium doped cesium iodide - which produces a flash of light when hit by a gamma ray photon (or anything else that can get at it).

Also called an avalanche photodiode.

A window, so the photons emerging from the transparent can get at the sensitive face of the photodiode

Avalanche photodiodes are tricky to bias.

The pulses do tend to be narrow. You need fast op amps and comparators, but they are widely available and not that expensive.

The minature Hamamatsu R7400U photomultiplier tubes are pretty compact (but not as compacts as an avalanche photodode) and need a higher voltage supply, but they aren't quite as tricky to bias as an avalanche photodiode.

Also on eBay, you can find "quantum energy" or "scalar energy" or "negative ion" pendants made out of a black volcanic rock. They're often touted as a way of protecting oneself from the (supposed) harmful effects of electromagnetic radiation.

They do generate "negative ions", in the form of beta-decay electrons... these are not your gentle after-the-rainstorm negative ions by any means!

By one report the radiation level is high enough that wearing one next to the skin for a year can exceed safe exposure levels and might even cause a mild radiation burn. I understand that Singapore has banned the import of such products for this reason.

The rock from which they are made appears to be rich in thorium and its decay products (perhaps something like monazite?).

I measured one on my home-made gamma spectrometry system and got a very recognizable thorium-decay signature (the isotope markers are from a best-efforts calibration).

Another useful calibration source (for higher-energy gammas from potassium-40) is a sack of potassium chloride water-softener recharge crystals from your local big-box home/hardware store (or, on a smaller scale, a box of Morton "Lite Salt" which is about half potassium chloride).

And, one can still find orange Fiestaware and green uranium glass at the occasional estate or garage sale or antique store.

Resolution of that setup looks really good -- is it written up anywhere?

-- john, KE5FX

Thanks for the information. I will try to find the black volcanic rock products. I recall an article describing these that got them kicked off Amazon (I think).

Your gamma spectrometry system sounds very interesting. Can you supply more information, such as what kind of scintillator crystal do you use, how big is it, and do you use a PMT or diode for the detector, what software do you use to generate the spectrum, how do you drive it, and any other details that might be interesting.

I use pure potassium chloride (KCl) in the form of Windsor Salt Free seasoning, but my Radiacode is not sensitive enough to detect the decay products. I left it sitting on three containers for several days but had no luck. I need a more sensitive detector, which is why yours is so interesting.

On a related topic, I have been trying to find out why the background gamma spectrum has a sharp rise at low energies, and where does it come from? Do you have any ideas?

Thanks,

Mike

why did somebody snip off the last letter of .PNG????

Ebay was a bust. Quantum Energy turned out to be a fishing reel. Scalar energy was some patterns on a piece of paper.

However, Amazon turned up lots of volcanic rock emi protectors. I got one titled

"Volcanic Stone Pendant,Energy Necklace Fashionable Health Care for Women for Daily Wear Quantum Science Scalar Energy Necklace Round Pendant for Men Woman Tourmaline Volcanic Stone Jewelry",

$15.86 FREE delivery August 22 - September 16

This one sounds like it will do the job. Thanks.

Mike

I don't think I've written up anything in detail - it's been a few years since I played with it much.

I have a couple of probes, both built with surplus commercial PMTs, one with a NaI(Tl) crystal and another with a BGO crystal.

The signal processing uses a charge-sensitive amplifier with some pulse shaping built in (I like the CSA architecture because it's not particularly sensitive to the cable length and capacitance). The original design was done by Richard Hester; I did some tweaks for my version and did a custom board layout. The files are in the

directory.

The biggest disadvantage to this circuit is that if I forget, and connect or disconnect the probe cable with the power on (or without letting it discharge to zero) the spike usually blows out the front-end JFET, and I have to cuss and pull it out of the air-wire socket and replace it :-)

The high-voltage supply is based on a flyback with diode voltage multiplier, based on a circuit I found some years ago intended for use with GM tubes.

The output of the CSA goes into a moderately-priced USB sound card line input.

I wrote the pulse detection and filtering code myself. The first version was strictly command-line (record audio to a .WAV file, then post-process to detect and bin the pulses and generate data files which could be fed to gnuplot). I then wrote a Linux GUI based on the FLTK toolkit which can read/process/display in real time.

The trickiest part was handling high pulse rates, where one pulse starts while the CSA is still recovering from the previous one. At some point I may sit down and try writing some DSP code to de-convolve the CSA's pulse shaping and turn the signal back into narrow impulses.