1. 1Research Institute for Science and Engineering, Waseda Univ. Japan
Improvement of charge resolution for CR-39 plastic nuclear track detector by the optimization for various conditions
S. Ota1,2, N. Yasuda2, S. Kodaira2, M. Kurano2, M. Sato1 and N. Hasebe1
1Research Institute for Science and Engineering, Waseda Univ. Japan
2National Institute of Radiological Sciences, , Japan 1st Sep. 2008, 24th ICNTS. Bologna.

2. Introduction Fragmentation reaction of Heavy Ion at 0.1–several GeV/n
The study has various scientific fields for applications
Our interest : application to galactic cosmic ray (GCR) propagation
C, N, O
Collision of heavy ions in GCR with interstellar medium during propagation  various secondary particles (fragment)
GCR composition changes from the source one (e.g. Fe Sub-Fe, C-O  Li-B) Fe -
The difference of composition in GCR & our galaxy  depends on path length & fragmentation cross section
Precise fragmentation cross section for various nuclei
 path length, source composition and the origin of GCR

3. Fragmentation cross section
So far, some investigators measured fragmentation cross sections for nucleus + nucleus reactions, but most of them…
Inclusive: A + B  C + anything? by e.g. Webber (1990),..
With the advance of analytical technique for understanding of heavy ion reaction (such as QMD, IC, VUU theory and so on),
More exclusive: A + B  C + D + E.. needed for verification
CR-39 is very feasible detector
Excellent position resolution multi-fragmentation.
Wide dynamic range (Z>~3)
Possibility of hybrid passive detector with nuclear emulsion C He B Be H
Li-C
Our plan: More exclusive measurement of the cross section for various GCR nuclei (from C to ultra-heavy ) using CR-39

4. Charge Resolution σ in CR-39
In experiment that projectile A + target B  fragments C, D, E., Fragmentation cross sections: determined by the number of ions A, C, D, E.. before & after the target.
The number of ions : counted from etch pit area distributions
It is the most critical in the measurement how clear electric charge (Z) can be identified!
AreaZ
Identification  represented by charge resolution (σ)
AreaZ-1
enlarged Mg Al Si
δAreaZ
δAreaZ-1
AreaZ, Z-1 : Means of etch pit area distributions with given Z and Z-1
δArea : larger standard deviation of AreaZ and Z-1 Mg Al Si
Full identification  σ < 0.1 – 0.15 c.u.
Si & fragments (Z ≥ 4)
It is found empirically σ is largely improved with etching time

5. So, we studied optimization of Etching time for the best σ
Experiment
So, we studied optimization of Etching time for the best σ
2 kinds of CR-39, BARYOTRAK (FUKUVI) & TD-1 (Harzlas)
C, O, Si, Ar, Fe, Kr and Xe: 0.2 – 0.5 GeV/n at HIMAC (NIRS).
Stacks were set up and bombarded with these ions (Z).
Charge resolution for produced fragment (Z-1) are studied
Etching Time (E. T.): 5 – 60 h (etched a CR-39, step by step)
Etch pits analysis: using High speed imaging microscope & track extraction software (Pit Fit)
Note: we focus on σ by single sheet of CR-39 in this work
CR-39s in front of the target: analyzed to calibrate REL & Etching time v.s. Etch pit area
CR-39s behind the target: analyzed for charge resolution (σ) for fragmentZ-1
Etchant: 7 N NaOH 70˚ C
CR-39: 5×1 cm2
(N. Yasuda et al., 2005)

6. Etching Time versus resolution (σ)
First, the result of relationship between E.T. & σ is shown
As E. T. increases, charge resolution (σ) gets better.
But as E. T. exceeds 20–30 h, σ becomes const..
Saturated σsatu up to Fe, σsatu  ~ 0.21±0.03 c.u.
σsatu is better, but not enough for full identification (i.e., 0.1 – 0.15 c.u.)
Threshold E. T. for saturation exists!
So we tried to study σsatu in more detail

7. REL versus Etch pit Area
Restricted Energy Loss (REL) v.s. Etch pit Area was calibrated for E. T. 5 – 30 h . Then we see how AreaZ-AreaZ-1 changes with E. T.
AreaZ
AreaZ
AreaZ-1
AreaZ-1
AreaZ-AreaZ-1 gets larger with E. T.!
TD-1
BARYO
AreaZ – AreaZ-1 becomes larger with E. T

8. Etching time versus δArea
Next, we studied on E. T. v.s. δArea
It was confirmed that δArea gets larger as E. T. increases
Whereas AreaZ–AreaZ-1 larger with E. T., δArea  larger wth E. T.. Then ratio keeps const.
Now we understand why σ becomes const. with E. T.
To improve σ, it is essential to suppress the increment of δArea
 So, we tried to identify the causes of δArea

9. Experiment: σSi-Al ~0.24 c.u.
Analysis of δArea (1)
We assumed following fluctuations as the causes of δArea
Fluctuation of ion energy  REL fluctuation: δAreaREL
Fluctuation of etch pit area in measurement  δAreaMeas
Fluctuation by inhomogeneity of CR-39 δAreainhomo
Registration sensitivity, amount of bulk etch and so on differ locally in a sheet of CR-39 (CR-39 used in this work: 5×1cm2)
Totally, δArea should be estimated by following expression
As an example, let’s try to estimate resolution
σSi-Al of 324 MeV/n Si and Al (40 h etching)
<- Si
Al ->
Frequency
Experiment: σSi-Al ~0.24 c.u.
Si δArea: ±69.3 um2
Area (um2)

10. Analysis of δArea (2) 1. Fluctuation of REL
(by JAEA et al. )
Energy loss through the stack was simulated using PHITS code
δREL/REL: ±0.1 % δAreaREL estimated from REL v.s. Area
2. Fluctuation of etch pit area in measurement
A sampled Si etch pit area was measured iteratively to estimate
3. Fluctuation by inhomogeneity in a sheet of CR-39
Standard deviation of means for Si etch pit area distributions at 4 divisions in a sheet of CR-39 (5×1cm2  into 4 parts)
4. Total of δArea  agreed with experiments in 5 %!
δAreaREL
δAreameas
δAreainhomo
δAreacalc
δAreaexp
δArea (um2)
4.4
64.3
13.4
65.8 (-5 %)
69.3
δAreameas from measurement accuracy, not from CR-39 itself.
Possibly, σ: estimated to reach c.u. from 0.24 c.u. if δAreameas reduces by improvement of meas. system.

11. Conclusion & future prospect
When we define charge resolution (σ) using Etch pit area distri. as following,
σ gets rapidly better as Etching time (E. T.) increases, but becomes constant after h E. T.
The reason for σ to be const.:Whereas AreaZ–AreaZ-1larger, δArea  larger wth E. T.. Ratio of them keeps const. with E.T..
Saturated σ: 0.21±0.03 c. u. for up to Fe-Mnnot enough
We studied δArea to obtain better σ and identified the causes of δArea as δAreaREL, meas and inhomoδAreacalc agreed with experi.
Possibility of full identification was shown.
As future work, improvement of δAreaREL, meas and inhomo and E. T. v.s. δAreaREL, meas and inhomo should be studied. Then optimum E. T. and the best σ for BARYOTRAK & TD-1 under present condition (7 N NaOH 70˚C) can be determined.