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Temperature dependence of three-quantum annihilation in organic crystals
Volume
75, number
3
TEMPERATURE
CHEMICAL
DEPENDENCE
PHYSICS
LETTERS
OF THREE-QUANTUM
1 November
ANNIHILATION
1980
lN ORGANIC CRYSTALS
T. GOWOREK, C. RYBKA, R. WASIEWICZ and J. WAWRYSZCZUK Institute of Physrcs, Mario Cune Sklodorvska Uiwemty. Reccwed
30 hfay 1980.
in fiil
2003I Lublin, Poland
form 25 July 1980
The three-quantum anmhdatlon mtenslty m molecular crystals was measured as a function of tempcwture. It wvz~sshown that the thermally mduced long-lned component observed earher m the tnne spectrum awes from ortho-Ps decay.
1. Introduction
compared with that in a standard (e.g. a metallic sample). In our case one can use as a “no positronium”
The posltron can, after moderation, capture an electron and form a bound state called posltroruum. It has been demonstrated that m many aromatlc mo-
standard the same sample at a sufficiently low temperature. Thus, there IS no need to exchange the samples
lecular crystals, posltromum atoms do not form - the tune spectrum of positrons anmhdating in the crystal shows a single exponential decay only, Lvlth a meanlife of about 300 ps, typlcal of free anmhilatlon. In crystals wth free volume type defects, e.g. m muted crystals or in crystals containing thermally produced defects, a long-lived component appears in the time spectrum. Its appearance is accompanied by a narrowing of the momentum spectrum of the annihdating e+e- prurs, thus it IS commonly accepted that this component is connected with the decay of locahzed o-Ps atoms, and not wth trapped positrons. A confirmation of the origin of the long-lived component can be made by measuring the three-quantum anmiulatlon probabihty. Formation of positronium should lead to an increase in the 3-y annihilation rate. Our aim urns to identify the nature of the long-lived component observed at elevated temperatures in pure organic sol&, using this method.
during the measurements, one can easily keep the same geometry, source corrections, etc., making it possible to detect even small vanations of 3y annihilation intenslty. The three-quantum annihJation rate in solid p-terphenyl and pyrene was measured as a function of tem-
perature. Terphenyl of scinttiation grade purity was refined by zone melting. The positron source, a30 i.Xi 22Na deposited on nickel foil, was sandwiched between two p-terphenyl single crystals cut in the primary cleavage plane. The source-sample unit was placed m a copper container, whose temperature was regulated to +l K. The sample was viewed by three scmtillation counters equally spaced by 120°, coplanar with the source. The triple coincidence rate was measured usmg standard electronic circuitry with a time resolution of 40 ns. The background was estimated by moving one counter out of the plane determined by the source and the other two counters. .4 typical counting rate was 3 X lob2 s-t. Three runs of
measurements were performed, two on heating and one on coohng, and the results were reproducible.
2. Experimental Measurements of three-quantum annihilation rates are usually performed as relative measurements, the mtensity of 37 decays m the mvestigated sample IS
3. Results and discussion Increase
of temperature
increases
the triple coinci569
Volume 75, number 3
1
CHEMICAL PHYSICS LETTERS
November
1980
f l
0
*f
b
TEMPERATURE.
TEMPERATURE , -C
Fig. 1. p-terphenyl (a) Temperature dependence of the longhvrd component mtensity. Tmngks denote angular correhtion data, Le. 31NvalueS. (b) Increase of the three-quantum ZUI&~IIXI mtenstty relative to the intenstty at room temperature. The broken curve represents the lntenslty expected from hfetlme measurements.
dence rate fsr (Fig. l), as can be expected in the case of positronium formation. The results are compared with calculations based on the lifetime measurement data. The temperature dependence of Is_, is not the same as that of the long-hved component intensity f, (fig. la) due to the changes of lifetime 72 with the temperature. Good agreement between the threequantum and lifetuTle data IS observed, indicating that all the long-hved component IS of ortho-Ps ongin. Slrmlar experunents were performed v&h polycrystrdhe samples of pyrene (fig- 2). The agreement between three-quantum anruhdation rate and hfetune data WLLS also satisfactory. The nature of the thermally produced positronium formation centers is not known. The short iifetime of the r2 component excludes the interpretahon of these
570
OC
Rg. 2 Temperature dependence of the three-quantum anmhdatlon mtenslty in pyrene The broken curve LSthe mtenslty expected from lIfetIme data.
centers as vacancies. The hfetune rises wth temperature [ 11, as UI plastic crystals [2,3]. While in this last case the sphttlng of the long-lived component into two components is possible (the longest 1s of vacancy ongu~), m our case this attempt failed. E.g. m the tune spectrum at 16O”C, with 3 X lo6 coincidences collected, the intensity ofthe vacancy-type component does not exceed 0.5%. The short hfetune r2 points to small size of posltron traps, thus one of the possible explanations of the thermal Ps centers is the orientationai disorder 111the me&urn, gwing nse to small unstable free volumes.
References
t11 R. W$chllk,
J. Wawrysznuk, C. Rybka and T. Goworek, Phys. Stat. Sol. 95b (1979) K113. 121 M. Eldrup, N.J. Pedcrsen and J.N. Sherwood, Phys. Rev. Letters 43 (1979) 1407. hl. Eldmp and J-N. Sherwood. Chem. 131 D. Lightbody, Phys , to be published
75, number
3
TEMPERATURE
CHEMICAL
DEPENDENCE
PHYSICS
LETTERS
OF THREE-QUANTUM
1 November
ANNIHILATION
1980
lN ORGANIC CRYSTALS
T. GOWOREK, C. RYBKA, R. WASIEWICZ and J. WAWRYSZCZUK Institute of Physrcs, Mario Cune Sklodorvska Uiwemty. Reccwed
30 hfay 1980.
in fiil
2003I Lublin, Poland
form 25 July 1980
The three-quantum anmhdatlon mtenslty m molecular crystals was measured as a function of tempcwture. It wvz~sshown that the thermally mduced long-lned component observed earher m the tnne spectrum awes from ortho-Ps decay.
1. Introduction
compared with that in a standard (e.g. a metallic sample). In our case one can use as a “no positronium”
The posltron can, after moderation, capture an electron and form a bound state called posltroruum. It has been demonstrated that m many aromatlc mo-
standard the same sample at a sufficiently low temperature. Thus, there IS no need to exchange the samples
lecular crystals, posltromum atoms do not form - the tune spectrum of positrons anmhdating in the crystal shows a single exponential decay only, Lvlth a meanlife of about 300 ps, typlcal of free anmhilatlon. In crystals wth free volume type defects, e.g. m muted crystals or in crystals containing thermally produced defects, a long-lived component appears in the time spectrum. Its appearance is accompanied by a narrowing of the momentum spectrum of the annihdating e+e- prurs, thus it IS commonly accepted that this component is connected with the decay of locahzed o-Ps atoms, and not wth trapped positrons. A confirmation of the origin of the long-lived component can be made by measuring the three-quantum anmiulatlon probabihty. Formation of positronium should lead to an increase in the 3-y annihilation rate. Our aim urns to identify the nature of the long-lived component observed at elevated temperatures in pure organic sol&, using this method.
during the measurements, one can easily keep the same geometry, source corrections, etc., making it possible to detect even small vanations of 3y annihilation intenslty. The three-quantum annihJation rate in solid p-terphenyl and pyrene was measured as a function of tem-
perature. Terphenyl of scinttiation grade purity was refined by zone melting. The positron source, a30 i.Xi 22Na deposited on nickel foil, was sandwiched between two p-terphenyl single crystals cut in the primary cleavage plane. The source-sample unit was placed m a copper container, whose temperature was regulated to +l K. The sample was viewed by three scmtillation counters equally spaced by 120°, coplanar with the source. The triple coincidence rate was measured usmg standard electronic circuitry with a time resolution of 40 ns. The background was estimated by moving one counter out of the plane determined by the source and the other two counters. .4 typical counting rate was 3 X lob2 s-t. Three runs of
measurements were performed, two on heating and one on coohng, and the results were reproducible.
2. Experimental Measurements of three-quantum annihilation rates are usually performed as relative measurements, the mtensity of 37 decays m the mvestigated sample IS
3. Results and discussion Increase
of temperature
increases
the triple coinci569
Volume 75, number 3
1
CHEMICAL PHYSICS LETTERS
November
1980
f l
0
*f
b
TEMPERATURE.
TEMPERATURE , -C
Fig. 1. p-terphenyl (a) Temperature dependence of the longhvrd component mtensity. Tmngks denote angular correhtion data, Le. 31NvalueS. (b) Increase of the three-quantum ZUI&~IIXI mtenstty relative to the intenstty at room temperature. The broken curve represents the lntenslty expected from hfetlme measurements.
dence rate fsr (Fig. l), as can be expected in the case of positronium formation. The results are compared with calculations based on the lifetime measurement data. The temperature dependence of Is_, is not the same as that of the long-hved component intensity f, (fig. la) due to the changes of lifetime 72 with the temperature. Good agreement between the threequantum and lifetuTle data IS observed, indicating that all the long-hved component IS of ortho-Ps ongin. Slrmlar experunents were performed v&h polycrystrdhe samples of pyrene (fig- 2). The agreement between three-quantum anruhdation rate and hfetune data WLLS also satisfactory. The nature of the thermally produced positronium formation centers is not known. The short iifetime of the r2 component excludes the interpretahon of these
570
OC
Rg. 2 Temperature dependence of the three-quantum anmhdatlon mtenslty in pyrene The broken curve LSthe mtenslty expected from lIfetIme data.
centers as vacancies. The hfetune rises wth temperature [ 11, as UI plastic crystals [2,3]. While in this last case the sphttlng of the long-lived component into two components is possible (the longest 1s of vacancy ongu~), m our case this attempt failed. E.g. m the tune spectrum at 16O”C, with 3 X lo6 coincidences collected, the intensity ofthe vacancy-type component does not exceed 0.5%. The short hfetune r2 points to small size of posltron traps, thus one of the possible explanations of the thermal Ps centers is the orientationai disorder 111the me&urn, gwing nse to small unstable free volumes.
References
t11 R. W$chllk,
J. Wawrysznuk, C. Rybka and T. Goworek, Phys. Stat. Sol. 95b (1979) K113. 121 M. Eldrup, N.J. Pedcrsen and J.N. Sherwood, Phys. Rev. Letters 43 (1979) 1407. hl. Eldmp and J-N. Sherwood. Chem. 131 D. Lightbody, Phys , to be published