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Registration temperature effect in polypropylene detectors
Na¢/ D'a¢~ p...,g,n, llfmm..Vol. 22, Nee 1--4. pp 93..-96. 1993 Bhevier Scumce Ltd itramd m Omm Ih'itaia. 0969-1107S~ $6.00+.00
Perl~mM}n
REGISTRATION TEMPERATURE EFFECT IN POLYPROPYLENE DETECTORS
P. Yu. Aim.*, A. YU. DIDYg*, L. I. K I I A ~ * , V. I. K ~ v * *Joint ~
and B. I. PUUOVt
for Nuclear R a s h : h , 141980. Dulma, RlUlSia;
and tlmtiam of l~/~ca m/Power Ballmedag,24~0, ~
X~n,ia
ABSTRACT The registration temperature e~ect (RTE) was investigated using two types of polypropy1erie (PP) detectors. It was found that the RTE depends on the content of radical scaveager in the polymer. The results are compared with published radiothermo]-minescence data and discussed in terms of rdaxation transitions.
KEYWORDS Polypropylene, track detectors, registration temperature effect, fission fragments
~TRODUCTION First studies of the dependence of the track response of polymers on temperature during registration were carried out with the aim to improve charge resolution of SSNTD's (O'Sullivan and Thompson, 1980). During following decade many works were done on this subject (see review of Durrani, 1991). Mostly, typical polymeric detector media such as polycarbonate or CR-39 were used in these experiments. It follows from a number of works that the track etch rate of heavy ions in polymers generally decreases with increasing registration temperature, but there are some exceptions, in particular, for relatively low-LET particles. The behaviour of polymeric SSNTDs at di~erent temperatures is rather complicated and the mechanism of the RTE is not dear in detail In the present paper the authors used another kind of polymeric detecting media - polypropylene films - - in order to extend RTE study on polymers with di~erent composition and different properties. An accumulation of experimental data for a wide range of polymers is important in order to conclude about the nature of the RTE.
93
94
P. YU. APEL et
al.
EXPERIMENTAL Two batches of Torayfan polypropylene filrn~ (Toray, Japan) with the thickness of 10 pm were used. According to ultraviolet absorption spectra, the t~lrne contained 0.1 and 0.5% phenolic antioxidant (Irgano~ 1010). Small samples (10xl0 cm) were mounted in thermal contact with a temperature-controlled copper holder. The holder was placed in the exposure vacuum chamber and the samples were irradiated by collimated fission fragments from the 2ssu target at temperatures lying in the range of 77 to 293 K. The uranium target was exposed to the thermal neutron flux (10 s cm-2s -1) from a nuclear reactor. The temperature of the holder was measured by a transistor. The pressure in the vacullrn chamber during irradiation depended on the temperature and varied from 1 Pa (room temperature) to 10 -4 Pa (77K). Each exposure was carried out after allowing the samples to outgass for not less than l h before irradiation. The exposure time was from 12 to 48 rain, the track density in the samples amounted to ca. 10e c m -2. Three series of exposures were performed. All irradiated samples were stored in air at room temperature for several months before processing. Round samples with diameter of 20 mm were cut from the irradiated sheets and etched in a conductometric cell. The breakthrough time was measured and converted into the value of track etch rate. Measurements were repeated several times for a given irradiation temperature. The processing of the samples of each series was completed within 1 month. 16N sulphuric acid saturated with CrOs was used as an etchant.
RESULTS AND DISCUSSION In Fig.1 the variation of the relative track etch rate VT depending on the registration temperature T is shown for two types of the PP films. In both cases, there is a maximum on the Vr vs T function at ca. 140K. The height of the maximum is smaller for the films with higher content of antioxidant. It seems plausible that there exists an immediate relationship between the content of additives in PP and the RTE value. As known, phenolic antioxidants are very effective in capturing radicals ('scavenger' reactions). They can shift the equilibrium between competing reactions of intermediates generated in latent ionizing particle tracks. In general, the radicals in a polymer can disappear in the processes of recombination, disproportionation and macromolecular decay. Also, reactions with various substances dissolved m the matrix, such as oxygen, moisture, additives, are possible. The recombination of the radicals leads to partial restoration and/or cross linking of polymer chains. This reaction is characterized by the highest value of activation energy in comparison with the other reactions mentioned above (Dubinskaja, 1978) and, as a result, the recombination process must be suppressed with decreasing registration temperature. From this point of view, the general increase of the track etch rate with decreasing irradiation temperature is understandable. But, if the polymer contains a scavenger, the main part of the radicals may be trapped and, therefore, the influence of the reactions between the radicals on the RTE value must be lower. In accordance with this concept, we observed sufficiently weaker influence of the irradiation temperature in the samples with high content of the antioxidant.
REGISTRATION TEMPERATURE EFFECT
~egistrcstion tempercsture,°C
RegistrQl"iontemperoture,"C -200
-100
95
-200
0
-100
0
3
0
2~
2~
e-
CU >
R~tst~hon femperoture,K
Regtsfrotmntemperature,K
Fig. 1. Relative track etch rate normalized to track etch rate at a registration temperature of 20°C as a function of the registration temperature. Polypropylene film with 0.1% (a) and 0.5% (b) antioxidant. Open and black circles represent different series of irradiation by fission fragments. The solid lines are drawn to guide the eye.
Since the changes in mobility of chemically reactive intermediates often correlate with relaxation trRnsitions in polymers, it is reasonable to compare our results with raAiothermoluminescence (RTL) curves. There are numerous RTL data for polypropylene in literature. In most cases two maxima at 130-140K and 220-250K were observed (Mozisek, 1970; Kuleshov and Nikolskij, 1991). The position of the former coincides very well with the peak in our VT vs T curves. Kuleshov and Nikolskij (1991) presented a strong evidence that this RTL peak is affected by the presence of oxygen. It was supposed that below this temperature alkyl ra~licals do not react with the oxygen molecules so as degradation of macromolecules due to oxidation does not occur. Similar explanation was offered by O'Sullivan et al.(1984) for anomalous point at the temperature of liquid nitrogen. Durrani (1991) critisized this assumption; his argument was, that the same temperature dependence has been observed in experiments performed in vacuum. Nevertheless, it should be noted that keeping the samples in vacuum for several hours could hardly provide complete elimination of oxygen from the polymer matrix. Usually, high vacuum in combination with annealing during a long time should guarantee the absence of oxygen. The 'high temperature' maximum drawn in Fig.lb is questionable. More detailed experiments at temperatures around -70°C are required to answer whether the Vr vs T function has the second maximum. If so, its position is not far from the glass transition temperature and corresponding RTL peak.
945
P. YU. A P F ~ ~ ~d.
CONCLUSION The correlation between the magnitude of the RTE and the content of a scavenger showed that the reactions of radicals in tracks contribute to the registration temperature dependence of the track etch rate. The position of the main maximum of the Vr vs T rune, ion coincides with that of the RTL curves. It seems reasonable to perform luther investigations of the RTE by using experimental methods allowing to detect relaxation transitions in polymers, together with the studies of track etehlng behaviour.
REFERENCES Durrani, S.A. (1991). The e~ect of irradiation temperature on the response of track recording crystalline and polymeric media: a brief review. ~ucl. TYacks Radiat. l e a s . , 19, 61-70. Kuleshov, I.V., and V.G.Nikolskij (1991). Radiothermoluminescence of polymers. Moscow, Khlmi• p.l12 (in Russian). Mozisek, M. (1970). Radiothermoluminescence of polyethylene and polypropylene. Int. J. App1. Radiat. Isot., 21, 11-16. O'Sullivan, D., and A.Thompson (1980). The observation of a sensitivity dependence on temperature during registration in solid state nuclear track detectors. ~£L-~, 4, 371-276. O'Sullivan, D., A.Thompson, J.A.AdAm~ and L.P.Beahm (1984). New results on the investigation of the variation of nuclear track detector response with temperature. N~d. "l~racksRadiat. leas., ~, 143-146.
Perl~mM}n
REGISTRATION TEMPERATURE EFFECT IN POLYPROPYLENE DETECTORS
P. Yu. Aim.*, A. YU. DIDYg*, L. I. K I I A ~ * , V. I. K ~ v * *Joint ~
and B. I. PUUOVt
for Nuclear R a s h : h , 141980. Dulma, RlUlSia;
and tlmtiam of l~/~ca m/Power Ballmedag,24~0, ~
X~n,ia
ABSTRACT The registration temperature e~ect (RTE) was investigated using two types of polypropy1erie (PP) detectors. It was found that the RTE depends on the content of radical scaveager in the polymer. The results are compared with published radiothermo]-minescence data and discussed in terms of rdaxation transitions.
KEYWORDS Polypropylene, track detectors, registration temperature effect, fission fragments
~TRODUCTION First studies of the dependence of the track response of polymers on temperature during registration were carried out with the aim to improve charge resolution of SSNTD's (O'Sullivan and Thompson, 1980). During following decade many works were done on this subject (see review of Durrani, 1991). Mostly, typical polymeric detector media such as polycarbonate or CR-39 were used in these experiments. It follows from a number of works that the track etch rate of heavy ions in polymers generally decreases with increasing registration temperature, but there are some exceptions, in particular, for relatively low-LET particles. The behaviour of polymeric SSNTDs at di~erent temperatures is rather complicated and the mechanism of the RTE is not dear in detail In the present paper the authors used another kind of polymeric detecting media - polypropylene films - - in order to extend RTE study on polymers with di~erent composition and different properties. An accumulation of experimental data for a wide range of polymers is important in order to conclude about the nature of the RTE.
93
94
P. YU. APEL et
al.
EXPERIMENTAL Two batches of Torayfan polypropylene filrn~ (Toray, Japan) with the thickness of 10 pm were used. According to ultraviolet absorption spectra, the t~lrne contained 0.1 and 0.5% phenolic antioxidant (Irgano~ 1010). Small samples (10xl0 cm) were mounted in thermal contact with a temperature-controlled copper holder. The holder was placed in the exposure vacuum chamber and the samples were irradiated by collimated fission fragments from the 2ssu target at temperatures lying in the range of 77 to 293 K. The uranium target was exposed to the thermal neutron flux (10 s cm-2s -1) from a nuclear reactor. The temperature of the holder was measured by a transistor. The pressure in the vacullrn chamber during irradiation depended on the temperature and varied from 1 Pa (room temperature) to 10 -4 Pa (77K). Each exposure was carried out after allowing the samples to outgass for not less than l h before irradiation. The exposure time was from 12 to 48 rain, the track density in the samples amounted to ca. 10e c m -2. Three series of exposures were performed. All irradiated samples were stored in air at room temperature for several months before processing. Round samples with diameter of 20 mm were cut from the irradiated sheets and etched in a conductometric cell. The breakthrough time was measured and converted into the value of track etch rate. Measurements were repeated several times for a given irradiation temperature. The processing of the samples of each series was completed within 1 month. 16N sulphuric acid saturated with CrOs was used as an etchant.
RESULTS AND DISCUSSION In Fig.1 the variation of the relative track etch rate VT depending on the registration temperature T is shown for two types of the PP films. In both cases, there is a maximum on the Vr vs T function at ca. 140K. The height of the maximum is smaller for the films with higher content of antioxidant. It seems plausible that there exists an immediate relationship between the content of additives in PP and the RTE value. As known, phenolic antioxidants are very effective in capturing radicals ('scavenger' reactions). They can shift the equilibrium between competing reactions of intermediates generated in latent ionizing particle tracks. In general, the radicals in a polymer can disappear in the processes of recombination, disproportionation and macromolecular decay. Also, reactions with various substances dissolved m the matrix, such as oxygen, moisture, additives, are possible. The recombination of the radicals leads to partial restoration and/or cross linking of polymer chains. This reaction is characterized by the highest value of activation energy in comparison with the other reactions mentioned above (Dubinskaja, 1978) and, as a result, the recombination process must be suppressed with decreasing registration temperature. From this point of view, the general increase of the track etch rate with decreasing irradiation temperature is understandable. But, if the polymer contains a scavenger, the main part of the radicals may be trapped and, therefore, the influence of the reactions between the radicals on the RTE value must be lower. In accordance with this concept, we observed sufficiently weaker influence of the irradiation temperature in the samples with high content of the antioxidant.
REGISTRATION TEMPERATURE EFFECT
~egistrcstion tempercsture,°C
RegistrQl"iontemperoture,"C -200
-100
95
-200
0
-100
0
3
0
2~
2~
e-
CU >
R~tst~hon femperoture,K
Regtsfrotmntemperature,K
Fig. 1. Relative track etch rate normalized to track etch rate at a registration temperature of 20°C as a function of the registration temperature. Polypropylene film with 0.1% (a) and 0.5% (b) antioxidant. Open and black circles represent different series of irradiation by fission fragments. The solid lines are drawn to guide the eye.
Since the changes in mobility of chemically reactive intermediates often correlate with relaxation trRnsitions in polymers, it is reasonable to compare our results with raAiothermoluminescence (RTL) curves. There are numerous RTL data for polypropylene in literature. In most cases two maxima at 130-140K and 220-250K were observed (Mozisek, 1970; Kuleshov and Nikolskij, 1991). The position of the former coincides very well with the peak in our VT vs T curves. Kuleshov and Nikolskij (1991) presented a strong evidence that this RTL peak is affected by the presence of oxygen. It was supposed that below this temperature alkyl ra~licals do not react with the oxygen molecules so as degradation of macromolecules due to oxidation does not occur. Similar explanation was offered by O'Sullivan et al.(1984) for anomalous point at the temperature of liquid nitrogen. Durrani (1991) critisized this assumption; his argument was, that the same temperature dependence has been observed in experiments performed in vacuum. Nevertheless, it should be noted that keeping the samples in vacuum for several hours could hardly provide complete elimination of oxygen from the polymer matrix. Usually, high vacuum in combination with annealing during a long time should guarantee the absence of oxygen. The 'high temperature' maximum drawn in Fig.lb is questionable. More detailed experiments at temperatures around -70°C are required to answer whether the Vr vs T function has the second maximum. If so, its position is not far from the glass transition temperature and corresponding RTL peak.
945
P. YU. A P F ~ ~ ~d.
CONCLUSION The correlation between the magnitude of the RTE and the content of a scavenger showed that the reactions of radicals in tracks contribute to the registration temperature dependence of the track etch rate. The position of the main maximum of the Vr vs T rune, ion coincides with that of the RTL curves. It seems reasonable to perform luther investigations of the RTE by using experimental methods allowing to detect relaxation transitions in polymers, together with the studies of track etehlng behaviour.
REFERENCES Durrani, S.A. (1991). The e~ect of irradiation temperature on the response of track recording crystalline and polymeric media: a brief review. ~ucl. TYacks Radiat. l e a s . , 19, 61-70. Kuleshov, I.V., and V.G.Nikolskij (1991). Radiothermoluminescence of polymers. Moscow, Khlmi• p.l12 (in Russian). Mozisek, M. (1970). Radiothermoluminescence of polyethylene and polypropylene. Int. J. App1. Radiat. Isot., 21, 11-16. O'Sullivan, D., and A.Thompson (1980). The observation of a sensitivity dependence on temperature during registration in solid state nuclear track detectors. ~£L-~, 4, 371-276. O'Sullivan, D., A.Thompson, J.A.AdAm~ and L.P.Beahm (1984). New results on the investigation of the variation of nuclear track detector response with temperature. N~d. "l~racksRadiat. leas., ~, 143-146.