1. [24th ICNTS @ Bologna (Italy) 04/09/2008]
Control of the detection threshold of CR-39 track detector for the measurement of ultra heavy nuclei in galactic cosmic rays
S. Kodaira1, N. Yasuda1, N. Hasebe2, T. Doke2, S. Ota2, and K. Ogura5
National Institute of Radiological Sciences
Waseda University
Nihon University

2. Goal of Cosmic Ray Research
☆ Origin: Where and How are galactic cosmic rays (GCRs) accelerated ?
Are the Young or Old?
☆ Acceleration: Is the temperature at the acceleration site Hot or Cold ?
☆ Propagation: How do GCRs propagate in the Galaxy ? 　

3. Scientific Objectives
Precise observation of ultra-heavy nuclei (Z>30) in galactic cosmic rays (GCRs)
Origin: Primary compositions of elements and isotopes in GCRs. The freshly-synthesized material of R-process is enhanced in GCRs ?　　　　　　　　　　　　　　　　　　
Injection mechanism: FIP or Volatility ?
Life-time of GCRs: Mean-life time, Delay time between nucleosynthesis and acceleration
Search for the nearby source
History of Galactic materials
Study of nucleosynthesis process and its site
Search for trans-Uranium nuclei predicted theoretically

4. Need SSTD with high threshold
Flux of Z<30 nuclei
>> Flux of trans-iron nuclei
Using high sensitive SSTD such CR-39 Z<30 nuclei are background tracks 　　 for the observation of ultra heavy nuclei Fe
Ultra heavy nuclei
HEAO-3-C3
Binns et al., (1989)

5. Objectives
PEW-x solution (15wt%KOH+xwt%C2H5OH+ (85-x)wt%H2O) has a possibility to:
- Make to desensitive the track registration sensitivity
of CR-39 track detector
- Improve charge resolution because of shrinking tracks
of rather lighter nuclei
- Improve surface condition after the etching
Somogyi & Hunyadi, (1979)
Ogura et al., (2001)
Optimization of PEW-X etching condition using high purity CR-39 (BARYOTRAK)
Verification of the performance of CR-39 (BARYOTRAK) by PEW-x solution

6. Beam exposure & Etching condition
-- Exposed heavy HIMAC---
Ion
Fe, Kr, Xe
Energy
< 500 MeV/n
(36 ≤ Z*/b ≤115)
--- Etching condition ---
Solution
1) PEW-x; x=35, 50, 65wt% for ethanol
2) 7N NaOH
Temp.
1) 50C, 2) 70C
Time
1~240 min

7. Variation of response curve for PEW-x concentration
70C Temp.
PEW-50 (50wt% for ethanol) is adequate to detect heavy ion

8. Characteristics of bulk etch rate for PEW-50
Our normal etching condition:
 7N NaOH 70C: Vb=2.1mm/hr
Vb is ~22 PEW-50 70C
~7 PEW-50 50C
min!

9. Response curve by PEW-50 etching

10. Surface condition after the PEW etching
PEW-50 70C
PEW-50 50C
B=95.3mm
B=33.2mm
B=22.0mm
B=84.0mm
B=39.0mm
B=33.2mm
Etching longer than roughly B=30mm makes extremely unclear edge of etch-pit

11. Charge resolution for Kr ion
PEW-50 70C
PEW-50 50C
Poor charge resolution is caused from “unclear edge“ of etch pit ?

12. Additional etching by 7N NaOH solution
7N NaOH 70C (8hr) etching after the post-etching of PEW-50
Only PEW-50 70C
Recover very clear edge of etch pit by additional 7N NaOH etching !
PEW-50 70C+7N NaOH 70C

13. Drastic improvement of charge resolution
PEW-50 70C
+ 7N NaOH addition
Preliminary

14. PEW+NaOH hybrid etching is very useful technique for:
Summary
Verified the performance in CR-39 (BARYOTRAK) detector etched by PEW-x solution:
① Response curve
- Suppress effectively light ion track and therefore
inclination of response curve becomes steep
- Variable response curve by changing ethanol
concentration (xwt%)
- Optimum ethanol concentration should be x=50wt%
- Optimum temperature should be 70C
- Threshold of Z/b~40 in case of PEW-50 70C
② Charge resolution
- Drastic improvement of shape of etch pit and
charge resolution by additional 7N NaOH
- Charge resolution for Kr: 0.2cu (only single layer)
PEW+NaOH hybrid etching is very useful technique for:
- Observation of ultra heavy nuclei in
galactic osmic rays
- Measurement of nuclear interaction
(charge changing cross section) of
ultra heavy ion

15. Backup Chart

16. PEW-50 50C
+ 7N NaOH
PEW-50 70C
+ 7N NaOH