[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

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 ? 　

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

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)

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

Beam exposure & Etching condition -- Exposed heavy ion @ 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

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

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

Response curve by PEW-50 etching

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

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

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

Drastic improvement of charge resolution PEW-50 70C + 7N NaOH addition sZ=0.30cu@Kr sZ=0.21cu@Kr Preliminary

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

Backup Chart

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