Re: Efficiency Determination

[Judd Sills <sillsj@GAT.COM>]

At 01:35 PM 11/26/96 -0600, you wrote:
>Judd Sills wrote:
>snip.....
>> Electroplated sources are good for this type of situation.  FYI, the Ludlum
>> reported efficiency specifications (2 Pi) for this beta-gamma pancake
probe are:
>> C-14    10%
>> Sr-90/Y-90      45%
>> Tc-99   38%
>> P-32    65%
>> Pu-239  30%
>> 
>> The 4 Pi geometries are one half of the above values.
>> 
>> 
>
>Dear Radsafers,
>
>Judd makes some very good points.  Thanks.
>
>Looking at the above efficiencies, there is an apparent inconsistency.
>Sr-Y-90 should have a substantially higher counting efficiency than P-32
>since Sr-Y-90 has 2 betas per decay, one of which (2.27 Mev) is quite a
>bit higher energy than that of P-32, and the other (0.546 MeV) ain't too
>shabby either.
>
>Are Ludlum's specs just wrong here, or am I missing something?
>
>Wes
>
Interesting observation Wes:
The Ludlum specs are reported by me correctly, though whether they are in fact
in the specifications correctly is anybody's guess.  The efficiencies do match
up well with the efficiencies that Ludlum reports for other G-M detectors
with the
same window.  I would guess that the answer may lie in the average
energy per decay for the two isotopes.  The average energy for the two
betas for Sr-90/Y-90 are 196 KeV and 935 KeV.  The probe window on the
44-9 is 1.7 mg/cm-2, and will have a more pronounced attenuation for the
Sr-90 beta.  If you weight average the two Sr-90/Y-90 betas, the average
beta energy that the probe would be detecting is about 565 KeV.  This is
lower than the average of 695 KeV for the P-32.  I have looked briefly at
Eberline
catalogs for similar G-M detectors, and unfortunately, they don't report an
efficiency for P-32.  The efficiency for Sr-90/Y-90 is however, reported to be
32% (4 Pi), or 64% 2 Pi.  It would suggest a problem somewhere since the window
thickness on the Eberline HP-210 probe is reported to be 1.4 - 2.0 mg/cm2.
Eberline's
reported efficiencies for Tc-99 (30% 2 Pi) and C-14 (12% 2 Pi) for the HP-210
probe.  There doesn't seem to be a good match.

I will attempt to get some answers to this, but welcome anyone out there who
can offer a handy answer.

Judd Sills