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372) Useful references for users of CR-39 chips

Ludwik Kowalski
Montclair State University, Montclair, NJ, 07055
April 13, 2009

1) g) Energy distributions of alpha particles from Am-241, after they  traverse mylar filters of several thicknesses, are reported (by Jan Jakubek) in

Figure 1, extracted from this reference, shows, for example, that after traversing 24 microns of mylar the mean energy is still 2.2 MeV. The distribution of energy is nearly gaussian. Jakubek’s Figure 2 should also be very useful.

= = = = = = = = = = = = = =
. .
 Figure 1                                          Figure  2 

******* Mylar films can be purchased at a company called "spi supplies" *******

P.O. Box 656
West Chester, PA 19381-0656

tel: (800)-2424-SPI or (610)-436-5400

It is called XFR Thin Mylar Support Films (for X-ray spectroscopy)

A spool of 6 microns (3 “ wide and 92 meters long) costs $73

************** They also have 3.6 and 2.5 micron mylar ***************

2) Those of us who work with CR-39 detectors should not forget about a powerful particle identification toll developed by Russian researchers. SPAWAR team is using this tool by observing how diameters of pits grow when the etching time is increased. A detail description is in two papers that A. Lipson et al. published, in the proceedings of our Catania conference (pages 163 and 182). I used their data to construct turves in Figure 3. Hopefully, this will be useful to some researchers.

= = = = = = = = = = = = = =

Figure 3

3) Someone wrote that alpha particle pits might have different sizes on CR-39 chips from different manufactirers. I have no experience with chips from manufactures other than Fukuvi. A little corner of each of my chip is always irradiated with alpha particles. And I always look at this corner, to make sure etching was succesful. A big change in size, for example by a factor of 1.5 would probably be noticed. I do not recall any big changes in sizes. Replying to the posted message I wrote:

“One thing that might produce Am-241 pits of different sizes on CR-39 from the same manufacturer (and from the same batch) are different average etching temperatures. I also suspect that an etching solution looses "potency" in time. My etching solutions are always stirred, to make sure the temperature is uniformly distributed within the beaker on the hot plate. I am sure you do the same , or something equivalent. . . . By the way, I suspect that an etching solution looses "potency" in time. My etching solutions are always stirred, to make sure the temperature is uniformly distributed within the beaker on the hot plate.”

4) Another CMNS researcher responded: “  It is correct that an etching solution can lose potency over time [especially if kept in an open beaker] many days. When NaOH etchant is exposed to air, carbon dioxide from the air reacts with NaOH to make sodium bicarbonate NaHCO3. As this reaction proceeds the concentration of NaOH falls and the etchant becomes weaker.  Holding other parameters such as time, temperature, and stirring constant the pit size that is reached in an experimental measurement becomes smaller. If one wishes to establish a reproducible correlation between particle energy and pit size, full strength etchant must be used for each measurement.”

5) Range energy curves,for example for alpha particles, seem to be very different for different materials, such as air, polyethylene, aluminum, or iron. But this is true only when ranges are expressed in units of distance. Express ranges in terms of mg/cm^2 and curves become very similar, especially when atomic numbers (Z) are not very different. This is illustrated below.
10 MeV alphas in gold: R=45.5 mg/cm^2
6 MeV alphas in gold: R=22.8
5 MeV alphas in gold: R=18.0
4 MeV alphas in gold: R=13.6
3 MeV alphas in gold: R=9.6

10 MeV alpha in nickel: R=22.6 mg/cm^2
6 MeV alphas in nickel: R=10.9
5 MeV alphas in nickel: R=8.5
4 MeV alphas in nickel: R=6.4
3 MeV alphas in nickel: R=4.5

10 MeV alphas in aluminum: R=16.5 mg/cm^2
6 MeV alphas in aluminum: R=7.5
5 MeV alphas in aluminum: R=5.8
4 MeV alphas in aluminum: R=4.2
3 MeV alphas in aluminum: R=2.9

10 MeV alphas in carbon: R=12.4 mg/cm^2
6 MeV alphas in carbon: R=5.4
5 MeV alphas in carbon: R=4.1
4 MeV alphas in carbon: R=2.9
3 MeV alphas in carbon: R=1.9

10 MeV alphas in hydrogen: R=4.19 mg/cm^2
6 MeV alphas in hydrogen: R=1.70
5 MeV alphas in hydrogen: R=1.24
4 MeV alphas in hydrogen: R=0.85
3 MeV alphas in hydrogen: R=0.53
A range-energy distance for aluminum, for example, can be used to predict how the energy changes as a function distance in mylar or iron, provided high accuracy is not needed.

To covert cm into mg/cm^2 one must know the density of the material. The density of aluminum is 2.7 g/cm^3. That means that 1 cm of aluminum translates into one gram per square centimeter, or 1000 mg/cm^2. The density of mylar is 1.39 g/cm^3. That means that 6 microns of mylar translates into 0.83 mg/cm^2. A thickness of a thin foil (actually it is its surface density) can be calculated my dividing its measured mass by its measured area. This is often more accurate than using a mechanical micrometer.

6) How does the size of a pit, due to an alpha particle, depends on the etching temperature of the 6.5 N NaOH electrolyte? My alpha particles were from an Am-241 source removed from a smoke detector. I do not know what the energy was, after particles traversed a layer of something that prevents contamination. It is probably 3 MeV or 4 MeV. To make my data useful at any energy (between ~3 MeV and ~ 8 MeV), diameters of pits are expressed in arbitrary units. The diameter at 60 C is 1 unit, be definition, after 12 hours of etching.

temp=60 C ---> diameter=1.0 units after 12 hours
temp=65 C ---> diameter=1.8 units after 12 hours
temp=70 C ---> diameter=3.1 units after 12 hours
temp=75 C ---> diameter=4.3 units after 12 hours

7) It would be interesting to know (in light of what was stated in point 4 above) what the effect of potency (concentration) of the etching solution is. Perhaps I will investigate this, if necessary. But here is a bit of useful information. One of the researchers who replicated a SPAWAR-type co-deposition experiment, under The Galileo Project, was Winthrop Williams. His CR-39 calibration chip, exposed to alpha particles of 5.45 MeV, was examined by Russian Scientists: A. Lipson, A. Roussetski and E. Saunin. I am reading their 2007 report, in Proceedings of our Catania workshop (page 163).

Williams’ chip was etched in NaOH solution of 6.5 N (for 3 hours at 68 C). Russian scientists also irradiated their CR-39 chip with alpha particles of 5.45 MeV. But their etching conditions were different (6 N NaOH, for 7 hours, at 70 C). In both cases CR-39 was from Fukuvi. The pits on the Russian chip were found to be larger than pits on the American chip. That is not surprizing, considering a large etching times difference. What is surprizing is that less than 2 hours was needed to compensate the etching times difference of 4 hours. By the way, Russian scientists did not study the effect of concentration of the NaOH sulution; additional etching for 1.75 hours was necessary to start sequential etching when mean sizes of pits, on chips to be compared, were identical.

To study the effect of the NaOH concentration, one can perform the following experiment. Two Fukuvi chips are exposed to alpha particles of the same energy. Then they are etched in the same oven, for example at 70C, for exactly the same time, in two freshly-made etching solutions, 6.0 N and 6.5 N. I have no doubt that pits etched in the more concentrated solution will be larger. But I have no idea by how much.

8) Referring to 4, one CMNS researcher wrote: “You should be using fresh NaOH solution for each etch.  We found close-fitting test tubes that enabled us to use only a few mL of fresh etchant for each CR-39 chip.” They probably have an oven with a thermostat, or perhaps tubes containing the etching solution are immersed into a beaker with water, which is standing on a hot plate.

9) I wrote: “Another possibility is to accumulate several CR-39 chips (keeping them in salty water, rather than in air) between the end of electrolysis and etching. That what I am going to do this summer. Following Oriani's protocol, I always drill a little hole in each chip and suspend chips in the stirred etching solution. That would really be essential if the goal were to compare sizes of pits on different chips.

In order to compare experimental results from different labs (I am still hoping for phase 2 of The Galileo Project), the etching temperatures must be specified to within 1 degree or so. The hot plate I am using does not allow me to do  this. How precise are typical ovens in chemistry labs? Etching for 12 hours at 70 C would produce much larger alpha pits than etching at 65 C (see unit 6 above).

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