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Preparation of cesium targets for gamma-spectroscopic studies
Nuclear Instruments and Methods in Physics Research B 171 (2000) 406±408
www.elsevier.nl/locate/nimb
Letter to the Editor
Preparation of cesium targets for gamma-spectroscopic studies S. Bhattacharyya a
a,1
, S.K. Basu
a,*
, S. Chanda a, P. Deb a, Md. Eqbal a, S. Kundu a, D. Joseph b
Department of Atomic Energy, Variable Energy Cyclotron Centre, Section-1, Block-AF, Bidhan Nagar, Calcutta 70064, India b Bhabha Atomic Research Centre, Mumbai 400085, India Received 5 May 2000; received in revised form 15 August 2000
Abstract A procedure to prepare monoisotopic cesium compound targets for gamma-spectroscopic experiments is described. Using this procedure, uniform targets up to thicknesses of 0.6±1.2 mg/cm2 were prepared and used for in-beam spectroscopic studies. The purity of the target was tested by energy dispersive X-ray ¯uorescence (EDXRF) measurements. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 29.25.)t; 81.15.g Keywords: Cesium; Nitrate; Compound target; vacuum deposition; Energy dispersive X-ray ¯uorescence
The high spin spectroscopy of many odd±even and odd±odd nuclei in the A 140 region has not been possible due to lack of suitable target± projectile combinations. One is rather constrained to use light heavy-ion projectiles with Z 6 8 and a limited set of targets around A 130 for the production of those nuclei with reasonable crosssections. The preparation of uniform cesium targets of thicknesses in the range 0.5±1.5 mg/cm2 is, thus, considered important for such investigations. Cesium is a monoisotopic element with
*
Corresponding author. Tel.: +91-33-337-1231; fax: +91-33334-6871. E-mail address: [email protected] (S.K. Basu). 1 Member, Radiation Safety and Systems Division, BARC, Mumbai, India.
Z 55 and A 133, belonging to Group I alkali metals of the periodic table of elements. It is highly reactive like sodium, having low melting point (28.4°C) [1] and is, therefore, unstable in normal atmospheric conditions. The preparation of selfsupporting or backed cesium target in elemental form is thus impossible and the target makers have always recommended the use of compound targets. In an earlier review by Muggleton [2], mention was made of cesium bromide targets, prepared by vacuum evaporation on thin backings. However, no details were available. In a recent article by Adair and Kobisk [3], it has been mentioned that targets of cesium on metal backing, with thickness range 10±1000 lg/cm2 , were prepared by the Isotope Research Materials Laboratory at ORNL, again by evaporation of a cesium salt. Though cesium halides have reasonable
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 4 2 6 - 2
S. Bhattacharyya et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 406±408
melting and boiling points (approximately in the range 625±680°C and 1250±1290°C, respectively) [1] and are very stable during vacuum evaporation, the halide targets are not suitable because of the likely contamination of the spectra from nuclear reactions on elements, such as chlorine, bromine and iodine. The other stable compounds, viz., the cesium oxides and nitrates have relatively lower melting points (around 400°C). However, these compounds decompose prior to boiling and, hence, the targets produced by vacuum evaporation of oxides and nitrates are not stable under normal atmospheric conditions. In this short note, we shall describe a method to prepare CsNO3 targets on a suitable high-Z backing, such as gold, by evaporation under controlled conditions. Such targets have been used by us recently for in-beam spectroscopic studies of near-spherical prometheum nuclei, close to A 140 [4,5], using 12;13 C projectiles. To our knowledge, there exists no such published report in the literature on the preparation of stable CsNO3 targets for gamma-spectroscopic studies, except one by Parker and Slatis [6], which relates to preparation of radioactive CsNO3 source on Al-backing for b-spectroscopy. In the present work, we have used ultraspecpure (99.99%) CsNO3 compound, supplied by Aldrich, USA, which is a very stable compound at room temperature and is slightly hygroscopic. As it decomposes before its boiling point is reached, it is, therefore, necessary to employ special technique so that there is no change in the composition of the CsNO3 sample during evaporation. In order to achieve this goal, we have adopted the following procedure before evaporating the sample. The chamber of the vacuum evaporation unit is ®rst evacuated to very high vacuum of the order of 10ÿ7 mbar pressure and dry nitrogen gas is ¯ushed in between quite a few times to minimise the amount of trapped oxygen and water molecules inside the chamber. The desired amount (approximately 5±6 mg) of the specpure CsNO3 powder, which has been put inside the chamber beforehand on an alumina-coated molybdenum boat, is then heated for 30 min under high vacuum of the order of 10ÿ7 mbar pressure in a controlled manner so that the temperature of the sample never exceeds 300± 310°C. After this, the sample is allowed to cool
407
down to room temperature under vacuum and dry nitrogen gas is again ¯ushed in the chamber to bring the chamber pressure to about 10ÿ5 mbar. At this pressure, the sample is heated slowly till the temperature reaches 490±500°C, when the sample melts without decomposition. At this point, it is and the evaporated in one go at the rate of 10A/s thickness is monitored. When the desired thickness is achieved, the heater power is switched o and the chamber is allowed to cool down to room temperature. It is found that at 10ÿ5 mbar pressure with the residual gas being predominantly nitrogen, there is absolutely no decomposition of CsNO3 sample and a good uniform deposit of CsNO3 takes place on the backing. Very uniform targets of CsNO3 of dierent thicknesses between 0.6 and 1.2 mg/cm2 , have been prepared following the above steps on gold backing of dierent thicknesses, mounted on a suitable target frame. As mentioned earlier, the targets were successfully used in in-beam spectroscopic studies [4,5] of 142;143 Pm using high-purity germanium detectors. It has been seen that there are not many strong contaminant peaks in the spectrum, except few arising from coulomb excitation of gold as well as from Au + C reaction. An energy dispersive X-ray ¯uorescence (EDXRF) spectrum (Fig. 1) was taken in order to ®nd out the trace amount of
Fig. 1. EDXRF spectrum of the cesium target used in the inbeam experiment, as referred to in the text.
408
S. Bhattacharyya et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 406±408
impurities, which might have gone into the target during evaporation. The spectra does not show many X-ray peaks except the ones coming from gold and cesium, which are appropriately labelled. The targets, after use, have been kept under normal atmospheric conditions and no signi®cant visual deteoriation has been observed. We conclude that the procedure followed by us to prepare CsNO3 targets by evaporation at 10ÿ5 mbar pressure with residual nitrogen gas inside the vacuum evaporation chamber, gives stable, uniform-backed targets suitable for gammaspectroscopic investigations. Acknowledgements S.B and S.K.B. acknowledge with thanks very fruitful discussion with Mr. D.C. Epraim of TIFR, Mumbai.
References [1] R.C. Weast (Ed.), Handbook of Chemistry and Physics, CRC Press, Cleveland, OH, 1979. [2] A.H.F. Muggleton, J. Phys. E. 12 (1979) 780. [3] H.L. Adair, E.H. Kobisk, Treatise on Heavy-Ion Science, in: D.A. Bromley (Ed.), Vol. 7, Plenum, New York, 1985, p. 119. [4] R. Palit et al., Nucl. Instr. and Meth. A 443 (2000) 386. [5] Sarmishtha Bhattacharyya et al., in: Proceedings of the Seventh International Conference on Nucleus±Nucleus Collisions (NN-2000), Strasbourg, France, July 2000. [6] W.C. Parker, H. Slatis, in: K. Siegbahn (Ed.), Alpha, Beta and Gamma-ray Spectroscopy, North-Holland, Amsterdam, 1965, p. 379.
www.elsevier.nl/locate/nimb
Letter to the Editor
Preparation of cesium targets for gamma-spectroscopic studies S. Bhattacharyya a
a,1
, S.K. Basu
a,*
, S. Chanda a, P. Deb a, Md. Eqbal a, S. Kundu a, D. Joseph b
Department of Atomic Energy, Variable Energy Cyclotron Centre, Section-1, Block-AF, Bidhan Nagar, Calcutta 70064, India b Bhabha Atomic Research Centre, Mumbai 400085, India Received 5 May 2000; received in revised form 15 August 2000
Abstract A procedure to prepare monoisotopic cesium compound targets for gamma-spectroscopic experiments is described. Using this procedure, uniform targets up to thicknesses of 0.6±1.2 mg/cm2 were prepared and used for in-beam spectroscopic studies. The purity of the target was tested by energy dispersive X-ray ¯uorescence (EDXRF) measurements. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 29.25.)t; 81.15.g Keywords: Cesium; Nitrate; Compound target; vacuum deposition; Energy dispersive X-ray ¯uorescence
The high spin spectroscopy of many odd±even and odd±odd nuclei in the A 140 region has not been possible due to lack of suitable target± projectile combinations. One is rather constrained to use light heavy-ion projectiles with Z 6 8 and a limited set of targets around A 130 for the production of those nuclei with reasonable crosssections. The preparation of uniform cesium targets of thicknesses in the range 0.5±1.5 mg/cm2 is, thus, considered important for such investigations. Cesium is a monoisotopic element with
*
Corresponding author. Tel.: +91-33-337-1231; fax: +91-33334-6871. E-mail address: [email protected] (S.K. Basu). 1 Member, Radiation Safety and Systems Division, BARC, Mumbai, India.
Z 55 and A 133, belonging to Group I alkali metals of the periodic table of elements. It is highly reactive like sodium, having low melting point (28.4°C) [1] and is, therefore, unstable in normal atmospheric conditions. The preparation of selfsupporting or backed cesium target in elemental form is thus impossible and the target makers have always recommended the use of compound targets. In an earlier review by Muggleton [2], mention was made of cesium bromide targets, prepared by vacuum evaporation on thin backings. However, no details were available. In a recent article by Adair and Kobisk [3], it has been mentioned that targets of cesium on metal backing, with thickness range 10±1000 lg/cm2 , were prepared by the Isotope Research Materials Laboratory at ORNL, again by evaporation of a cesium salt. Though cesium halides have reasonable
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 4 2 6 - 2
S. Bhattacharyya et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 406±408
melting and boiling points (approximately in the range 625±680°C and 1250±1290°C, respectively) [1] and are very stable during vacuum evaporation, the halide targets are not suitable because of the likely contamination of the spectra from nuclear reactions on elements, such as chlorine, bromine and iodine. The other stable compounds, viz., the cesium oxides and nitrates have relatively lower melting points (around 400°C). However, these compounds decompose prior to boiling and, hence, the targets produced by vacuum evaporation of oxides and nitrates are not stable under normal atmospheric conditions. In this short note, we shall describe a method to prepare CsNO3 targets on a suitable high-Z backing, such as gold, by evaporation under controlled conditions. Such targets have been used by us recently for in-beam spectroscopic studies of near-spherical prometheum nuclei, close to A 140 [4,5], using 12;13 C projectiles. To our knowledge, there exists no such published report in the literature on the preparation of stable CsNO3 targets for gamma-spectroscopic studies, except one by Parker and Slatis [6], which relates to preparation of radioactive CsNO3 source on Al-backing for b-spectroscopy. In the present work, we have used ultraspecpure (99.99%) CsNO3 compound, supplied by Aldrich, USA, which is a very stable compound at room temperature and is slightly hygroscopic. As it decomposes before its boiling point is reached, it is, therefore, necessary to employ special technique so that there is no change in the composition of the CsNO3 sample during evaporation. In order to achieve this goal, we have adopted the following procedure before evaporating the sample. The chamber of the vacuum evaporation unit is ®rst evacuated to very high vacuum of the order of 10ÿ7 mbar pressure and dry nitrogen gas is ¯ushed in between quite a few times to minimise the amount of trapped oxygen and water molecules inside the chamber. The desired amount (approximately 5±6 mg) of the specpure CsNO3 powder, which has been put inside the chamber beforehand on an alumina-coated molybdenum boat, is then heated for 30 min under high vacuum of the order of 10ÿ7 mbar pressure in a controlled manner so that the temperature of the sample never exceeds 300± 310°C. After this, the sample is allowed to cool
407
down to room temperature under vacuum and dry nitrogen gas is again ¯ushed in the chamber to bring the chamber pressure to about 10ÿ5 mbar. At this pressure, the sample is heated slowly till the temperature reaches 490±500°C, when the sample melts without decomposition. At this point, it is and the evaporated in one go at the rate of 10A/s thickness is monitored. When the desired thickness is achieved, the heater power is switched o and the chamber is allowed to cool down to room temperature. It is found that at 10ÿ5 mbar pressure with the residual gas being predominantly nitrogen, there is absolutely no decomposition of CsNO3 sample and a good uniform deposit of CsNO3 takes place on the backing. Very uniform targets of CsNO3 of dierent thicknesses between 0.6 and 1.2 mg/cm2 , have been prepared following the above steps on gold backing of dierent thicknesses, mounted on a suitable target frame. As mentioned earlier, the targets were successfully used in in-beam spectroscopic studies [4,5] of 142;143 Pm using high-purity germanium detectors. It has been seen that there are not many strong contaminant peaks in the spectrum, except few arising from coulomb excitation of gold as well as from Au + C reaction. An energy dispersive X-ray ¯uorescence (EDXRF) spectrum (Fig. 1) was taken in order to ®nd out the trace amount of
Fig. 1. EDXRF spectrum of the cesium target used in the inbeam experiment, as referred to in the text.
408
S. Bhattacharyya et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 406±408
impurities, which might have gone into the target during evaporation. The spectra does not show many X-ray peaks except the ones coming from gold and cesium, which are appropriately labelled. The targets, after use, have been kept under normal atmospheric conditions and no signi®cant visual deteoriation has been observed. We conclude that the procedure followed by us to prepare CsNO3 targets by evaporation at 10ÿ5 mbar pressure with residual nitrogen gas inside the vacuum evaporation chamber, gives stable, uniform-backed targets suitable for gammaspectroscopic investigations. Acknowledgements S.B and S.K.B. acknowledge with thanks very fruitful discussion with Mr. D.C. Epraim of TIFR, Mumbai.
References [1] R.C. Weast (Ed.), Handbook of Chemistry and Physics, CRC Press, Cleveland, OH, 1979. [2] A.H.F. Muggleton, J. Phys. E. 12 (1979) 780. [3] H.L. Adair, E.H. Kobisk, Treatise on Heavy-Ion Science, in: D.A. Bromley (Ed.), Vol. 7, Plenum, New York, 1985, p. 119. [4] R. Palit et al., Nucl. Instr. and Meth. A 443 (2000) 386. [5] Sarmishtha Bhattacharyya et al., in: Proceedings of the Seventh International Conference on Nucleus±Nucleus Collisions (NN-2000), Strasbourg, France, July 2000. [6] W.C. Parker, H. Slatis, in: K. Siegbahn (Ed.), Alpha, Beta and Gamma-ray Spectroscopy, North-Holland, Amsterdam, 1965, p. 379.