Zn-65 Spectrum Mystery

[isoenzyme]

Sorry for the long post, but I'm hoping that someone here will be able to tell me if there is something inherently bad about putting a Zn-65 source next to a CsI(Tl) crystal or where the mystery peak is coming from in my measured spectrum.

First, Zn-65 is a beta emitter that also emits a nice gamma centered around 1,111.5 keV. Should be easy to measure, but every time I try any background spectra I collect after measuring Zn-65 is highly unusual. To make this phenomenon as clear to illustrate as possible I will provide the spectra (and counting rate vs. time) plots for the four sources used to calibrate energies:

Spectra and Counting Rate Plots for Lu-164, Na-22, Cs-137 and Co-60 sources used to calibrate energy vs arbitrary units in this experiment.

After using these collected spectra to calibrate for energies, the following background spectra was collected:

Background spectra and counting rate plot for the empty GS-STANDUP obtained BEFORE the Zn-65 sample was measured.

The following spectrum for Zn-65 was subsequently collected:

Zn-65 gamma spectrum (600 seconds) obtained with a 2" CsI(Tl) detector

001 Zn-65 Spectra.png (42.82 KiB) Viewed 13763 times

The counting rate vs. time is also provided (to be complete and for reasons to be discussed later):

Zn-65 spectrum and counting rate vs. time plot.

001 Zn-65.png (83.41 KiB) Viewed 13763 times

The following spectrum and counting rate vs. time plot was collected immediately after removing the Zn-65 source (and placing it in a lead pig and locating it 30 feet away!):

Background spectra and counting rate plot for the empty GS-STANDUP obtained AFTER the Zn-65 sample was measured.

If I disassemble my setup, place the detector back in the foam-padded carrying case and give it a week then everything seems to work OK again. I'm hoping that someone with LOTS more experience than I have will be able to tell me:

(1) Why the background data is so strange after exposure of the scintillating crystal to Zn-65 when it behaved so "normally" before exposure?

(2) Does anybody know what the "mystery peak" around 500 keV is due to? If this was an artifact from a badly dialed GS-USB-PRO, bad cable or the like, wouldn't it show up in the calibration spectra as well? Zn-65 decays to Cu-65 which is, to the best of my knowledge, stable and doesn't produce any gamma rays.

Anybody have an obvious answer to what is a "Scooby Doo Mystery" to me?

[Sesselmann]

Christopher,

Good observation, CsI(Tl) is an excellent scintillation material and has good resolution, but it is known to exhibit afterglow or luminescence. I am not 100% clear about how or why this effect is more prominent in CsI, but many papers have been written on this subject.

It sounds like this is may be you are observing.

Steven

[cicastol]

(1) Why the background data is so strange after exposure of the scintillating crystal to Zn-65 when it behaved so "normally" before exposure?

(2) Does anybody know what the "mystery peak" around 500 keV is due to? If this was an artifact from a badly dialed GS-USB-PRO, bad cable or the like, wouldn't it show up in the calibration spectra as well? Zn-65 decays to Cu-65 which is, to the best of my knowledge, stable and doesn't produce any gamma rays.

Anybody have an obvious answer to what is a "Scooby Doo Mystery" to me?

As Steven said it is caused by afterglow effect , 511KeV is the annihilation peak due to pair production of the main photopeak (Ey>1.022MeV)

[isoenzyme]

Ciro and Steve:
Many thanks for explaining it to me! Your kind help is very much appreciated.

[GEOelectronics]

I'll reply to:
"(2) Does anybody know what the "mystery peak" around 500 keV is due to?"

before reading other's response.

I believe it is exactly 511 keV, formed by positron annihilation.

George Dowell

Sorry for the long post, but I'm hoping that someone here will be able to tell me if there is something inherently bad about putting a Zn-65 source next to a CsI(Tl) crystal or where the mystery peak is coming from in my measured spectrum.

First, Zn-65 is a beta emitter that also emits a nice gamma centered around 1,111.5 keV. Should be easy to measure, but every time I try any background spectra I collect after measuring Zn-65 is highly unusual. To make this phenomenon as clear to illustrate as possible I will provide the spectra (and counting rate vs. time) plots for the four sources used to calibrate energies:
001 Calibration Standards.png

After using these collected spectra to calibrate for energies, the following background spectra was collected:
001 Initial Background.png

The following spectrum for Zn-65 was subsequently collected:
001 Zn-65 Spectra.png

The counting rate vs. time is also provided (to be complete and for reasons to be discussed later):
001 Zn-65.png

The following spectrum and counting rate vs. time plot was collected immediately after removing the Zn-65 source (and placing it in a lead pig and locating it 30 feet away!):
001 Post-Zn Background.png

If I disassemble my setup, place the detector back in the foam-padded carrying case and give it a week then everything seems to work OK again. I'm hoping that someone with LOTS more experience than I have will be able to tell me:

(1) Why the background data is so strange after exposure of the scintillating crystal to Zn-65 when it behaved so "normally" before exposure?

(2) Does anybody know what the "mystery peak" around 500 keV is due to? If this was an artifact from a badly dialed GS-USB-PRO, bad cable or the like, wouldn't it show up in the calibration spectra as well? Zn-65 decays to Cu-65 which is, to the best of my knowledge, stable and doesn't produce any gamma rays.

Anybody have an obvious answer to what is a "Scooby Doo Mystery" to me?

[GEOelectronics]

OK now I've read the other posts and will add this:

Zn-65 decays by "EC" or Electron Capture. True that IS one form of beta decay, but not the common one. EC causes an orbital electron to enter the nucleus, where it interacts with a proton, changing that proton to a neutron. This of course changes the atomic number Z, and that always means a new element has formed. (Cu-65). Upon this transition a positive electron (b^+) is ejected from the nucleus, giving almost immediate rise to direct positron-electron annihilation (outside the nucleus). Also at that moment your 1.115 Mev Gamma ray is released from the nucleus, as the nucleus seeks a ground or unexcited state. As the Gamma ray exits through the electron shells, sometimes it interacts with the electrons in the K and L shells causing them to move out of the atom. This of course leaves the electron shell unstable, so those must be replaced. When other electrons fill those vacant spots, they give up some kinetic energy to "fit" that new spot, giving rise to characteristic X-Rays being Ka= 8.05 keV, Kb=8.90 keV, La = .93 keV Lb= 0.95 keV.

Of course this is just a guess.
George Dowell

[GEOelectronics]

"Zn-65 decays to Cu-65 which is, to the best of my knowledge, stable and doesn't produce any gamma rays."

Actually, Cu-65 emits a Gamma Ray 1.115 MeV and a plethora of X-Rays, but only does it once.

Zn-65 emits only a Positron, no gamma rays or X-Rays.

Alpha and Beta (all forms) are decay modes (a decay mode being defined as a transmutation of element (Z#) not necessarily A#

My previous post explains that I think, if questions, just ask.

George Dowell

[GEOelectronics]

"The following spectrum and counting rate vs. time plot was collected immediately after removing the Zn-65 source (and placing it in a lead pig and locating it 30 feet away!): "

To test Steve's theory, set up same outfit, use Co-60 until the counts reach same level (about 10k counts it looks like), remove the Co-60 then run background for same time factor (run time). See if there is residual. If yes, then afterglow ( I never heard or this, but seems logical). If no, maybe source leakage (doubtful)

George Dowell

[luuk]

Hi Christopher,
I have tested a lot’s of crystals but never seen this afterglow phenomenon you describe here.
Yes, it is true that crystals e.g. CsI(Tl) and CsI(Na) are known of having sometimes afterglow but that only happened when radiated with a strong Uv, Fl or daylight not by a simple test source unless your source is extreme strong, but I expect that in your case this is not so.

I asked around but no one at work ever hear of this effect you got, so I did a simple test I took a 2x2” CsI(Tl) mounted it on a 2” pmt and put our strongest Cs-137 sources direct in contact with the crystal the strength of the sources are in total ± 234KBq.
But first I did a non shielded background test my ROI was set during the tests from 5 KeV to 2 MeV that gave ± 85cps, background , after that I exposed the crystal for 50 minutes to 234KBq.
The sources are strong so I calculated the cps it gave approximately 5.1k cps that is in my vision a lot!
Immediately after I removed the sources I did again a background test and got ±84cps so actually the same result as before the exposure.
I know it is just one test , so no statistic but I for me it proves what I expected it is not possible to produce afterglow with a test source.
What did happened during your tests I don’t know but in my opinion it is not afterglow you have seen.

Luuk

[Sesselmann]

Christopher,

I suggest this needs further investigation to find the root cause.
Some pretty crazy physics would be going on for the crystal to continue seeing a 1.1 MeV gamma after removing the source. Even if it were an annihilation peak from positrons you would expect this to be spontaneous.

I found this paper on ZN65 : https://www.tandfonline.com/doi/pdf/10. ... 5809171024

Steven