Low Level Gamma Radiation (2023 Update)

Don't the dynodes wear out after some numbers of coulombs?

I made it to the Louisiana State Science Fair with an alpha scintillator, a 931A in a pipe with a bit of glow paint scraped from a clock. First Place in Physics! The competition was mediocre.

The alphas made giant pulses, way above background.

Next year I made it to the Nationals in Baltimore with a CRT thing. Didn't win anything but hung out with Amory Lovins.

Personal preference. I don't like the idea of feeding 800 volts into electronics. Caps break down, pc traces arc from moisture or dirt, things spark over as you mentioned earlier.

I'll run my cathode at -800 volts where nothing can be damaged.

Incidentally, a 10nF cap discharges from 800V to zero in about 50ms.

People say modern leds are crazy brilliant. I wonder if one will glow at

People often complain about leds staying lit on leakage currents in wiring, but nobody measures the current.

About 1000 coulombs per square centimeter of photocathode, give or take. They also age out after five years or so because all the volatile stuff like caesium metal migrates around inside. If a gamma counter gets up to 1000 C/cm**2, I suspect Mike would have other problems. ;)

Fun.

Yup.

Don't know him.

Cheers

Phil Hobbs

It doesn't matter which end gets the high voltage. Why take chances by putting the high voltage on the anode. It works just as well by putting the voltage on the cathode. I'm not going to be using it long enough to worry about ion migration.

Toyota Principle. Poka-Yoke: Catch mistakes before they occur.

The high sensitivity means the HV supply has to be well regulated.

One site mentioned the HV has to be adjusted for each PMT to get the spectrum lines to align with the corresponding MeV values, which makes sense. I suppose the system would have to be recalibrated as the PMT ages.

I wonder if that is because of the mechanism of operation, and the large number of cascaded sections.

As I understand it, electrons hit a dynode and eject a number of electrons. These electrons accelerated under the influence of the voltage between electrodes, the same as in the old vacuum tubes. They hit the next electrode and eject more electrons. The number of electrons depends on the impact velocity, so the higher the voltage, the more electrons you get.

This process is repeated for each dynode, so the effect is multiplied each time. This is why the PMT is so sensitive to the applied voltage.

And this is why it is so difficult to keep the spectrum aligned with the known energy levels.

No. By then they had failure modes well under control.

The only time I have ever known it happen was with a much bigger high power early model turbo pump (one of the largest available at the time). Fortunately it failed at lunchtime and there was no-one in the lab. It was pumping a big chamber about 1m^3 and something went horribly wrong. The academic experimental setup was intrinsically not fail safe.

It shredded the blackout curtains, imploded the lab windows and doors. The bang was heard all over the site and people went running to see what had happened. Titanium rotor blades were stuck about 2" into concrete, and went through breezeblock walls in a worryingly large kill zone. Some were also stuck in the casing walls. Miraculously no-one was harmed.

The replacement included external Kevlar armour and a host of other safety measures to prevent a recurrence from harming experimenters.

Yes, that's right. The secondary yield generally isn't quite proportional to the bias per stage, because at higher bias the incident electrons penetrate deeper, so that their secondaries have to make their way out through a thicker layer of material on average, and still have enough energy to escape when they hit the surface.

There are negative electron affinity (NEA) photocathodes, where essentially any free electron that reaches the surface will escape. They have higher quantum yield, as you'd expect, but are also slower because it takes awhile for all those low-energy secondaries to bounce around before reaching the surface. I'd expect tubes with NEA dynodes to follow the power law more closely.

Well, and the fact that the energy resolution of CsI is the pits compared with something like intrinsic germanium. On the plus side, it's cheap and doesn't need liquid nitrogen.

Back in the days before really pure silicon and germanium crystals were available, they used to be "lithium drifted", i.e. lithium interstitials were introduced to bind to impurity trap states. Those ones had to be kept at 77K continuously, or else they'd be spoiled in a few days.

Cheers

Phil Hobbs

Provided you can insulate the can from the photocathode.

Cheers

Phil Hobbs

Add some source with a known narrow line, and servo on that.

Yikes.

Cheers

Phil Hobbs

Whoa! Having no people in the room makes that a fun story, the imploded windows, shredded curtains etc. - hard to match. My closest electronics related event compares to that like stumbling in the street to a plane crash, I am somewhat envious :). [Once, almost 30 years ago, I reversed the power on a new board with plenty of tantalum caps which exploded machine-gun like, pretty loud at that. Lucy was in the next room and had frozen thinking she might have lost me; unfroze with a huge sigh of relief some 10 seconds later when I grasped what had happened and started to chain-swear].

Thanks very much for the lucid explanation.

I'll find out soon. Thanks

There are a few reasons we do not ship internationally. Main one is ITAR and having to figure out what is and isn't can be a pain and it's not worth it for us. Second would be shipping issues. We have no control over the shipping and a lot of items would get lost in the past and then it's on us to refund. I know there are some new ebay programs which reduce this risk and it is something we are considering for the future. I am really happy we were able to save this tube from the scrap heap and that so many are finding it useful.

True, better safe than sorry when it comes to that sort of thing. It's probably safe enough to ship them to Canada. But I wouldn't want to risk it with China or Russia, given that I don't even know the ECCN #.

Very cool little gadget, in any case!

-- john, KE5FX