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Investigation of the crystal scintillations in the VUV region
Nuclear Instruments and Methods in Physics Research A288 (1990) 659-661 North-Holland
659
Letter to the Editor
INVESTIGATION OF THE CRYSTAL SCINTILLATIONS IN THE VUV REGION A.F. BUZULUTSKOV, L.K. TURCHANOVICH and V.G. VASILCHENKO Institute for High Energy Physics, Serpukhov, USSR
Y.L. JANSONS, Z.A. RACHKO and V.J. VALBIS Institute of Solid-State Physics, Riga, USSR
N.M. KHAIDUKOV
Institute of Common and Inorganic Chemistry, Moscow, USSR
Received 6 October 1989
Some characteristics of the new crystal scintillators K2YF5 , KLuF4 and KLu 2 F7 emitting in the vacuum ultraviolet region have been studied.
Solid scintillation proportional counters (SSPC), combining high energy resolution of scintillation detectors and convenience of the wire chamber readout, have recently been widely used in high energy physics and nuclear medicine (see, for example, ref. [11). Therefore, the search for new solid scintillators, emitting in the sensitivity range of the photoionizing vapours TMAE, thriethylamine (TEA), ethylferrocene, etc ., is of utmost importance . The scintillators emitting in the TEA sensitivity range (i.e., in the VÜV region) may turn out to be most promising, since TEA is very easy to handle and has a high vapour pressure at room temperature. The spectra of the first solid scintillators, KMgF3 and KCaF3, emitting in the VUV region, are presented in ref. [2] . The mechanism of emission is also described in this paper and the authors call it cross-luminescence. KMgF3 crystal light yield measurements have been reported in ref. [3], and the possibility of its coupling to a proportional chamber, working with a CH4 + TEA mixture, has been shown. However, the radiation lengths of KMgF3 and KCaF3 are rather large (7 cm). Therefore the search for scintillators with a higher average atomic number is of great interest . In the present work we studied K 2YF5 , KLuF4, KLu 2F7 , and KYbF4 crystals which belong to the double chemical compound of KF with fluorides of rareearth elements. To obtain the crystals we used the hydrothermal procedure. The crystals had an irregular shape and their characteristic sizes were several millimeters. The KMgF3 crystal, 7.5 mm high and 12 mm in 0168-9002;90/$03 .50 © Elsevier Science Publishers B.V. (North-Holland)
diameter, was also used for the relative light yield measurements. In the light yield measurements, the scintillations were induced by ß-particles from a radiative g°6 Ru source and were detected by two photomultiplier tubes (fig. 1). PM-171 with a MgF2 window and a semitransparent multialkali photocathode was used. The crystal, together with the PMs, were put into a sealed box and flushed with argon or air . The emission spectrum part occurring in the VUV range was estimated from the comparison of the results obtained with argon and air . The signal from one of the PMs (the top one in fig . 1) was used for pulse-height analysis, and the signal from the second one (trigger PM) was used to generate a strobe pulse for the integrating unit.
~1
106 RU PU-171
Fig. 1. Experimental facility for the light yield measurements .
A.F. Buzulutskov et al. / Crystalscintillations in the VU V region
66 0
Table 1
N ar bxni ts
Crystal
500
Density [g/cm] Radiation length [cm] Light yield [rel. unit] Light fraction with It <180 nm
400
300
200
BaF2 fast KMgF3 component
K2YF5
KLuF4
4.89
3.16
3.1
4.8
2.05
7.72
5.3
2.0
2.3
1
0.4
0.5
0
0.5
0.5
0.7
100
Fig . 2. PM-171 pulse-height spectra obtained from crystal scintillations due to the excitation by 1.5-MeV electrons. By increasing the triggering PM discrimination threshold one may select electrons of the highest energy from the ß-spectrum. The measurements performed with the KMgF3 crystal have shown that the maximum threshold corresponds to an average energy deposited in the crystal of 3 MeV [3] and the minimum threshold to 1.5 MeV, i.e., to the average energy of the ß-spectrum.. The results presented below were obtained under identical conditions at the minimum discrimination threshold . In this case, not only the scintillations themselves were observed, but also Cherenkov radiation in the range of small amplitudes, which mainly occurs at the PM's entrance windows. Its contribution was measured separately, and then it was subtracted . The measurements showed that KYbF4 does not scintillate. However, in the K2YF5 and KLuF4 crystals some scintillation was observed at a level considerably higher than the weak Cherenkov radiation, the decay time being much iess than the PM-pulse duration (<< 30 ns). The scintillation pulse-height spectra for these crystals are presented in fig. 2 together with the KMgF3 crystal sputrum . The quantum efficiency of the PM-171 photocathode changes weakly in the VLJV range. Therefore, using the peak positions of the distributions, one can estimate the relative light yield of the scintillations . Thpcp data
ara e-~PPCPntP~i
in tatela 1
ltha vctim~tinn
accuracy is 30%) together with the results of measurements of the crystal density, the part of the spectrum belonging to the VUV range and calculated values for the radiation lengths . As can be seen from the table, the most promising scintillator is KLuF4. The possibility to couple the new scintillators to a proportional wire chamber was investigated on the basis of the technique described in ref. [3] . In ;.his case the top PM under investigation in fig . 1 was replaced by a proportional chamber with a MgF2 window, working
with a CH4 +'7 .5%-TEA mixture . The scintillations of the K 2YF5 and KLuF4 crystals were detected in the chamber, and their amplitude was at a level of several photoelectrons, which is comparable to the average amplitude from the KMgF3 crystal, equal to 7 photoelectrons for 1.5 MeV deposited energy [3] . A more detailed study of the spectrum and decay kinetics of these crystals was undertaken when a 7-keV electron beam excited the crystals. To 1o so, the technique described in ref. [4] was used. The decay time of cross-luminescence was less than 2 ns (resolution limit). Fig. 3 presents the scintillation spectra for the K 2YF5 and KLuF4 crystals. From the comparison of the results from fig . 3 and table 1 one can notice, that the emission spectrum is harder for electrons with lower energies . This can be explained by the absorption of the proper radiation in the samples and the different depths of the conversion regions for 7-keV and 1 .5-MeV electrons. Cross-luminescence was also observed for the KLu 2 F7 crystal whose density was > 5 g/cm3. Its emission spectrum differs weakly from those presented in fig. 3, and the light yield is 1.5 times less than for KLuF4. In conclusion it should be noted that more detailed studies are necessary for double chemical compounds with KF, KMgF3, KCaF3 , K2YF5 , KLuF4 aid KLu 2 F7, and first of all, for their radiation resistance, trans-
.e ô 0.5
5 .0
6.0
70
6.0
E(eV)
9.0
Fig. 3. Emission spectra of the crystals obtained due to excitation by 7-keV electrons .
A.F. Buzulutskov et al. / Crystal scintillations in the VUV region
parency and decay time. Since scintillation processes in these crystals are of a cross-luminescence nature [2], their decay time should be < ns. This can serve as the basis for their application in the time-of-flight technique. The most promising scintillators for high energy physics and nuclear medicine are KMgF3 and KLuF4 , the first one for its relatively high light yield and low cost and the second one because of its small radiation length. Note that at present the technique of growing crystals from the melt is being worked out, which would make their production much cheaper and simpler. The considered crystals seem to be interesting as possible materials for lasers in the UV and VUV range. The idea of producing light crystals, scintillating in the VUV region, of the potassium--beryllium-fluoride type and solid solutions with LiF, similar to Rbx K r _x F, seems very attractive. Such crystals may be used for the construction of fast hodoscope detectors .
661
Acknowiedgements Tne authors express their gratitude to V.I . Kryshkin, V .I . Rykalin and R.M. Sulyaev for useful discussions and support of the work.
References [1] G. Charpak, V. Peskov, D. Scigocki and J. Valbis, preprint CERN-EP/89 (1989) . [2] J.A. Valbis, Z.A. Rachko and J.L. Jansons, Opt. and Spectrosc . (USSR) 64 (1988) 1196. [3] A.F. Buzulutskov, V .G. Vasilchenko and L.K. Turchanovich, IHEP, p-print 88-167 (Serpukhov, 1988). [4] J.L. Jansons and Z.A. Rachko, Phys. Status Solidi A53 (1979) 121.
659
Letter to the Editor
INVESTIGATION OF THE CRYSTAL SCINTILLATIONS IN THE VUV REGION A.F. BUZULUTSKOV, L.K. TURCHANOVICH and V.G. VASILCHENKO Institute for High Energy Physics, Serpukhov, USSR
Y.L. JANSONS, Z.A. RACHKO and V.J. VALBIS Institute of Solid-State Physics, Riga, USSR
N.M. KHAIDUKOV
Institute of Common and Inorganic Chemistry, Moscow, USSR
Received 6 October 1989
Some characteristics of the new crystal scintillators K2YF5 , KLuF4 and KLu 2 F7 emitting in the vacuum ultraviolet region have been studied.
Solid scintillation proportional counters (SSPC), combining high energy resolution of scintillation detectors and convenience of the wire chamber readout, have recently been widely used in high energy physics and nuclear medicine (see, for example, ref. [11). Therefore, the search for new solid scintillators, emitting in the sensitivity range of the photoionizing vapours TMAE, thriethylamine (TEA), ethylferrocene, etc ., is of utmost importance . The scintillators emitting in the TEA sensitivity range (i.e., in the VÜV region) may turn out to be most promising, since TEA is very easy to handle and has a high vapour pressure at room temperature. The spectra of the first solid scintillators, KMgF3 and KCaF3, emitting in the VUV region, are presented in ref. [2] . The mechanism of emission is also described in this paper and the authors call it cross-luminescence. KMgF3 crystal light yield measurements have been reported in ref. [3], and the possibility of its coupling to a proportional chamber, working with a CH4 + TEA mixture, has been shown. However, the radiation lengths of KMgF3 and KCaF3 are rather large (7 cm). Therefore the search for scintillators with a higher average atomic number is of great interest . In the present work we studied K 2YF5 , KLuF4, KLu 2F7 , and KYbF4 crystals which belong to the double chemical compound of KF with fluorides of rareearth elements. To obtain the crystals we used the hydrothermal procedure. The crystals had an irregular shape and their characteristic sizes were several millimeters. The KMgF3 crystal, 7.5 mm high and 12 mm in 0168-9002;90/$03 .50 © Elsevier Science Publishers B.V. (North-Holland)
diameter, was also used for the relative light yield measurements. In the light yield measurements, the scintillations were induced by ß-particles from a radiative g°6 Ru source and were detected by two photomultiplier tubes (fig. 1). PM-171 with a MgF2 window and a semitransparent multialkali photocathode was used. The crystal, together with the PMs, were put into a sealed box and flushed with argon or air . The emission spectrum part occurring in the VUV range was estimated from the comparison of the results obtained with argon and air . The signal from one of the PMs (the top one in fig . 1) was used for pulse-height analysis, and the signal from the second one (trigger PM) was used to generate a strobe pulse for the integrating unit.
~1
106 RU PU-171
Fig. 1. Experimental facility for the light yield measurements .
A.F. Buzulutskov et al. / Crystalscintillations in the VU V region
66 0
Table 1
N ar bxni ts
Crystal
500
Density [g/cm] Radiation length [cm] Light yield [rel. unit] Light fraction with It <180 nm
400
300
200
BaF2 fast KMgF3 component
K2YF5
KLuF4
4.89
3.16
3.1
4.8
2.05
7.72
5.3
2.0
2.3
1
0.4
0.5
0
0.5
0.5
0.7
100
Fig . 2. PM-171 pulse-height spectra obtained from crystal scintillations due to the excitation by 1.5-MeV electrons. By increasing the triggering PM discrimination threshold one may select electrons of the highest energy from the ß-spectrum. The measurements performed with the KMgF3 crystal have shown that the maximum threshold corresponds to an average energy deposited in the crystal of 3 MeV [3] and the minimum threshold to 1.5 MeV, i.e., to the average energy of the ß-spectrum.. The results presented below were obtained under identical conditions at the minimum discrimination threshold . In this case, not only the scintillations themselves were observed, but also Cherenkov radiation in the range of small amplitudes, which mainly occurs at the PM's entrance windows. Its contribution was measured separately, and then it was subtracted . The measurements showed that KYbF4 does not scintillate. However, in the K2YF5 and KLuF4 crystals some scintillation was observed at a level considerably higher than the weak Cherenkov radiation, the decay time being much iess than the PM-pulse duration (<< 30 ns). The scintillation pulse-height spectra for these crystals are presented in fig. 2 together with the KMgF3 crystal sputrum . The quantum efficiency of the PM-171 photocathode changes weakly in the VLJV range. Therefore, using the peak positions of the distributions, one can estimate the relative light yield of the scintillations . Thpcp data
ara e-~PPCPntP~i
in tatela 1
ltha vctim~tinn
accuracy is 30%) together with the results of measurements of the crystal density, the part of the spectrum belonging to the VUV range and calculated values for the radiation lengths . As can be seen from the table, the most promising scintillator is KLuF4. The possibility to couple the new scintillators to a proportional wire chamber was investigated on the basis of the technique described in ref. [3] . In ;.his case the top PM under investigation in fig . 1 was replaced by a proportional chamber with a MgF2 window, working
with a CH4 +'7 .5%-TEA mixture . The scintillations of the K 2YF5 and KLuF4 crystals were detected in the chamber, and their amplitude was at a level of several photoelectrons, which is comparable to the average amplitude from the KMgF3 crystal, equal to 7 photoelectrons for 1.5 MeV deposited energy [3] . A more detailed study of the spectrum and decay kinetics of these crystals was undertaken when a 7-keV electron beam excited the crystals. To 1o so, the technique described in ref. [4] was used. The decay time of cross-luminescence was less than 2 ns (resolution limit). Fig. 3 presents the scintillation spectra for the K 2YF5 and KLuF4 crystals. From the comparison of the results from fig . 3 and table 1 one can notice, that the emission spectrum is harder for electrons with lower energies . This can be explained by the absorption of the proper radiation in the samples and the different depths of the conversion regions for 7-keV and 1 .5-MeV electrons. Cross-luminescence was also observed for the KLu 2 F7 crystal whose density was > 5 g/cm3. Its emission spectrum differs weakly from those presented in fig. 3, and the light yield is 1.5 times less than for KLuF4. In conclusion it should be noted that more detailed studies are necessary for double chemical compounds with KF, KMgF3, KCaF3 , K2YF5 , KLuF4 aid KLu 2 F7, and first of all, for their radiation resistance, trans-
.e ô 0.5
5 .0
6.0
70
6.0
E(eV)
9.0
Fig. 3. Emission spectra of the crystals obtained due to excitation by 7-keV electrons .
A.F. Buzulutskov et al. / Crystal scintillations in the VUV region
parency and decay time. Since scintillation processes in these crystals are of a cross-luminescence nature [2], their decay time should be < ns. This can serve as the basis for their application in the time-of-flight technique. The most promising scintillators for high energy physics and nuclear medicine are KMgF3 and KLuF4 , the first one for its relatively high light yield and low cost and the second one because of its small radiation length. Note that at present the technique of growing crystals from the melt is being worked out, which would make their production much cheaper and simpler. The considered crystals seem to be interesting as possible materials for lasers in the UV and VUV range. The idea of producing light crystals, scintillating in the VUV region, of the potassium--beryllium-fluoride type and solid solutions with LiF, similar to Rbx K r _x F, seems very attractive. Such crystals may be used for the construction of fast hodoscope detectors .
661
Acknowiedgements Tne authors express their gratitude to V.I . Kryshkin, V .I . Rykalin and R.M. Sulyaev for useful discussions and support of the work.
References [1] G. Charpak, V. Peskov, D. Scigocki and J. Valbis, preprint CERN-EP/89 (1989) . [2] J.A. Valbis, Z.A. Rachko and J.L. Jansons, Opt. and Spectrosc . (USSR) 64 (1988) 1196. [3] A.F. Buzulutskov, V .G. Vasilchenko and L.K. Turchanovich, IHEP, p-print 88-167 (Serpukhov, 1988). [4] J.L. Jansons and Z.A. Rachko, Phys. Status Solidi A53 (1979) 121.