[ RadSafe ] Re: Suitable Gross alpha-beta counting system

welch at jlab.org welch at jlab.org
Sun Feb 22 21:20:26 CST 2009


Not to argue with Bob's response, but to me a Liquid scintillation counter
seems to be overkill if the only thing you're going to do with it is leak
check guages, unless perhaps you operate a lab in which you are going to
be leak checking many of these devices for many customers.

The nice thing about leak testing (at least in the U.S.) is that the
detection limits required are not too difficult to meet, even with
relatively modest equipment, unless as Bob mentioned you are dealing with
some isotopes that only emit conversion electrons.  But, again, I assume
if you're dealing with density guages, the nuclides will emit
easy-to-detect radiations.

LSC are generally not cheap, and you must care for them routinely, keep a
supply of cocktail, deal with the waste, etc., etc.

My recommendation (again, assuming you are only doing occasional testing
on a reasonably small number of sources with beta, gamma and alpha
emissions) would be to look for a simple, combination counter, perhaps one
of the dual-phosphor systems, using an organic plastic scintillator with
alpha sensitive ZnS layer.  These are sensitive to beta-gamma and alpha. 
They are generally relatively small, lightweight, sometimes portable units
that could be stuck in a closet when not needed.  Any instrument used for
this purpose (including LSC) will need to be calibrated to respond in a
known way to the emissions of interest, and you will need to be careful
about the calibration/efficiency testing to make sure you are using
appropriate standards for this purpose.  For these wipe counters, the
calibration sources are reasonably inexpensive.

All systems are not the same.  Check out the detector configurations and
sensitivities.  Some use a very thin plastic disc, which reduces gamma
sensitivity.  But in most cases, your gamma-emitting nuclides will be beta
emitters too - you should make sure you know what you're trying to detect.

Ludlum, Protean and Thermo/Fisher (and probably others) make dual phosphor

There are of course other approaches, but you have to consider all the
factors you are dealing with.

Keith Welch

> ------------------------------
> Message: 7
> Date: Sat, 21 Feb 2009 11:48:29 -0800 (PST)
> From: Falah Abu-Jarad <abujarad2 at yahoo.com>
> Subject: [ RadSafe ] Suitable Gross alpha-beta counting system
> To: radsafe at radlab.nl
> Message-ID: <569632.78162.qm at web36506.mail.mud.yahoo.com>
> Content-Type: text/plain; charset=utf-8
> Dear All
> I am looking for an advice to suggest:
> (1) The suitable system/model (alpha-beta counting system) to use it for
> wipe samples analysis from nuclear gauges in industry.
> (2) Simple Gamma counting system for the same. (Not HPGe)
> (3) Can LSC be used to detect alpha-beta –gamma from the same sample?
> (4) Good reference for the above
> Regards
> Falah
> ------------------------------
> Message: 8
> Date: Sat, 21 Feb 2009 14:34:21 -0700
> From: "BobCat" <bobcat167 at earthlink.net>
> Subject: RE: [ RadSafe ] Suitable Gross alpha-beta counting system
> To: <radsafe at radlab.nl>
> Message-ID: <000701c9946c$2a53e5d0$7efbb170$@net>
> Content-Type: text/plain;	charset="utf-8"
> You don't say what type of nuclear gauges you are talking about (which
> radionuclides). I would seriously consider LSC especially if there are
> lower energy emitters involved, but probably for higher energy ones as
> well.  LSC is the only dependable technique for sensitive detection of
> low-energy conversion electrons and low-energy beta emitters. The
> efficiency for alpha particles and medium- to high-energy beta particles
> approaches 100% so corrections applied to account for detection
> efficiency/quench are generally relatively small (perhaps 20-30% as
> compared with proportional counting where corrections for efficiency and
> self-absorption effects can approach 70-90%) and thus there is
> intrinsically lower levels of measurement bias and uncertainty.  It is
> fast and easy - just stick the swipe in the vial and squirt in LS
> cocktail, and it runs itself using a sample changer. It also provides a
> spectrum and allows one to discriminate against lower energy activity (at
> least to some !
>  extent). No counting gas is required...
> There are two really only two downsides:  1) You create waste.  2) the
> sample is immersed in cocktail and not as readily available for subsequent
> analysis if there is any question about the identity of which
> radionuclides are present.
> Best of luck,
> Bob Shannon
> QRS - Denver
> 303-432-1137

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