Greetings from Victoria, BC Canada + Cs137 spectrum with odd peak

[Conor Whyte]

Just wanted to say hello and show you some spectroscopy that I've been doing.

I have a weak Cs137 source ( an old spark gap tube from the mid 60s) that produces a clean 32KeV and 662KeV lines, but recently noticed a peak to the right of the 662KeV is on the increase.
I thought it was background noise, but it shows up in a similar location to Cs134... which to me doesn't seem to make sense.

IMG_4274.jpg

Recent sampling shows this peak increasing over time... more pictures to follow shortly.


I am using a Scionix 38B57 1.5" NaI/Tl detector on 800V.

Any ideas what might be occurring here?

[Conor Whyte]

So more testing reveals that this peak is showing on any spectrum that I am taking.... I am confused as to what this might be.

I have tried pulling spectrum from different computers, and get the same results. All end up with this odd peak near 800KeV.
Could this be RF getting into the audio cable? stumped..
Just for some more background info... the Cs137 tube has around ~300nCi of Cs137 left in the tube and is reasonably weak.

The other spectra in the pics are from a strong Am241 source from 1m distance away. Notice that the parasitic peak is still showing up around 750-800KeV.

Theremino MCA driver v3.3 is being used as the HV source.

[Peter-1]

Hello Cesium,
I have a small probe from fukushima and can see the same peak.

I think this is Cs134.
Peter

[Sesselmann]

Hi and welcome to the forum, first for the protocol, we all use our real names on this forum, so unless you are genuinely Dr. Cesium, could you either change the handle or add a signature on your posts showing your real name.

As for the mystery peak, it is not from the Cs137 source, so it might be worth doing a background spectrum to see if it is still there, and if so, you might have something unexpected in your lab which is emitting that peak. Keep us posted..

Steven

[Conor Whyte]

Sorry, and thank you for the response.

I suspect the peak is now some form of interference as I have this peak showing up in multiple localities across every source I own, including the background.
I am suspecting it is either the driver, or audio cable picking up some kind of interference.

[Peter-1]

Hello
I made a quick test with only the background and with my Cs137 from fukushima. And now ?

Peter

[Conor Whyte]

The source is a sparkgap tube from the 60s, so I would not expect Cs134 to show up. Additionally I see this peak in NORM samples, a K40 source and just running background at multiple localities.
It's got to be some sort of error with either the driver /( probably a case grounding issue) or audio cable grounding issue or both? This problem also seems to be getting worse.

[Sesselmann]

Conor,

It is possible to get a false peak from tiny HV arcs on the board, in a cable or in a connector, but these can usually be identifies as they have regular intervals and don't appear random like natural radiation. It doesn't appear to show on the background (red line), which indicates to me that it may not be a hardware problem after all.

If it's coming from the spark gap we need to be looking for potential isotopes with a relatively long half life and a high intensity gamma between 790 and 800 keV.

http://nucleardata.nuclear.lu.se/toi/radSearch.asp

Not too many come up... Pu ?

Steven

[Conor Whyte]

Sorry Steven, the erroneous peak shows up in all my spectra, not just the spark gap tube. This is why I am now quite positive it's a hardware issue of somesort. The cables are Pomona and are of high quality, so they are not suspect.


I also notice when I turned up the audio and listened the play through, I hear what I think is a random discharge every couple of seconds, like a malfunctioning MOSFET (partially damaged) -- A rushing high frequency static buildup, and then a discharge pulse, then the cycle repeats.. This is all very faint but melodical.

--- May be looking at trying another high voltage source.

[Conor Whyte]

Well the problem seems to be solved now. I tried upping the voltage slightly to 845V and the parasitic peak dissipated quickly. So, it appears that the theremino PMT adapter has a stability problem at its lowest setting of 800V. Measuring the output at the lowest voltage drift about 10v over the course of ~30minutes.
upping the voltage helped stabilize the output and the drift was minimal, even after an hour.

Weird...