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[Phys-L] Re: Survey Meters

Brian,
If your goal is nothing else but to detect alpha particles then the
approach you described should work. Depending on the PMT (photo
multiplier tube) and a chain of resistors that often comes with it (to
evenly distribute the applied HV among dynodes) you will need a power
supply delivering several mA at about 1000 V (preferably adjustable
between 800 and 1200 V). To do spectroscopy you would need a very
stable voltage. But for your purpose a simple power supply would be
sufficient. Expect each alpha particle to produce a pulse of about one
volt or so. Unless you are satisfied with observing pulses with an
oscilloscope (via a simple CR coupling) some way of counting pulses
will be needed. In the old days it would a mechanical counter with a
power amplifier based on one tube. A more modern solution would be to
use, for example, Vernier interface box and software. It is a good
project if one is willing to spend time and money. And a good student
project, I think. Showing that radioactive dust concentrates on a
screen of any monitor or TV can be very exciting. Also "discovering"
how radioactivity comes down in several hours.

P.S.
As you certainly know, the PMT, the scintillator and the source should
be in total darkness when the HV is applied.

Ludwik Kowalski
Let the perfect not be the enemy of the good.

On Oct 18, 2005, at 7:18 PM, Brian Whatcott wrote:

I see that this URL offers raw slabs of plastic scintillator,
apparently 2 inch thick, at $2 per sq inch.

<http://www.oetech.com/neutron14.html>

Moreover, I read that one has seen on offer at eBay,
PMTs suited to the purpose at $10 on occasion.

But I have one or two on the shelf, come to think of it.....

Then, the HT supply and processor still need to be found.
Now if a prior generation of Amateur Scientist author were still
alive and kicking, I would be reading "how to" details, right now....

Brian Whatcott

At 09:28 PM 10/17/2005, you wrote:
Old GM counters, even those designed for beta rays, often had thin
enough windows to detect alpha particles. The NaI mounted on the PMT
is
for gamma rays. The NaI crystal, the PMT and its power supply
(usually
not portable) are likely to be much more expensive than a GM tube and
its simple battery-operated HV unit (as in yellow civil defense
dosimeters).
Ludwik Kowalski
Let the perfect not be the enemy of the good.

On Oct 17, 2005, at 9:52 PM, Brian Whatcott wrote:

Thanks for the check on unit comparisons.
In browsing for meter schematics, I noticed that an iodide
scintillator
into a PMT seems to be the preferred way of achieving a sensitive
detector.
And I was piqued to notice a nuclear spectrum analyzer. Not quite
sure
of its
operational modus: pulse height binning?

Brian W

At 12:30 PM 10/17/2005, you wrote:
If FS (Plessey) most sensitive scale is one cGy/hr then my calc.
result
is the old analog would be 500 times more sensitive. Like
photometry
a
proliferation of unites makes conversion difficult.

cGy ~= cSv one Sv = 100 rem [is r on meter rem or rads?]


note air dose ~= 0.7 tissue.

so (if no error) no surprise.

bc


Brian Whatcott wrote:
Remembering that several years ago, I sampled the decay of
radio active dist wiped from the terminal screen
(a thread of Luwik's as I recall) I bought a Plessey PDRM 82
survey meter as a replacement for the Victoreen OCD: CDV-700

The Victoreen was a yellow tin box with an analog meter,
an earphone and a tube on a lead. It had a check source stuck
on the side of the can.
The Plessey is an orange high impact plastic case with a
digital display.
The Victoreen was scaled 0.5mr/hr and 300 counts/min
with a X100 X10 X1 sensitivity switch.

The Plessey scale is marked cGy/h (IN AIR) - and it turns out,
the sensitivity is way lower than the Victoreen, so I don't get
a reading from screen dust. Pity - it looks SO cool!



Brian Whatcott Altus OK Eureka!