[ RadSafe ] Measuring alphas with GM? Re: Survey MDAs vs. ScanMDAs

[Glenn R. Marshall]

Ludlum also quotes 19% efficiency for Tc-99 (4-pi), and I've received
pancakes from them with reported 4-pi efficiencies in excess of 20%.
Everyone else gets 10-12%.  I called their tech support people once and
asked about it.  The person on the other end of the phone said they put
the source inside the recess of the probe, right up against the screen.
Perhaps he was pulling my leg; if so, would somebody please tell me?  I
don't put much stock in advertised efficiency.  In the field, under
normal industrial conditions, alpha efficiency on a pancake is usually
pretty crappy.

 

Glenn 

  _____  

From: JGinniver at aol.com [mailto:JGinniver at aol.com] 
Sent: Thursday, May 11, 2006 3:26 PM
To: Glenn R. Marshall; Grant.NIXON at mdsinc.com; HALEEM at cua.edu
Cc: rreif at whoi.edu; radsafe at radlab.nl
Subject: Re: [ RadSafe ] Measuring alphas with GM? Re: Survey MDAs vs.
ScanMDAs

 

Hmmm,

pancake GMs are not as bad as many people think at detecting Alphas.

 

Thermo quote 0.9 cps per Bq per cm2 for Pu-238 for the 900EP15
(contamination monitor)  see
http://www.thermo.com/com/cda/product/detail/1,1055,21854,00.html , this
is generally fitted with an LND pancake GM e.g. 7311, 7312, 7313 or 7314
(depending on the connectors required and operating voltage).

 

Ludlum quotes 15% (4pi) efficiency for Pu-239 for the 44-9, as this and
other similar probes (e.g. Eberline HP260, Bicron PGM, Tech Assoc P15)
all use a 15.5 cm2 pancake geiger similar to those listed above from LND
(e.g. Canberra, St Gobain etc.) the performance for alpha monitoring
will be broadly similar.

 

Regards,

    Julian