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Effect of radiation on the stability of plastic scintillators
.loornal of Nuclear Energv Parts A/B, 1965.Vol. 19. pp. 490 to 494.
LETTER
Effect of radiation
Pergamon
Press
Ltd.
Prmted
m Northern
Ireland
TO THE EDITORS
on the stability
of plastic
scintillators*
(Received 2 September 1963) 1NTRODUCTlON THE study of ageing processes in plastic scintillators (solutions of luminophors in transparent polymers) is not only of practical importance to scintillator technology but is interesting also because such processes are a particular aspect of the general problem of research into polymer materials. Not much work has been done on ageing processes in plastic scintillators due to the action of radiation. CHARLESBY’~~~’ has described the effect of y-radiation on organic scintillators and ROZMANand TSIMMER’~+’ have studied the effect of ionizing radiations on the stability of polystyrene plastic scintillators. There is considerable practical and theoretical interest in studying the action of radiation on the stability of systems of the type polymer + luminophor having various chemical characteristics. Such studies are needed for the purpose of selecting systems having the best possible stability with respect to radiation and also to discover the mechanism by which plastic scintillators suffer radiation damage. In the work described here, a study was made of those luminophors most widely used in scintillator technology, namely derivatives of 1,3-oxazole, 1,2-ethylene, Az-pyrazoline and also the polyphenyl derivatives. The polymers used were the vinyl derivatives of benzene, toluene and xylene. EXPERIMENTAL
PROCEDURE
The plastic scintillators were prepared by high-temperature block polymerization of the appropriate monomer containing the dissolved luminophors inside sealed ampoules in an atmosphere of nitrogen. Nitrogen, containing not more than 0.03 per cent of oxygen, was used as the scavenging agent. From the plastic blocks so obtained, cylindrical samples of standard size (diameter 18 mm, height 10 mm) were turned off on a lathe and afterwards carefully polished. The plastic scintillators were irradiated on the K-1600 installation of the Ukrainian SSR Academy of Sciences Physical Chemistry Institute in a radiation field giving a dose rate of 115 rads/sec, as determined by the ferrosulphate method. The irradiations were conducted under strictly identical geometrical conditions in glass ampoules at a distance of 100 mm from the source.? The procedures used for making spectrophotometric and photoluminescence investigations and for measuring the light yield of the plastic scintillators during the radioactive irradiations have been described in detail elsewhere .(5*B) The method of measuring the self-transmission of plastic scintillators which had been exposed to radiation was taken from the work of ROZMANand TSIMMER’~‘. RESULTS It can be seen from Table 1 that polystyrene plastic scintillators containing a variety of luminophors are stable with respect to irradiations in air for doses of y-radiation up to nearly 5 x 104-loj rads.1 For higher doses the plastic scintillators suffer appreciable damage which at first causes a reduction in the light yield. The damage suffered by a polystyrene plastic scintillator depends on the nature of the luminophor. For identical y-irradiations (105-lo6 rads), the most stable plastic scintillators are those containing derivatives of ethylene or the polyphenyl series; the least stable are the heterocyclic derivatives and derivatives of Aa-pyrazoline. The most stable systems (Table 2) are the three component systems containing two luminophors. The secondary additive used in the systems we studied * Translated by D. L. ALLAN from Atomnuya Energiyu 17, 67 (1964). ‘1The authors take this opportunity of thanking A. M. KABAKCHIand his co-workers of the Department of Radiation Chemistry of the Ukr. SSR Academy of Sciences Institute of Physical Chemistry for assisting with the irradiation of these samples. $ The criterion of stability was taken to be the change in light yield during irradiation by a Yo source with an intensity 4.1 mC compared with that for an unirradiated sample.
491
Letter to the edttors TABLE I.-CHANGE IN LIGHT YIELDOF POLYSTYRENE PLASTICSCINTJLLATORS INCORPORATING VARIOUSLUMINOPHORS J,cint (%) ;‘-ray dose (rads) 4 ,‘. 10” 10” I 0” 4 _~_~~
c,,,,
( “a)
Luminophor I -phenyl-2-(4-biphenylyl)ethylene (PBP) p-terphenyl (PPP) 2,.5-diphenyloxazole-I,3 (PPO) 2-(4-biphenylyl)-5-phenyloxazole-l,3 (BPO)
25.5 ~38 59
76 76 83
36
43
81
48 8
53* --16
70 52
I.2 2 I.5
m-16 -25 38
I.5 1.5 2 :- 0.1
1,3,5-triphenylpyrazoline (ZP .-- 4”) @erphenyl t POPOPi
* For ZP-Az the radiation dose was -2 :t lo6 rads. .b POPOP--I ,4-di-2-(5-phenyI)-oxazolylbenzole-I ,3. was the luminophor POPOP (0.1 per cent). However, in this case, the nature of the primary luminophor is of considerable importance (see Table 2): the most stable plastic scintillators were those the most damaged were those containing in which the principal luminophor was p-terphenyl: I .3-oxazole derivatives as the principal luminophor. An analysis of the data given in Table 3 shows TABLE 2.-CHANGE OF LIGHT YIELD 0~ POLYSTYRENE SCINTILLATORS WHICH RECEIVEDA GAMMA-RAYDOSE -4 )\ IO” rads (SECONDARY ADDITIVE-POPOP-0.1 g/l) Primary additive
C‘,,, ( ~3
PPP PBE PPO BP0
2.0 I.2 I.5 I.5
J.c,nt o’d) 59 63 69 68
that the stability of a plastic scintillator depends only on the nature of the secondary luminophor and that the more stable plastic scintillators are those for which the secondary additives are derivatives of the ethylene series (BBE). As has already been said, the main interest is in the study of the stability of plastic scintillator~ with various polymer bases. In view of the very limited supplies of high-purity monomers, stability TABLE 3.--CHANGE ot LIGHT YIELDDUE 'r0 C~AMMA-IRRADIAT~~N 0~ POLYSTYRENE SCINTILLATORS INCORPORATING VARIOUSSECONDARY ADDITIVES Luminophors Primary Secondary
-__
p-terphenyl p-terphenyl p-terphenyl p-terphenyl PBE
BBE BBO POPOP ZP-12 BBE
6
10,’ 8 0 0 0 14
10:’ 0 0
I 17.5
J,Ci!ll (“k) Gamma-ray dose (rads) 10” 2 IO6 4 I 10” 6 0 4
2 ~-25.5
21 22 35 40
37 -44 54 48.5 45
10” 8 51 52
IO” 56 57 66
54.5
comparisons with respect to ;Grradiations of plastic scintillators prepared from different bases were carried out for only one dose -4 x IO” rads. The samples were irradiated in air and in a vacuum medium simultaneously, the residual gas pressure in the latter case being 4 x lo--” mm Hg. Data on the change in the light yield of the scintillator materials obtained from measurements made immediately after the irradiations are given in Table 4. The luminophors used were PPP(2 per cent) and POPOP (0.1 per cent).
492
Letter to the editors
As these data show, the plastic scintillators prepared using the various polymers, all of which received exactly the same absorbed y-ray doses, can be arranged in the following order with respect to the degree of damage: polystyrene > polyvinyltoluene > polyvinylxylene. Differences in the light yields of the plastic scintillators irradiated in different media (vacuum, air) are unimportant. Light yield results for these same scintillators obtained from measurements made at various times after the irradiations imply that, for a given dose of radiation, there is a partial (polystyrene) or complete (polyvinylxylene) recovery of the original scintillator characteristics after a definite time. TABLE
‘t--CHANGE
SCINTILLATORS
OF WHICH
DOSE
-4
LIGHT
-59 -47 -38
Polystyrene Polyvinyltoluene Polyvinylxylene
A
OF
PLASTIC
GAMMA-RAY
x lo8 rads In air
Polymer base
YIELD
RECEIVED
Jqcint (%) In vacuum -61 -46 -39
The picture is different if the scintillators are irradiated and then kept under vacuum after the irradiation. Figure 1 shows light yield graphs for polystyrene and polyvinylxylene scintillators which were stored after irradiation in air and in vacuum for ~20 days. The first thing we notice is that the change in scintillation efficiency is insignificant for sclntillators stored in vacuum for 20 days compared with the change for similar samples where the measurement was made immediately after the irradiation; after 20 days in vacuum the blocks remained yellow and the absorption spectra of the samples were virtually no different from those of similar samples measured immediately after irradiation. After opening the ampoules to air a rapid colour change developing from the surface
t,
doys
in light yield of plastic scintillators as a function of time after FIG. 1 .-Change ;j-irradiation in vacuum: 1, 2, 3-polystyrene, polyvinyltoluene, polyvinylxylene, respectively (storage in air); 1’ and 3’ polystyrene and polyvinylxylene (storage in vacuum). inwards towards the interior of the block was observed, together with an increase in the scintillation indices ; 40 days after opening the samples (60 days after the irradiation) the scintillators had scintillation indices close to those of samples opened up immediately after the irradiation and thereafter maintained in air. It seems therefore that oxygen plays an extremely important role in the recovery of the scintillation characteristics of samples during the period following the irradiation. A comparison of the curves in Fig. 2, which give the light yield of the plastic scintillators as a function of storage time after irradiation for different storage temperatures, clearly shows that the scintillation indices change most rapidly when the scintillators are kept at an elevated temperature,
Letter to the editors
t.
493
days
2.-Change in light yield of plastic scintillators as a function of time after irradiation for storage at different temperatures: >:- 0°C; ?? - -5°C; A- -60°C. FIG.
but when kept at a temperature which J is quite small. Because the plastic az?because the intensity of yellowing toluene > polyvinylxylene, an estimate
is not particularly low (even wO@C), the rate scintillators turned yellow as a result of exposure could be arranged in the order: polystyrene was made of the change in light transmission
TABLE 5.-TRANSMISSION RECEIVING A GAMMA-RAY
Polymer base
of change of to radiation, > polyvinyl(Table 5) for
OF PLASTIC SCINTILLATORS DOSE OF -4 X 10firads
Transmission 2 days after irradiation (%) Irradiated in Irradiated in air vacuum
Polystyrene Polyvinyltoluene Polyvinylxylene
52 64 75
50 58 72
the light from the self-luminescence of the samples relative to that for unirradiated samples. Also, on the basis of the absorption and photoluminescence characteristics, the coefficient of absorption 11was calculated (Table 6). The results show that the transmission to the light of self-luminescence TABLE
6.--ABSORPTION
COEFFICIENTS OF PLASTIC SCINTILLATORS
RECEIVING A GAMMA-RAY
Polymer base Polystyrene Polyvinyltoluene Polyvinylxylene
Before irradiation 0.5 0.6 0.7
DOSE
~4
x 106rads
After irradiation in air /L’ A&,, = IC’IIL 1.55 I.20 1.05
3 2 I.5
diminishes in the direction: polystyrene < polyvinyltoluene < polyvinylxylene and, for the polymers we investigated, a somewhat larger transmission is observed for samples irradiated in air. The values of ,u given in Table 6 indicate that the changes in ib caused by exposure to radiation are not all the same but depend on the type of scintillator. Here also the magnitudes change in the direction polystyrene > polyvinyltoluene > polyvinylxylene. Figure 3 presents the absorption spectra, before and after irradiation, of polymer samples having a height of 10 mm. The spectra show that, as a result of the action of the radiation, products due to the radiation damage are formed inside the polymer which absorb very strongly in the short-wave region : a new absorption band is produced with Amax lying in the interval 330-340 m,r/. A rather strong absorption is observed in the visible region up to 530 m/b. In view of the fact that, following exposure to radiation, the colour of the samples changes with the passage of time when they are stored in air, the transmission of the polymers was measured at definite intervals of time after irradiation. It was found that the light absorption of the polymers in the long-wave region fell off a little with time; there was no change of absorption in the short-wave
494
Letter to the editors
band which arises as a result of the irradiation. The change in light absorption for the long-wave absorption band differed from polymer to polymer. One can say, therefore, that one of the causes of the unequal damage suffered by scintillator plastics is that identical absorbed :)-ray doses cause different amounts of damage to the different polymer bases. The considerable differences in the absorption coefficients of different scintillators incorporating the same luminophor may be explained by the differences in the shielding effect when the damage to the polymer occurs as a result of the transfer of energy from the polymer to the luminescent additive. Such an effect has been discussed in detail by GARDEN and ERSTEIN”’ for the case of polymethylmethacrylate.
FIG.
3.-Absorption sample;
spectra of polystyrene (sample with h = 10 mm): I-unirradiated 2-sample which received a y-ray dose -4 x lo6 rads.
From the results of this work the following conclusions can be made: (i) The minimum dose of y-radiation which does not cause substantial changes to the scintillation characteristics of the plastic scintillator samples is 05 x 105-lo5 rads. (ii) The light yield of the plastic scintillators falls off with increasing radiation dose. (iii) The scintillation characteristics recoverwhen the samples are stored inairafter the irradiation. (iv) The structural peculiarities of the luminophors play an important part in the ageing processes of plastic scintillators; the most stable plastic scintillators are those containing as luminophors derivatives of arylethylenes and polyphenyls. (v) When two luminophors are incorporated, the increase in the efficiency of the energy transfer from the polymer to the luminescent additive gives a large shielding effect. (vi) The nature of the polymer base plays an important part in the ageing processes of plastic scintillators; for identical absorbed doses and for the same luminophors, plastic scintillators emplovinea,-, oolvvinvlxvlene as the base suffer the most damaee. , , V. D. BEZUCLII J
”
L. L. NAGORNAYA REFERENCES
1. CHARLESBYA. J. Polynz. Sci. 11, 513, 521 (1953). 2. CHARLESBYA. Nucleonics 12, 18 (1954).
3. 4. 5. 6. 7.
ROZMAN I. M. and TSIMMERK. G. Atomnaya Energiya 2,54 (1957). ROZMAN I. M. and TSIMMERK. G. Atomnaya Energiya 2,20 (1957). KILIMOV A. P., NAG~RNAYA L. L. and ZADOROZHNIIB. B. Prib. tekh. eksp. No. 2, 34 (1957). NARGORNAYA L. L., BEZUGLII V. D. and DEMCHENKON. P. Opt. Spektroskop. XIII, 518 (1962). GARDEN D. and ERSTEINL. J. them. Phys. 34, 1653 (1961).
LETTER
Effect of radiation
Pergamon
Press
Ltd.
Prmted
m Northern
Ireland
TO THE EDITORS
on the stability
of plastic
scintillators*
(Received 2 September 1963) 1NTRODUCTlON THE study of ageing processes in plastic scintillators (solutions of luminophors in transparent polymers) is not only of practical importance to scintillator technology but is interesting also because such processes are a particular aspect of the general problem of research into polymer materials. Not much work has been done on ageing processes in plastic scintillators due to the action of radiation. CHARLESBY’~~~’ has described the effect of y-radiation on organic scintillators and ROZMANand TSIMMER’~+’ have studied the effect of ionizing radiations on the stability of polystyrene plastic scintillators. There is considerable practical and theoretical interest in studying the action of radiation on the stability of systems of the type polymer + luminophor having various chemical characteristics. Such studies are needed for the purpose of selecting systems having the best possible stability with respect to radiation and also to discover the mechanism by which plastic scintillators suffer radiation damage. In the work described here, a study was made of those luminophors most widely used in scintillator technology, namely derivatives of 1,3-oxazole, 1,2-ethylene, Az-pyrazoline and also the polyphenyl derivatives. The polymers used were the vinyl derivatives of benzene, toluene and xylene. EXPERIMENTAL
PROCEDURE
The plastic scintillators were prepared by high-temperature block polymerization of the appropriate monomer containing the dissolved luminophors inside sealed ampoules in an atmosphere of nitrogen. Nitrogen, containing not more than 0.03 per cent of oxygen, was used as the scavenging agent. From the plastic blocks so obtained, cylindrical samples of standard size (diameter 18 mm, height 10 mm) were turned off on a lathe and afterwards carefully polished. The plastic scintillators were irradiated on the K-1600 installation of the Ukrainian SSR Academy of Sciences Physical Chemistry Institute in a radiation field giving a dose rate of 115 rads/sec, as determined by the ferrosulphate method. The irradiations were conducted under strictly identical geometrical conditions in glass ampoules at a distance of 100 mm from the source.? The procedures used for making spectrophotometric and photoluminescence investigations and for measuring the light yield of the plastic scintillators during the radioactive irradiations have been described in detail elsewhere .(5*B) The method of measuring the self-transmission of plastic scintillators which had been exposed to radiation was taken from the work of ROZMANand TSIMMER’~‘. RESULTS It can be seen from Table 1 that polystyrene plastic scintillators containing a variety of luminophors are stable with respect to irradiations in air for doses of y-radiation up to nearly 5 x 104-loj rads.1 For higher doses the plastic scintillators suffer appreciable damage which at first causes a reduction in the light yield. The damage suffered by a polystyrene plastic scintillator depends on the nature of the luminophor. For identical y-irradiations (105-lo6 rads), the most stable plastic scintillators are those containing derivatives of ethylene or the polyphenyl series; the least stable are the heterocyclic derivatives and derivatives of Aa-pyrazoline. The most stable systems (Table 2) are the three component systems containing two luminophors. The secondary additive used in the systems we studied * Translated by D. L. ALLAN from Atomnuya Energiyu 17, 67 (1964). ‘1The authors take this opportunity of thanking A. M. KABAKCHIand his co-workers of the Department of Radiation Chemistry of the Ukr. SSR Academy of Sciences Institute of Physical Chemistry for assisting with the irradiation of these samples. $ The criterion of stability was taken to be the change in light yield during irradiation by a Yo source with an intensity 4.1 mC compared with that for an unirradiated sample.
491
Letter to the edttors TABLE I.-CHANGE IN LIGHT YIELDOF POLYSTYRENE PLASTICSCINTJLLATORS INCORPORATING VARIOUSLUMINOPHORS J,cint (%) ;‘-ray dose (rads) 4 ,‘. 10” 10” I 0” 4 _~_~~
c,,,,
( “a)
Luminophor I -phenyl-2-(4-biphenylyl)ethylene (PBP) p-terphenyl (PPP) 2,.5-diphenyloxazole-I,3 (PPO) 2-(4-biphenylyl)-5-phenyloxazole-l,3 (BPO)
25.5 ~38 59
76 76 83
36
43
81
48 8
53* --16
70 52
I.2 2 I.5
m-16 -25 38
I.5 1.5 2 :- 0.1
1,3,5-triphenylpyrazoline (ZP .-- 4”) @erphenyl t POPOPi
* For ZP-Az the radiation dose was -2 :t lo6 rads. .b POPOP--I ,4-di-2-(5-phenyI)-oxazolylbenzole-I ,3. was the luminophor POPOP (0.1 per cent). However, in this case, the nature of the primary luminophor is of considerable importance (see Table 2): the most stable plastic scintillators were those the most damaged were those containing in which the principal luminophor was p-terphenyl: I .3-oxazole derivatives as the principal luminophor. An analysis of the data given in Table 3 shows TABLE 2.-CHANGE OF LIGHT YIELD 0~ POLYSTYRENE SCINTILLATORS WHICH RECEIVEDA GAMMA-RAYDOSE -4 )\ IO” rads (SECONDARY ADDITIVE-POPOP-0.1 g/l) Primary additive
C‘,,, ( ~3
PPP PBE PPO BP0
2.0 I.2 I.5 I.5
J.c,nt o’d) 59 63 69 68
that the stability of a plastic scintillator depends only on the nature of the secondary luminophor and that the more stable plastic scintillators are those for which the secondary additives are derivatives of the ethylene series (BBE). As has already been said, the main interest is in the study of the stability of plastic scintillator~ with various polymer bases. In view of the very limited supplies of high-purity monomers, stability TABLE 3.--CHANGE ot LIGHT YIELDDUE 'r0 C~AMMA-IRRADIAT~~N 0~ POLYSTYRENE SCINTILLATORS INCORPORATING VARIOUSSECONDARY ADDITIVES Luminophors Primary Secondary
-__
p-terphenyl p-terphenyl p-terphenyl p-terphenyl PBE
BBE BBO POPOP ZP-12 BBE
6
10,’ 8 0 0 0 14
10:’ 0 0
I 17.5
J,Ci!ll (“k) Gamma-ray dose (rads) 10” 2 IO6 4 I 10” 6 0 4
2 ~-25.5
21 22 35 40
37 -44 54 48.5 45
10” 8 51 52
IO” 56 57 66
54.5
comparisons with respect to ;Grradiations of plastic scintillators prepared from different bases were carried out for only one dose -4 x IO” rads. The samples were irradiated in air and in a vacuum medium simultaneously, the residual gas pressure in the latter case being 4 x lo--” mm Hg. Data on the change in the light yield of the scintillator materials obtained from measurements made immediately after the irradiations are given in Table 4. The luminophors used were PPP(2 per cent) and POPOP (0.1 per cent).
492
Letter to the editors
As these data show, the plastic scintillators prepared using the various polymers, all of which received exactly the same absorbed y-ray doses, can be arranged in the following order with respect to the degree of damage: polystyrene > polyvinyltoluene > polyvinylxylene. Differences in the light yields of the plastic scintillators irradiated in different media (vacuum, air) are unimportant. Light yield results for these same scintillators obtained from measurements made at various times after the irradiations imply that, for a given dose of radiation, there is a partial (polystyrene) or complete (polyvinylxylene) recovery of the original scintillator characteristics after a definite time. TABLE
‘t--CHANGE
SCINTILLATORS
OF WHICH
DOSE
-4
LIGHT
-59 -47 -38
Polystyrene Polyvinyltoluene Polyvinylxylene
A
OF
PLASTIC
GAMMA-RAY
x lo8 rads In air
Polymer base
YIELD
RECEIVED
Jqcint (%) In vacuum -61 -46 -39
The picture is different if the scintillators are irradiated and then kept under vacuum after the irradiation. Figure 1 shows light yield graphs for polystyrene and polyvinylxylene scintillators which were stored after irradiation in air and in vacuum for ~20 days. The first thing we notice is that the change in scintillation efficiency is insignificant for sclntillators stored in vacuum for 20 days compared with the change for similar samples where the measurement was made immediately after the irradiation; after 20 days in vacuum the blocks remained yellow and the absorption spectra of the samples were virtually no different from those of similar samples measured immediately after irradiation. After opening the ampoules to air a rapid colour change developing from the surface
t,
doys
in light yield of plastic scintillators as a function of time after FIG. 1 .-Change ;j-irradiation in vacuum: 1, 2, 3-polystyrene, polyvinyltoluene, polyvinylxylene, respectively (storage in air); 1’ and 3’ polystyrene and polyvinylxylene (storage in vacuum). inwards towards the interior of the block was observed, together with an increase in the scintillation indices ; 40 days after opening the samples (60 days after the irradiation) the scintillators had scintillation indices close to those of samples opened up immediately after the irradiation and thereafter maintained in air. It seems therefore that oxygen plays an extremely important role in the recovery of the scintillation characteristics of samples during the period following the irradiation. A comparison of the curves in Fig. 2, which give the light yield of the plastic scintillators as a function of storage time after irradiation for different storage temperatures, clearly shows that the scintillation indices change most rapidly when the scintillators are kept at an elevated temperature,
Letter to the editors
t.
493
days
2.-Change in light yield of plastic scintillators as a function of time after irradiation for storage at different temperatures: >:- 0°C; ?? - -5°C; A- -60°C. FIG.
but when kept at a temperature which J is quite small. Because the plastic az?because the intensity of yellowing toluene > polyvinylxylene, an estimate
is not particularly low (even wO@C), the rate scintillators turned yellow as a result of exposure could be arranged in the order: polystyrene was made of the change in light transmission
TABLE 5.-TRANSMISSION RECEIVING A GAMMA-RAY
Polymer base
of change of to radiation, > polyvinyl(Table 5) for
OF PLASTIC SCINTILLATORS DOSE OF -4 X 10firads
Transmission 2 days after irradiation (%) Irradiated in Irradiated in air vacuum
Polystyrene Polyvinyltoluene Polyvinylxylene
52 64 75
50 58 72
the light from the self-luminescence of the samples relative to that for unirradiated samples. Also, on the basis of the absorption and photoluminescence characteristics, the coefficient of absorption 11was calculated (Table 6). The results show that the transmission to the light of self-luminescence TABLE
6.--ABSORPTION
COEFFICIENTS OF PLASTIC SCINTILLATORS
RECEIVING A GAMMA-RAY
Polymer base Polystyrene Polyvinyltoluene Polyvinylxylene
Before irradiation 0.5 0.6 0.7
DOSE
~4
x 106rads
After irradiation in air /L’ A&,, = IC’IIL 1.55 I.20 1.05
3 2 I.5
diminishes in the direction: polystyrene < polyvinyltoluene < polyvinylxylene and, for the polymers we investigated, a somewhat larger transmission is observed for samples irradiated in air. The values of ,u given in Table 6 indicate that the changes in ib caused by exposure to radiation are not all the same but depend on the type of scintillator. Here also the magnitudes change in the direction polystyrene > polyvinyltoluene > polyvinylxylene. Figure 3 presents the absorption spectra, before and after irradiation, of polymer samples having a height of 10 mm. The spectra show that, as a result of the action of the radiation, products due to the radiation damage are formed inside the polymer which absorb very strongly in the short-wave region : a new absorption band is produced with Amax lying in the interval 330-340 m,r/. A rather strong absorption is observed in the visible region up to 530 m/b. In view of the fact that, following exposure to radiation, the colour of the samples changes with the passage of time when they are stored in air, the transmission of the polymers was measured at definite intervals of time after irradiation. It was found that the light absorption of the polymers in the long-wave region fell off a little with time; there was no change of absorption in the short-wave
494
Letter to the editors
band which arises as a result of the irradiation. The change in light absorption for the long-wave absorption band differed from polymer to polymer. One can say, therefore, that one of the causes of the unequal damage suffered by scintillator plastics is that identical absorbed :)-ray doses cause different amounts of damage to the different polymer bases. The considerable differences in the absorption coefficients of different scintillators incorporating the same luminophor may be explained by the differences in the shielding effect when the damage to the polymer occurs as a result of the transfer of energy from the polymer to the luminescent additive. Such an effect has been discussed in detail by GARDEN and ERSTEIN”’ for the case of polymethylmethacrylate.
FIG.
3.-Absorption sample;
spectra of polystyrene (sample with h = 10 mm): I-unirradiated 2-sample which received a y-ray dose -4 x lo6 rads.
From the results of this work the following conclusions can be made: (i) The minimum dose of y-radiation which does not cause substantial changes to the scintillation characteristics of the plastic scintillator samples is 05 x 105-lo5 rads. (ii) The light yield of the plastic scintillators falls off with increasing radiation dose. (iii) The scintillation characteristics recoverwhen the samples are stored inairafter the irradiation. (iv) The structural peculiarities of the luminophors play an important part in the ageing processes of plastic scintillators; the most stable plastic scintillators are those containing as luminophors derivatives of arylethylenes and polyphenyls. (v) When two luminophors are incorporated, the increase in the efficiency of the energy transfer from the polymer to the luminescent additive gives a large shielding effect. (vi) The nature of the polymer base plays an important part in the ageing processes of plastic scintillators; for identical absorbed doses and for the same luminophors, plastic scintillators emplovinea,-, oolvvinvlxvlene as the base suffer the most damaee. , , V. D. BEZUCLII J
”
L. L. NAGORNAYA REFERENCES
1. CHARLESBYA. J. Polynz. Sci. 11, 513, 521 (1953). 2. CHARLESBYA. Nucleonics 12, 18 (1954).
3. 4. 5. 6. 7.
ROZMAN I. M. and TSIMMERK. G. Atomnaya Energiya 2,54 (1957). ROZMAN I. M. and TSIMMERK. G. Atomnaya Energiya 2,20 (1957). KILIMOV A. P., NAG~RNAYA L. L. and ZADOROZHNIIB. B. Prib. tekh. eksp. No. 2, 34 (1957). NARGORNAYA L. L., BEZUGLII V. D. and DEMCHENKON. P. Opt. Spektroskop. XIII, 518 (1962). GARDEN D. and ERSTEINL. J. them. Phys. 34, 1653 (1961).