Gamma Spectrometry Forum

Left the scintillator on the drainboard (no castle yet) for a while and came back to this spectrum of the background radiation.

What do I have here? and what is that only obvious peak with little above it? Tl-208 at 2.64MeV?
I am clueless and can use some instruction.

Daryl,

Fortunately what you are seeing there is not a mystery isotope on your drain board, it's just caused by the volume setting being set too high.

The Line in audio port measures voltages in the range -1.736 to +1.736 volts, and PRA software converts this to -100 to +100 Arbitrary units, which in turn can be divided into 65,536 channels (16 bit). https://en.wikipedia.org/wiki/Line_level

What you are seeing here are counts from pulses exceeding 1.736 volts as they pile up in the last bins.

Those really high value pulses up to 160 arb.u are most likely anomalies caused by doubles.

A rule of thumb for a good volume setting is to have K40 (1460 keV) in the middle of the screen at 50 arb.u.

Steven

Steven,

Excellent! Thank you very much!

I found the volume control. It was set to 15 (out of 100) and I backed it up to 5. The peaks no longer clip in the Audio window. They are really quite small and negative (pointing down).

I repeated the exercise and have lost that peak, but still do not know where I am. This was 640,000 pulses collected over 21,000 seconds. That’s around 1800cpm which is really close to where this scintillator reads when on my Ludlum 3.

Where am I? or better yet, is K40 in this picture? Do I have to build a banana castle?

Daryl

Daryl,

The fact that you have to have your volume set to 5% indicates that you are using a MIC port rather than a Line In port. If your computer doen't have a Line In port you may want to consider buying an external USB sound card.

It's always hard to diagnose a spectrum without seeing your settings, but it looks as if your first spectrum was shown in linear format and the second spectrum in log format. I also can't recall what kind of detector you are using which makes it harder.

Have you set your pulse height threshold to a negative number?

Can you show me the above spectrum in linear mode?

Can you take a screen clipping of the Settings panel?

Steven

Steven,

Both these are shown in log. Yes, I'm using the mic in, I do not have a line in. I am overdue for a new computer.

I am using PRA 15.0.1.0 The detector is an Irad 2inch Bicron BC-412 with an XP5215 PMT. The GS1100A is set to 1000v.

Here are the settings I used along with views in linear with them dialed up to where you can actually see the curves. If I use shape tolerance I get nothing.

I sign for a new home at the end of the month and will be able to unpack an external sound card at that time.

Thank you very much for walking me through this educational experience,

Daryl

I think I might be making progress - even if it doesn't look pretty.

I cranked the GS-1100A up to 1100v because the detector is supposed to perform better for lower energies.

The first one is on the drain board.
The second one is on the drain board with a smoke detector part sitting on the end of the detector.

Would that bump be around 59.54KeV ??

25asa is slow.

Using shape tolerance and finally getting results - after viewing the video for the sixth time.

My pulses in the Audio Input window are negative, so pulse shape acquisition gives me negatives in the pulse shape window.

Daryl,

Nice to see progress :)

Good you have the pulse height threshold correctly set for negative pulses at -0.3 √

On the screen shot you didn't have the "Use shape tolerance" box ticked, this needs to be ticked otherwise you will see nothing. Obviously you also have to perform the Pulse Shape Acquisition run as described in the manual.

Finally your last problem is your choice of scintillator BC412, it's a plastic scintillator and great for counting gamma, but unfortunately pretty useless for spectrometry. The pulse height resolution for BC412 is so low that gamma peaks will show up as camel humps at best.

You need to get yourself a detector or at least a new crystal suitable for spectrometry, NaI(Tl) gives you best value for money, but there are others that can be fun to play with.

Here is an extensive list of scintillation materials and there properties, so you can compare.

http://scintillator.lbl.gov


I also found this page with a NaI vs. Plastic Comparison spectrum.

https://am241.wordpress.com/tag/plastic-scintillator/

Steven

AHA!! Right film and developer, but wrong lens!

Okay, I'll be looking for a scratch and dent sale for one of those.