The CR-39 detectors of alpha particles are also
suitable for noncontroversial student projects, for
example, a study of distribution of radon; see my
appendix below. No darkness is required, as with
photographic emulsions. The unique advantage of
CR-39, as illustrated by R. Oriani and J. Fisher, is
that a chip can be “developed” twice.
Suppose the first development produced 37 tracks
near a recognizable scratch on the detector’s surface.
This is the background due to previous long time
exposure to radon, cosmic rays, etc. The chip is then
used in an experiment and developed again. The
previously seen tracks are still visible but an additional
set of new tracks appears, for example, 10. Then we
know that new tracks are real; they were created when
the experiment was performed. This approach is much
better that using two different chips, that is: getting 47
from one chip (background plus signal), getting 37 from
another (background only) and subtracting these two
numbers to get 10. The net result obtained in that way
can not be distinguished from statistical fluctuations.
Ludwik Kowalski
Appendix:
The detector, called CR-39; is a sheet of transparent
plastic. Alpha particles, for example, from radon, create
latent tracks which can be made visible (through a
microscope) by developing pieces of CR-39 in hot
KOH. These detectors are commercially available
from several suppliers. One place to obtain them is:
Track Analysis Systems Ltd
H H Wills Physics Lab
Tyndall Avenue
Bristol
BS8 1TL
U.K.
Tel (0)117 9260353
Fax (0)117 9251723
web www.tasl.co.uk
e-mail apf@tasl.co.uk
Here is what this supplier wrote to me in January
2003. “Thank you for your inquiry. We do not have
any agents in the US -- we sell directly to customers.
The prices depend entirely on what you require.
If you can say the sizes and quantities you are
interested in, I will be pleased to make a specific
quote. For example: pieces 25x25 mm, with an
engraved (incrementing) number, cost 1.75 US
dollars each. We can quote for sizes from
10x10 mm up to 275x280 mm. Our material is
carefully produced to high quality standards, and
each batch of production is tested for background
and track response. For the US, we have a
minimum order quantity of 100 US dollars.”
To gain experience I exposed a CR-39 chip to a
241Am source of alpha particle (removed from a
broken fire alarm detector). After an exposure of 30
seconds the chip was kept in hot KOH (70 degrees C)
for six hours. Then it was washed in water and placed
under an ordinary microscope (magnification 40).
Pits due to alpha particles were visible in the exposed
area while the surrounding areas had much lower
track densities.