Re: Re[2]: Measurement of Neutron Exposure In NPP?

[Darryl Kaurin <dkaurin@eden.rutgers.edu>]

Dear RADSAFERS,

Regarding bubble dosimeters to detect neutrons,

I used some in 1990 as part of a masters degree project to compare the
direct component versus the scattered neutron component in a room
from an unshielded PuBe neutron source.  They were BD100Rs which were
supplied by Siemens Gamma Sonics.  They worked quite well, but as Ted
Bohn mentioned, counting bubbles became quite tedious.  Siemens Gamma
Sonics did make an apparatus to count the bubbles,  I remember seeing
it at the 1991 Health Physics annual meeting, and I believe the price
was $27,000 including the computer.  The price most likely has come
down in recent years.  They were easy to use, since you just count the
number of bubbles and multiply by the bubble per mrem value given by
the supplier to obtain dose.  The temperature in the room was
constant, so the temperature correction was a single value.

The dosimeters I used had several disadvantages.  The temperature
correction problem of the dosimeters was solved by adding a
temperature/pressure apparatus to the dosimeter, but I don't know how
well it works. The recompression device had a small barrel that gave
my fingers blisters after repeated use, but I think they made the
whole device bigger, so it works better.  I dropped one on the vertex
(they are shaped like a test tube) and it gave me a uniform
distribution of bubbles throughout the gel (a mechanical shock will
induce bubble formation).  The biggest disadvantage was that the
sensitivity changed with recompression number, which I showed with a
t-test of the sensitivity as a function of recompression number with
repeated measurements.  I did this with 6 bubble dosimeters, and it
occured with all dosimeters.  Several years later, I talked with Harry
Ing, from Chalk River Laboratories, who was involved with the
development of the bubble dosimeter we used.  He mentioned that the
decrease in sensitivity had since been corrected.
My work was not published, nor peer reviewed, so it should be treated
as anecdotal.

Apfel Enterprises also makes several types of bubble dosimeters, one
type has a gel that when a bubble is formed, a popping noise can be
heard, and he uses an acoustical sensor integrated with a
microprocessor to count the number of "pops".  He also had a syringe
type dosimeter, in which the expanding bubbles would move the syringe
piston, and you determined the dose from the calibration marks on the
transparent syringe barrel.

Darryl Kaurin
dkaurin@eden.rutgers.edu