At 04:50 PM 10/6/2012, Jed Rothwell wrote:
CR39 is very hard to use. It is not for dummies or beginners. That's the take home lesson I learned after listening to 2 days of discussion on CR32 by experts.
CR39 is difficult to *interpret*. It's also a pain to etch. As normally used, CR39 is thick. As it is etched, deeper and deeper layers are shown. I think that a phase-contrast microscope is used, because CR39 is a clear plastic, one is essentially viewing pits on the surface. Focus is critical.
There is a reason people invented electronic particle detectors and stopped using the analog ones such as CR39. A lot of reasons, actually.
Actually, I think the solid state nuclear track materials like CR39 are more recent than electronic detectors like Geiger counters. But I'm not sure.
I am not saying the old techniques are inferior, but they are harder. To say they are inferior would be like saying that RTDs are better than mercury thermometers. That is true in some ways but not so true in other ways. It is complicated.
SSNTDs are accumulating detectors, so they can integrate radiation over a long period of time. You can detect, with CR-39, radiation that would be indistiguishable from noise with electronic detectors.
There are instructions for using CR-39 in the Galileo project protocol documents. http://newenergytimes.com/v2/projects/tgp/TGP0-V5.1b%20Package.zip
The Galileo Project report has some images and comments about interpreting CR-39. In addition to the replication attempts reported by Krivit, there was an Earthtech replication.
Some work with CR-39 that I've seen used thin layers. I looked to see if I could find such material, but then I found LR-115.
LR-115 is a more recently invented nuclear track detector material. It is cellulose nitrate, and suffers damage from ionizing radiation like CR-39, and is etched with sodium hydroxide like CR-39, except that a lower concentration, lower temperature, and less time are required. The material contains a red colorant. It comes coated on a 100 micron clear polyester base, and in 6 micron or 12 microns of detector thickness.
A track that penetrates the whole surface layer will then show up as clear against the red material.
I found it difficult to buy CR-39. I was able to buy LR-115 from Dosirad in France; I'm reselling the 6 micron material. For the kits, I cut it down to 1 x 1.5 cm pieces. I sell a 9 x 12 cm sheet for $30.
LR-115 has a different response to radiation than CR-39. Tracks at energies above about 4 MeV may not sufficiently ionize the material with LR-115, at least Pam Mosier-Boss indicated this in an email.
However, Am-241 sources from ionizing smoke detectors produce beautiful, clear tracks in CR-39.
Now, Axil asked about the detection of neutrons. Neutrons, of course, don't leave tracks in SSNTDs. However, passing through a material containing hydrogen (like the detector materials), neutrons will collide with and eject protons from their positions in the plastics. These protons leave tracks, and are used, then, to infer the presence of neutrons. Most of the tracks shown in the CR-39 images are rather obviously proton knock-on tracks.
SPAWAR, however, focused largely on rare triple-tracks, produced when a neutron impacts a carbon nucleus and breaks it into three alpha particles.
In the first experimental run with one of my kits, done by a high school student, there are no clear tracks that were produced. The material was placed such that *only* neutron-caused tracks would be likely. It is possible that proton knock-on tracks were not energetic enough to produce tracks all the way through the detector layer, and more work is needed to characterize and determine the best etching protocol. However, LR-115 is sold for neutron detection, and we did follow the standard etching procedure, so the most likely explanation of the results is that neutrons were not generated.
Nevertheless, it will be important to see some comparison results with LR-115 exposed to a known and calibrated neutron source.
(In this experiment, the detectors were a bit further away from the cathode wire than the back sides of the SPAWAR CR-39 chips that showed so many back-side tracks. It's possible that for some unknown reason we did not set up a nuclear reaction. As far as I know, this was the first attempt to replicate the SPAWAR neutron findings. There will be others. Deeper analysis of the detector chips, which were partly damaged from unknown causes, is proceeding.)
Back to CR-39, there has been work with thin layers of the plastic, created by evaporating solvent with the plastic dissolved in it. I think it's worth investigating that. It's been used to image radiation from biological cells that have been tagged with a radioistope, a radioautograph on a microscopic scale.
An advantage of creating detector material like this would be that it could be completely fresh, no problem with background radiation from storage. And thin material then produces crisp radiation images.
