[ RadSafe ] RE: radsafe Digest, Vol 199, Issue 3

[Tracy, James W.]

Hi Elaine,

Is there significant dead time in your count?  If so, my guess is that you are having pulse pileup problems because of a high count rate. Increasing your source to detector distance can help with this.  Reducing your time constant can also help with this (but will increase your FWHM and will require calibration). The manual for the amplifier should discuss pulse pile up.  Also, is your pole zero set properly? This could result in pulse pileup issues at lower count rates.

The following link has an article on high count rate gamma spectroscopy.

http://www.canberra.com/literature/974.asp

If you have one of the newer all digital systems, pulse pileup shouldn't be a problem.

I hope this helped.

Jim Tracy
NIST
Health Physics

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Message: 5
Date: Fri, 5 Jun 2009 11:49:26 -0600
From: "Marshall, Elaine" <emarshall at lrri.org>
Subject: [ RadSafe ] Energy Shift
To: <radsafe at radlab.nl>
Message-ID:
	<CC21FAE71BFC48449018DD0FA2C54322018A1E1F at bali.lobos.lrri.org>
Content-Type: text/plain;	charset="us-ascii"

We have been observing something interesting.

 

We are working with Ce-141 in varying concentrations.   We counted two
samples yesterday on three different pieces of equipment - LSC, gamma
counter and also ionization chamber connected to a MCA.  With the lower
activity (about 3000 cpm), we see nice peaks on the MCA at 36 keV and
145 keV as expected.  In this instance, the relative intensity of the 36
keV peak is about 2X that of the 145 keV peak.  However with the higher
activity samples (5 uCi) in the same geometry, the higher energy peak is
shifted to 158 keV and the 36 keV peak can not be distinguished from the
significant scatter.  Count rates in the first 30 channels are orders of
magnitude greater than the 158 keV peak.  

 

I am confused as to why we would see the energy shift (from 145 to 158)
on the MCA for the higher activity samples and wonder if anyone can shed
some light on this.

 

Thanks!

 

Elaine T. Marshall, CHP

Radiation Safety Officer

Lovelace Respiratory Research Institute

2425 Ridgecrest Drive SE

Albuquerque, NM  87108

(505) 348-9578

emarshall at lrri.org