About selecting energy range on the spectrometer

[toex]

Hi everyone, I am doing an experiment about alpha spectrum. I am using Model P-2000/1 (don't know if this matters), a NaI(Tl) scintillation probe. I selected energy range 3-8 MeV at the front panel and waited for 3000 counts at the peak, it took ~800 seconds. But when I selected energy range 4-6 MeV and did the same measurement, it took me ~17000 seconds to have 3000 counts at the peak! Does anyone know why? Thanks a lot!

 

[mfb]

Was the peak fully contained within 4-6MeV?
Are you sure that the count was just the peak, not events in the whole range?
If it is not one of these things: No idea. Might be some strange effect from the setup.

 

[ramses22]

I'm not sure if I'm right but NaI(Tl) detector isn't good choice for alpha. IMHO its in endcap made by some metal maybe Al (hygroscopic material) so alpha particles cannot penetrate through this "cover". Alpha is mostly measured (spectrometry measurement) in vacuum chambers with some kind of semiconductor detectors. Are u sure u have NaI(Tl)? There are another scintilitor Zn(S)which is used for some kind of alpha spectroscopy. And regarding your question have to agree with mfb, u have narrower window so its possible that part of peak is out of your rang. BTW do u know what nuclide are u measuring?

 

[toex]

Was the peak fully contained within 4-6MeV?
Are you sure that the count was just the peak, not events in the whole range?
If it is not one of these things: No idea. Might be some strange effect from the setup.

1. Yes, the peak was fully contained in 4-6 MeV.
2. Yes, the count was just the peak.
3. Thanks for your reply :)

I'm not sure if I'm right but NaI(Tl) detector isn't good choice for alpha. IMHO its in endcap made by some metal maybe Al (hygroscopic material) so alpha particles cannot penetrate through this "cover". Alpha is mostly measured (spectrometry measurement) in vacuum chambers with some kind of semiconductor detectors. Are u sure u have NaI(Tl)? There are another scintilitor Zn(S)which is used for some kind of alpha spectroscopy. And regarding your question have to agree with mfb, u have narrower window so its possible that part of peak is out of your rang. BTW do u know what nuclide are u measuring?

Sorry, I mixed them up. I was not using NaI(Tl), I was using a charged silicon detector to measure Po-210 inside a vacuum chamber. Thank you for replying!
Another question, this time I was using a NaI(Tl) to measure the gamma spectrum of Cd-109. As it has two very nearby peaks(22.1 & 24.9 keV), I selected a very big COARSE value on the MCA so that the two peaks can be resolved. But when I chose the maximum COARSE value, the counting almost stop (I assume it is very long dead time?), what was happening? Also, when the Na(Tl) was measuring nothing, the spectrum showed two significant peaks and there were 10000 counts in 10 seconds! I don't think this comes from the background. Anyone know what was it?