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Effect of pressure on track registration in CR-39 plastic
Nuclear Instruments and Methods North-Holland, Amsterdam
in Physics
Research
B23 (1987) 367-368
367
EFFECT OF PRESSURE ON TRACK REGISTRATION IN CR-39 PLASTIC J. DRACH, Department Received
M.H. of Physics,
23 October
SALAMON,
M. SOLARZ
University of California, Berkeley,
and P.B. PRICE CA 94720, USA
1986 and in revised form 15 December
1986
CR-39 plastic track detectors were exposed to 1.3 GeV/u 56Fe at various pressures (0.0, 0.4, 1.0 atm of air) and temperatures (- 100 to + 20 o C). The response of CR-39 was found to depend upon pressure as well as temperature.
1. Introduction One might naively expect that the response of nuclear track-detecting plastics to particles of charge Z and velocity /3 should depend upon only those two parameters, since the prompt excitation and ionization energy distribution around the track of the slowing particle is solely a function of Z and p (for a given medium). The response is characterized by the reduced etch rate s = k’r/Vo (where Vr is the particle track etch rate and Vo is the general, bulk etch rate of the plastic) which determines the geometry of the etched track [l]; were s to be determined by the prompt dose distribution alone, then s would be a function solely of Z and @, for a given plastic and fixed chemical etching conditions. Several years ago, however, it was found that s has an additional, mild dependence on the temperature of the plastic at the time of exposure [2]; this “registration temperature effect” has been firmly established [3-81, although no convincing explanation for the effect has been advanced. We report here the response of CR-39 that was exposed to 1.3 GeV/u 56Fe ions while under a variety of pressures (0.0, 0.4, 1.0 atm of air) and temperatures ( - 100, - 60, - 20, + 20 o C). In addition to observing a registration temperature effect, we also find that s has an equally strong dependence upon the (air) pressure. These two effects appear to be only weakly coupled, leading to tentative conclusions regarding the role of oxygen in the registration temperature effect. The observed pressure effect has an obvious impact on trackmeasurement experiments conducted in space.
2. Data and discussion Our intention was to measure the effect of temperature and pressure at the time of particle irradiation on 0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
the response of CR-39, and if possible to see if these effects varied with the magnitude of particle energy loss; this study was motivated by an upcoming Space Shuttle cosmic ray experiment [8], where substantial pressure and temperature variations might occur. For this study, stacks of 675 pm-thick sheets of CR-39 doped with dioctyl phthalate (DOP) and a antioxidant (Uniroyal Nauguard 445), interleaved with 125 pm-thick Cu foils, were installed in three hermetic, aluminum containers filled with dry air (water vapor content 11.4 ppm) at pressures of 0.0, 0.4, and 1.0 atm. The hermetic containers were stored for two days at room temperature after which they were exposed to 56Fe ions at LBL’s Bevalac. At the time of exposure, the containers were held at a variety of temperatures (nominally - 100, - 60, -20, and + 20° C) within an environmental chamber of wide operating temperature range. The interleaved Cu foils within the stacks ensured that temperature inhomogeneities were below lo C at the time of exposures; stack temperatures were monitored with thermisters embedded within the center of each stack. A polyethylene absorber placed in front of the stacks fragmented about 25% of the incident Fe nuclei, thereby providing data on 1.3 GeV/u heavy ions of charge up to and including Z = 26. The irradiated samples remained in their hermetic containers for 35 days after exposure before being removed and etched in 6.25N NaOH at 40 o C. The results are shown in fig. 1. The registration temperature effect is clearly present, and is qualitatively similar to that observed earlier, viz. the existence of a response peak at low temperature (here, between - 60 o and - 100 QC). Quantitative differences between our data and earlier data do exist, and are consistent with the observation [6,9] that the curves of etch rate versus registration temperature do vary with the ionization energy of the particles creating the tracks (higher ionization energy results in a steeper temperature dependence). Although
J. Druch et al. / Effect of pressure on truck regmtration
368
z”l---
III = 10
atm
a = 0.0
atm
. 0
16
14
=04atm
I
2~26
12 -60" 120" -100" -20"
Z=24
llllllllill -60" +20* -100" -20"
-60" +20 -100" -20
-60" +20" -100" -20
1"' -60" +20-100" -20"
T (“C)
Fig. 1. Reduced etch rates s for 1.3 GeV/u 56Fe and its fragments as a function of temperature and pressure at the time of exposure. Note that the magnitudes of the shifts in s can be related to apparent charge shifts by comparing the s curves of neighboring fragments. Measurement errors of s are typically +O.Ol. The inset on the upper right shows the ratios of the reduced etch rates (for the Z = 26 data) to their values at + 20 o C for each pressure; the asterisk denotes the common normalization point.
time of oxygen out of the CR-39 detectors in vacuum is 5 1 day [ll], at the time of exposure the diffused oxygen content of the CR-39 sheets was near its equilibrium value at each of the three gas pressures. Therefore, the observed pressure dependence may simply be due to long-term thermal fading hypothetically enhanced in the reduction or absence of “oxygen-setting” of the latent track. If this is the case, the observed pressure dependence will vary with the time the plastics are kept at pressure after exposure. With our limited range of ionization energy density ( < 30% for Z = 22 to 26) we are not able to discern any significant variation in the temperature or pressure effects with ionization energy density. We thank R. Claxton for her help in this study and Irving Fatt for a measurement of the diffusion coefficient of oxygen in CR-39. This work was supported by NASA Contract NAS l-17806.
References 111 R.L. Fleischer, P.B. Price and R.M. Walker, Nuclear Tracks in Solids (University of California Press, 1975).
121 D. O’Sullivan and A. Thompson, Nucl. Tracks 4 (1980)
there is already evidence in the literature for an oxygen pressure effect in polymers [lo], in these results we see for the first time the dramatic dependence of pressures on CR-39(DOP) response. We cannot label this a “registration pressure effect” because it is unknown whether the effect is due to pressure at the time of exposure, or the pressure history (over days) before and/or after formation of the latent track. It is intriguing that the temperature dependence or the reduced etch rate s roughly maintains its form, independent of pressure. (The points of maximum etch rate do shift with pressure, however.) This argues against the earlier hypothesis [3] that the decline in s with decreasing temperature at temperatures below - 70 o C is due to the removal of available oxygen within the stack; it is probable that ambient oxygen plays little role in the observed registration temperature effect. The true explanation for the temperature dependence probably lies in the complex chemistry of energy redistribution immediately after exposure, with a variety of activation energies giving rise to a temperature-dependent end-state for the latent track. Since the diffusion
271. 131 A. Thompson, D. O’Sullivan, J.H. Adams and L.P. Beahm, Proc. 11th Int. Conf. Solid State Nuclear Track Detectors, Bristol (1981) p. 171. [41 A. Thompson, D. O’Sullivan, J.H. Adams and L.P. Beahm, 18th Int. Cosmic Ray Conf. Bangalore, vol. 9 (1983) p. 407. 151 D. O’Sullivan, A. Thompson, J.H. Adams and L.P. Beahm, Nucl. Tracks 8 (1984) 143. WI A. Thompson and D. O’Sullivan, Nucl. Tracks 8 (1984) 567. 171 R. Hamasalci, T. Hayashi and T. Doke, Nucl. Tracks 9 (1984) 149. PI J. Drach et al., 19th Inter. Cosmic Ray Conf., La Jolla, vol. 2 (1985) p. 131. [91 J.H. Adams and L.P. Beahm, Nucl. Tracks 12 (1986) 387. DOI Indication of an oxygen pressure effect in Lexan polycarbonate is discussed in ref. [2], and table 2-5 of ref. [l] lists qualitative reports of various polymer etch rate sensitivites to oxygen pressure. Such an effect in Lexan was also seen by J.H. Chan and P.B. Price, Phys. Rev. Lett. 35 (1975) 539. [ll] I. Fatt, private communication. For other gas diffusion data, see I. Fatt in Soft Contact Lenses, ed., M. Ruben (Wiley, New York, 1978).
in Physics
Research
B23 (1987) 367-368
367
EFFECT OF PRESSURE ON TRACK REGISTRATION IN CR-39 PLASTIC J. DRACH, Department Received
M.H. of Physics,
23 October
SALAMON,
M. SOLARZ
University of California, Berkeley,
and P.B. PRICE CA 94720, USA
1986 and in revised form 15 December
1986
CR-39 plastic track detectors were exposed to 1.3 GeV/u 56Fe at various pressures (0.0, 0.4, 1.0 atm of air) and temperatures (- 100 to + 20 o C). The response of CR-39 was found to depend upon pressure as well as temperature.
1. Introduction One might naively expect that the response of nuclear track-detecting plastics to particles of charge Z and velocity /3 should depend upon only those two parameters, since the prompt excitation and ionization energy distribution around the track of the slowing particle is solely a function of Z and p (for a given medium). The response is characterized by the reduced etch rate s = k’r/Vo (where Vr is the particle track etch rate and Vo is the general, bulk etch rate of the plastic) which determines the geometry of the etched track [l]; were s to be determined by the prompt dose distribution alone, then s would be a function solely of Z and @, for a given plastic and fixed chemical etching conditions. Several years ago, however, it was found that s has an additional, mild dependence on the temperature of the plastic at the time of exposure [2]; this “registration temperature effect” has been firmly established [3-81, although no convincing explanation for the effect has been advanced. We report here the response of CR-39 that was exposed to 1.3 GeV/u 56Fe ions while under a variety of pressures (0.0, 0.4, 1.0 atm of air) and temperatures ( - 100, - 60, - 20, + 20 o C). In addition to observing a registration temperature effect, we also find that s has an equally strong dependence upon the (air) pressure. These two effects appear to be only weakly coupled, leading to tentative conclusions regarding the role of oxygen in the registration temperature effect. The observed pressure effect has an obvious impact on trackmeasurement experiments conducted in space.
2. Data and discussion Our intention was to measure the effect of temperature and pressure at the time of particle irradiation on 0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
the response of CR-39, and if possible to see if these effects varied with the magnitude of particle energy loss; this study was motivated by an upcoming Space Shuttle cosmic ray experiment [8], where substantial pressure and temperature variations might occur. For this study, stacks of 675 pm-thick sheets of CR-39 doped with dioctyl phthalate (DOP) and a antioxidant (Uniroyal Nauguard 445), interleaved with 125 pm-thick Cu foils, were installed in three hermetic, aluminum containers filled with dry air (water vapor content 11.4 ppm) at pressures of 0.0, 0.4, and 1.0 atm. The hermetic containers were stored for two days at room temperature after which they were exposed to 56Fe ions at LBL’s Bevalac. At the time of exposure, the containers were held at a variety of temperatures (nominally - 100, - 60, -20, and + 20° C) within an environmental chamber of wide operating temperature range. The interleaved Cu foils within the stacks ensured that temperature inhomogeneities were below lo C at the time of exposures; stack temperatures were monitored with thermisters embedded within the center of each stack. A polyethylene absorber placed in front of the stacks fragmented about 25% of the incident Fe nuclei, thereby providing data on 1.3 GeV/u heavy ions of charge up to and including Z = 26. The irradiated samples remained in their hermetic containers for 35 days after exposure before being removed and etched in 6.25N NaOH at 40 o C. The results are shown in fig. 1. The registration temperature effect is clearly present, and is qualitatively similar to that observed earlier, viz. the existence of a response peak at low temperature (here, between - 60 o and - 100 QC). Quantitative differences between our data and earlier data do exist, and are consistent with the observation [6,9] that the curves of etch rate versus registration temperature do vary with the ionization energy of the particles creating the tracks (higher ionization energy results in a steeper temperature dependence). Although
J. Druch et al. / Effect of pressure on truck regmtration
368
z”l---
III = 10
atm
a = 0.0
atm
. 0
16
14
=04atm
I
2~26
12 -60" 120" -100" -20"
Z=24
llllllllill -60" +20* -100" -20"
-60" +20 -100" -20
-60" +20" -100" -20
1"' -60" +20-100" -20"
T (“C)
Fig. 1. Reduced etch rates s for 1.3 GeV/u 56Fe and its fragments as a function of temperature and pressure at the time of exposure. Note that the magnitudes of the shifts in s can be related to apparent charge shifts by comparing the s curves of neighboring fragments. Measurement errors of s are typically +O.Ol. The inset on the upper right shows the ratios of the reduced etch rates (for the Z = 26 data) to their values at + 20 o C for each pressure; the asterisk denotes the common normalization point.
time of oxygen out of the CR-39 detectors in vacuum is 5 1 day [ll], at the time of exposure the diffused oxygen content of the CR-39 sheets was near its equilibrium value at each of the three gas pressures. Therefore, the observed pressure dependence may simply be due to long-term thermal fading hypothetically enhanced in the reduction or absence of “oxygen-setting” of the latent track. If this is the case, the observed pressure dependence will vary with the time the plastics are kept at pressure after exposure. With our limited range of ionization energy density ( < 30% for Z = 22 to 26) we are not able to discern any significant variation in the temperature or pressure effects with ionization energy density. We thank R. Claxton for her help in this study and Irving Fatt for a measurement of the diffusion coefficient of oxygen in CR-39. This work was supported by NASA Contract NAS l-17806.
References 111 R.L. Fleischer, P.B. Price and R.M. Walker, Nuclear Tracks in Solids (University of California Press, 1975).
121 D. O’Sullivan and A. Thompson, Nucl. Tracks 4 (1980)
there is already evidence in the literature for an oxygen pressure effect in polymers [lo], in these results we see for the first time the dramatic dependence of pressures on CR-39(DOP) response. We cannot label this a “registration pressure effect” because it is unknown whether the effect is due to pressure at the time of exposure, or the pressure history (over days) before and/or after formation of the latent track. It is intriguing that the temperature dependence or the reduced etch rate s roughly maintains its form, independent of pressure. (The points of maximum etch rate do shift with pressure, however.) This argues against the earlier hypothesis [3] that the decline in s with decreasing temperature at temperatures below - 70 o C is due to the removal of available oxygen within the stack; it is probable that ambient oxygen plays little role in the observed registration temperature effect. The true explanation for the temperature dependence probably lies in the complex chemistry of energy redistribution immediately after exposure, with a variety of activation energies giving rise to a temperature-dependent end-state for the latent track. Since the diffusion
271. 131 A. Thompson, D. O’Sullivan, J.H. Adams and L.P. Beahm, Proc. 11th Int. Conf. Solid State Nuclear Track Detectors, Bristol (1981) p. 171. [41 A. Thompson, D. O’Sullivan, J.H. Adams and L.P. Beahm, 18th Int. Cosmic Ray Conf. Bangalore, vol. 9 (1983) p. 407. 151 D. O’Sullivan, A. Thompson, J.H. Adams and L.P. Beahm, Nucl. Tracks 8 (1984) 143. WI A. Thompson and D. O’Sullivan, Nucl. Tracks 8 (1984) 567. 171 R. Hamasalci, T. Hayashi and T. Doke, Nucl. Tracks 9 (1984) 149. PI J. Drach et al., 19th Inter. Cosmic Ray Conf., La Jolla, vol. 2 (1985) p. 131. [91 J.H. Adams and L.P. Beahm, Nucl. Tracks 12 (1986) 387. DOI Indication of an oxygen pressure effect in Lexan polycarbonate is discussed in ref. [2], and table 2-5 of ref. [l] lists qualitative reports of various polymer etch rate sensitivites to oxygen pressure. Such an effect in Lexan was also seen by J.H. Chan and P.B. Price, Phys. Rev. Lett. 35 (1975) 539. [ll] I. Fatt, private communication. For other gas diffusion data, see I. Fatt in Soft Contact Lenses, ed., M. Ruben (Wiley, New York, 1978).