The following are some notes I made regarding use of CR-39, and other
SPAWAR related things.
It may be possible to gain some discriminatory information of
particle type by depositing very thin layers of materials on a CR-39
detector, and then removing them prior to NaOH etching. This would
tell something about the ballistic collision cross section of the
particles with the thin layer chosen, and, if recoil interaction with
the layer is occurring, should provide many more elliptical tracks
than without the thin layer.
Thin but covering layers of vacuum deposited metals might work, as
they can readily be removed by acid bath. To protect from an
electrolyte, it might be necessary to coat some surfaces with thin
protective coatings which can be removed by appropriate choice of
solvent that does not also affect CR-39. It may be possible to use
such a protective coating material, even in lieu of a vacuum
deposited metal, as a supporting matrix for some kinds of materials,
like barium, boron, or possibly materials that can be neutron
activated and then removed for separate long term counting by other
means.
It may be feasible to make an integrating neutron counter by adhering
a boron containing layer covered piece of CR-39 to the back of the
CR-39 detector. This provides a layer of boron sandwiched between
two pieces of CR-39. This then uses the main CR-39 detector as a
moderator, the boron layer as an alpha generator, and the two
surfaces adjacent to the boron layer for etching to look for alphas
from the boron layer.
It may be of use to use BC-720 plastic from Bicron Inc. as a
scintillation detector. It could be appended to the CR-39, or used
instead of the CR-39, as a base for the cathode layer, especially
using an initial silver layer, and simultaneously as part of the
electrolysis cell wall, in order to readily position the
photomultiplier tube near it.
There are of course endless combinations of means for discriminating
particle types using these kinds of surface treatments and
sandwiching techniques. Some of these approaches can be used with
film detectors as well.
Something that may also be of utility is re-curing CR-39 prior to
use, especially if it is old, and especially if tracks are uncovered
upon successive etchings and there is concern they may be due to
neutrons. Cosmic ray secondaries cause tracks originating within the
CR-39. In any case it is desirable to have as clean as possible an
etching, and to be able to make use of old CR-39. The following
manufacturer's bulletin contains a cure cycle for CR-39.
http://tinyurl.com/yp6ld5
http://corporateportal.ppg.com/NR/rdonlyres/
3161A365-5C86-484F-97B6-74059920D2B6/0/CR39.pdf
It is also noteworthy that the electrolytic cell operating
temperature, thermal gradients in the CR-39 during cell operation,
and the duration of the run, can have an effect on the nature of the
CR-39 tracks detected by etching. If the cell operates at
temperatures sufficient to fully or partially cure tracks, then the
size and length of tracks which are detected will be affected by
highly localized temperature differences and duration of track
existence. Locally hot zones, say near hot cathode wire surfaces,
may actually reduce track diameters detected for particles in close
vicinity, by curing them to some degree before etching. In a hot
electrolyte, pe-existing tracks, say from cosmic rays, may be cured
out of existence at the surface, but still exist at depth.
These CR-39 curing issues may or may not affect a given experiment,
but certainly may be worthy of consideration when designing or
evaluating experimental controls or models of track etching.
It is notable that CR-39 curing, which depends on temperature and
duration, not only can affect track geometry according to the CR-39
history and experimental conditions, but also during the etching
process itself. The curing process should be capable of annealing
tracks altogether, or reducing their diameters or even etching
rates. One manufacturer's recommended curing process ranges in
temperature between 44 Deg. C to 90 Deg. C, and for a total duration
of 19 hours (see http://tinyurl.com/yp6ld5). These process numbers
are all right in the range of the approximately 70 deg. C etching
temperature process used in the SPAWAR experiments. Precision
temperature control and etching time control are thus critical to the
etching process, and the effects of the CR-39 curing rate likely
should be investigated and may be a parameter to be considered in any
comprehensive model of track etching.
A sample problem, due to a short etching once well above 70 deg. C
and then later less than 70 deg. C is that cosmic ray caused tracks
might appear in the second etch that don't in the first etch. That
is because the surface layer tracks can anneal out due to the high
temperature.
The price of CR-39 is quite bothersome for amateurs like me though.
It would be good to find a cheaper alternative! It could be that
curing your own CR-39 would be of use, and that would provide some
possibly very handy control of layer thickness. It would be pretty
neat to be able to use ultra thin layers, possibly separated by
discriminating materials, whether the layers are created by build-up
and curing, or by stacking and pressing. It is also notable that
thin metal coated plastics can be and are produced commercially in
roll form in large quantities cheaply. The problem then is to find
and prove out an alternative to CR-39. This kind of effort is
unfortunately probably not economically justified, unless maybe it
could result in improved dosimeters.
CR-39 is made by curing a monomer, i.e. forming cross-linked
polymerization. Track formation is due to breaking of the polymer
bonds by ionization. It is to me self evident the same thermal
process of polymerization should restore those broken bonds, or at
least restore some bonds to the point the etching process differs
significantly. The degree of annealing effectiveness of the original
curing protocol, or other protocols created specifically for "re-
curing", require verification, if nothing else as part of
experimental control.
Despite the somewhat speculative nature on the positive side, i.e.
the notion of use of re-curing is effective as a means of annealing
out tracks from older material, the strong potential that such
annealing effects of re-curing exist seems to me sufficient to
justify controlling and evaluating for such effects when making
deductions regarding observed tracks.
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
