RE: Floor monitor - background increasing

[Marco Caceci <mcaceci@radal.com>]

Sorry if I jump in a bit late, but my grain of salt:



not the detector, the electronics

proportional counters use <very> high input impedance opamps, to measure 

very low currents -

typically the 8500A (Datel-Intersil, now Dawn) CMOS chip, bad in all specs 

except some quite spectacular 10fA bias at 25 C

good manufacturers even screen these units to 3 fA

anyhow, the only thing wrong is that in MOS (and JFET) technology bias 

increases <dramatically> with temperature, if I remember right one factor 

of ten every three degrees Celsius - really - three log units per 10 

degrees

some people cool their chips with Peltiers, but I fear not in this case

so I would suspect the chip bias rather than tempco in the HV, which should 

be well and easily compensated in a good design

why does the current stay high on cooling? thermal inertia? self heating?

in aging machines stray currents due to condensation on the PCB (of water 

and alcohols) aided by poor layout have been known to knock out the specs  

 upon cooling:

...sticking a silica gel pouch in the case probably would not hurt...

(pH meters I was making in Spore and Mauritius would not work at all in 

Rhode Island until we let them dry a couple days, they arrived all wet 

inside)



simple morale: happens all the time: people trust machines too much

we build physical theories to explain strange observations when often these 

are just instrumental artifacts

guys get fried because their GMs all saturate and read zero

etc.



Marco Caceci

http://radal.com





> -----Mensaje original-----

> De:	Cehn@AOL.COM [SMTP:Cehn@AOL.COM]

> Enviado el:	viernes, 24 de agosto de 2001 20:26

> Para:	radsafe@list.vanderbilt.edu

> Asunto:	Floor monitor    - background increasing

>

> Thanks for your reponses to my post (see below).  The cause of the 

increasing

> background appears to be a temperature effect on the detector.  Ludlum 

admits

> that for these gas filled proportionals, efficiency goes up with 

temperature.

>

>

> I would have guessed it would go down, based on the decreasing density of 

the

> counting gas with increasing temperature.  But other factors (not yet

> revealed) are at work.

>

> The lesson is choose your high voltage carefully.  If you're on a flat

> plateau, temperature effects should be minimal.

>

> Oringinal post:

> "I have a Ludlum 43-37 floor monitor surveying a large concrete pad -

> outdoors.  On very hot days (>105), the instrument background goes up. 

 We

> take it into a cool trailer, cool it down, and background stays up.  Any

> ideas?

>

> My favorite theory so far is radon coming off the concrete in the heat, 

and

> the daughters sticking to the detector.  That would also account for the

> backgrounds returning to normal the next morning."

>

> Joel I. Cehn, CHP

> cehn@aol.com

> Oakland, CA

>  <<Archivo: ATT00009.htm>> 

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