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Coulomb fragmentation of InCl and InI
Volume
31B, number 2
PHYSICS
COULOMB
LETTERS
FRAGMENTATION
19 January 1970
OF
InCl
AND
In1
M. SCHUMACHER, J. WEISS * and H. LANGHOFF ** II. Physikalisches Institut der Unioewitaet Goettingen, Germany Received
15 December
1969
The molecules of InCl and In1 break up by electron capture in I141nm. The energy distributions of the Cd-fragments were investigated by resonance scattering of the 538 keV y quanta emitted after the electron capture.
Initiated by electron capture or beta decay the Coulomb fragmentation of molecules [1,2] leads to fragments having considerable velocities. Since very little is known about the velocity spectra, Cd-fragments from InCl and In1 molecules were investigated. Following the electron capture in I141nm a y-y cascade with E.,,l = 724 keV and Ey2 = 558 keV is emitted. Information on the velocity of the Cd-fragments is contained in the shapes of the y lines. The shape of y2, supplied by gaseous sources of InCl or In1 was investigated by the observation of resonance scattering of ~2 by cadmium in ccincidence with the preceding yl (fig. 1). The emission of yl leads to a Doppler shift onY2 of AE = -ErlEr2 cos~$/m,c~, where @ is the angle between yl and ~2, m, the mass of the molecule, or in case of break-up, the mass of the radioactive fragment. In order to achieve coincidence of the centers of the 558 keV emission and absorption lines one should have cos C$= - m, Er2 /maEyl, with ma the mass of the radioactive
atom.
By variation
of cos@
the
shape of the emission line can be scanned in the region between about 0 eV and 6 eV off the center. The sources were prepared by combining indium metal of high specific activity with chlorine and iodine, respectively. An appreciable surplus of In was provided in order to ascertain the formation of the monohalides. The sources were distilled into quartz ampoules and heated to 6400C and 76OoC, respectively. A uniform distribution of the activity in the ampoules indicated that the sources were completely vaporized. * Present address: ERNO Raumfahrttechnik GmbH, Bremen, Germany. ** Present address: Bartol Research Foundation, Swarthmore, Pa, USA.
For each source the cross sections of resonant scattering for nine different angles 6 were determined. From singles measurements it was ascertained that no decomposition of the source took place during the measurements. Since the lifetime of the 558 keV level is known from Coulomb excitation experiments [3] to be 7-2 = 13.2 f 0.6 ps, the experimental cross sections allow the calculation of N(E)dE, the number of y quanta emitted by the source into the energy interval dE around E. The results for the different angles are shown in fig. 2. Previous investigations [e.g.41 proved that after electron capture the molecule breaks up in almost all cases. This justifies to place m, = ms, hence, the cos 6 scale in fig. 2 may be converted into the energy scale given below. In order to discuss the results, we first assume that the source consisted of 1141nm atoms. Then, N(E) to be expected can be calculated (fig, 2, curve a). The rectangular shape of the y emission line due to the neutrino recoil is distorted by the thermal motion of the absorbing and emitting nuclei and by the angular resolution of the experimental device. The experimental points clearly indicate a much broader line, which can only be attributed to the effect of Coulomb explosion of the molecules. For a simple description it will be assumed that the time for the emission of all Auger electrons is much shorter than the lifetime 51, ~1 = 2 ps of the second excited state in I 14Cd. If the Cd-atom has received a charge 21, the halogen atom a charge 22 and the average distance of the atoms [S] at the explosion is d = 2.75 A for In1 and d = 2.40 A for InCl, the final velocity of the Cd fragment will lead to a width of the 558 keV emission line of AE Clb = 2 (~-2 with CY= 4.0 eV and 2.9 eV, 61
Volume 31B. number 2
PHYSICS
LETTERS
19
Cd-SCATTERER OR COMPARISONSn-SCATTERER
i
January 1970
InN
Fig. 1. Experimental arrangement. respectively, Since the Cd fragment gains 90% of its final velocity within 1 ps after charging, which is short compared to ~2 = 13 ps, the maximum hEclb will be observable in the experiment. Consequently, if P(Zz, Z,), i.e. the probability for the creation of 21 and 22, is known, N(E) can be calculated. Measurements of the charge distribution after electron capture and internal conversion have been performed for the noble gas nuclei 7QKr (ref. 7) and *sIXem (ref. 8). By a rough interpolation the following char e distribution, PCd(Zl), for an isolated lf4Cd-atom was obtained: PCd = 1.70/r, l.Qo/,, 4.10/r, 4.5%, lO.S%, 17.0%, 21.5%, 16.5%, lO.S%, 5.70/c, 2.70/c, 1.50/c, 0.90/c, 0.8% for Zl = 0, 1, 2, etc. , respectively. The N(E) shown in fig. 2, curve b, was Calculated by assuming P(Z1,22) = PCd(Zl+l) for Z2=landP(Z~,Z2)=OforZ2f, i.e., the Auger cascade proceeds uninfluenced by the halogen atom and thereafter one charge is transferred to the halogen atom. Curve c represents the maximum broadening obtainable from the initial charge distribution Pcd(Z1) by assuming that half of the charges are transferred to the halogen atom. Finally, to obtain curve d the width of ~2 was further increased by assuming that the presence of the halogen atom raises the total number of charges created by 50%. The experimental results seem to favour the assumptions of curve c or d indicating that the halogen atom becomes highly charged. Probably, more charges are created in the Cd-halide than in the Cd-atom. A detailed investigation of the
*****
62
ll
0
-1
ll
2
4
6
cos# 9
Fig. 2. Energy distribution, N(E), quanta.
IO E[eVl
of the 558 keV
charge spectrum is necessary for a more quantitative interpretation of the present results. The authors are indebted to Professor Dr. A. Flammersfeid for his interest and support of this investigation.
References 1. -F. R. Metzger, Phys. Rev. Letters 18(1967) 434; Phys. Ref. 171 (1968) 1257. 2. M. Berman and &. B.‘Beard, Phys. Rev. Letters 22 (1969) 753. 3. W. T. Milner et al., Nucl. Phys. A129 (1969) 687. 4. A. H. Snell, in Alpha, beta-, and gamma-ray spectroscopy, ed. K. Siegbahn (North-Holland, Amsterdam, 1965). F. K. MC Gowan, R. L. Robinson, P. H. Stelson and J. L. C. Ford, Nucl. Phys. 66 (1965) 97. A. H. Barrett and M. Mandel, Phys. Rev. 109 (1958) 1572. A. H. Snell, F. Pleasonton and S. L. Need, Phys. Rev. 116 (1959)1548. F. Pleasonton and A. H. Snell, Proc. Roy. Sot. A241 (1957) 141.
31B, number 2
PHYSICS
COULOMB
LETTERS
FRAGMENTATION
19 January 1970
OF
InCl
AND
In1
M. SCHUMACHER, J. WEISS * and H. LANGHOFF ** II. Physikalisches Institut der Unioewitaet Goettingen, Germany Received
15 December
1969
The molecules of InCl and In1 break up by electron capture in I141nm. The energy distributions of the Cd-fragments were investigated by resonance scattering of the 538 keV y quanta emitted after the electron capture.
Initiated by electron capture or beta decay the Coulomb fragmentation of molecules [1,2] leads to fragments having considerable velocities. Since very little is known about the velocity spectra, Cd-fragments from InCl and In1 molecules were investigated. Following the electron capture in I141nm a y-y cascade with E.,,l = 724 keV and Ey2 = 558 keV is emitted. Information on the velocity of the Cd-fragments is contained in the shapes of the y lines. The shape of y2, supplied by gaseous sources of InCl or In1 was investigated by the observation of resonance scattering of ~2 by cadmium in ccincidence with the preceding yl (fig. 1). The emission of yl leads to a Doppler shift onY2 of AE = -ErlEr2 cos~$/m,c~, where @ is the angle between yl and ~2, m, the mass of the molecule, or in case of break-up, the mass of the radioactive fragment. In order to achieve coincidence of the centers of the 558 keV emission and absorption lines one should have cos C$= - m, Er2 /maEyl, with ma the mass of the radioactive
atom.
By variation
of cos@
the
shape of the emission line can be scanned in the region between about 0 eV and 6 eV off the center. The sources were prepared by combining indium metal of high specific activity with chlorine and iodine, respectively. An appreciable surplus of In was provided in order to ascertain the formation of the monohalides. The sources were distilled into quartz ampoules and heated to 6400C and 76OoC, respectively. A uniform distribution of the activity in the ampoules indicated that the sources were completely vaporized. * Present address: ERNO Raumfahrttechnik GmbH, Bremen, Germany. ** Present address: Bartol Research Foundation, Swarthmore, Pa, USA.
For each source the cross sections of resonant scattering for nine different angles 6 were determined. From singles measurements it was ascertained that no decomposition of the source took place during the measurements. Since the lifetime of the 558 keV level is known from Coulomb excitation experiments [3] to be 7-2 = 13.2 f 0.6 ps, the experimental cross sections allow the calculation of N(E)dE, the number of y quanta emitted by the source into the energy interval dE around E. The results for the different angles are shown in fig. 2. Previous investigations [e.g.41 proved that after electron capture the molecule breaks up in almost all cases. This justifies to place m, = ms, hence, the cos 6 scale in fig. 2 may be converted into the energy scale given below. In order to discuss the results, we first assume that the source consisted of 1141nm atoms. Then, N(E) to be expected can be calculated (fig, 2, curve a). The rectangular shape of the y emission line due to the neutrino recoil is distorted by the thermal motion of the absorbing and emitting nuclei and by the angular resolution of the experimental device. The experimental points clearly indicate a much broader line, which can only be attributed to the effect of Coulomb explosion of the molecules. For a simple description it will be assumed that the time for the emission of all Auger electrons is much shorter than the lifetime 51, ~1 = 2 ps of the second excited state in I 14Cd. If the Cd-atom has received a charge 21, the halogen atom a charge 22 and the average distance of the atoms [S] at the explosion is d = 2.75 A for In1 and d = 2.40 A for InCl, the final velocity of the Cd fragment will lead to a width of the 558 keV emission line of AE Clb = 2 (~-2 with CY= 4.0 eV and 2.9 eV, 61
Volume 31B. number 2
PHYSICS
LETTERS
19
Cd-SCATTERER OR COMPARISONSn-SCATTERER
i
January 1970
InN
Fig. 1. Experimental arrangement. respectively, Since the Cd fragment gains 90% of its final velocity within 1 ps after charging, which is short compared to ~2 = 13 ps, the maximum hEclb will be observable in the experiment. Consequently, if P(Zz, Z,), i.e. the probability for the creation of 21 and 22, is known, N(E) can be calculated. Measurements of the charge distribution after electron capture and internal conversion have been performed for the noble gas nuclei 7QKr (ref. 7) and *sIXem (ref. 8). By a rough interpolation the following char e distribution, PCd(Zl), for an isolated lf4Cd-atom was obtained: PCd = 1.70/r, l.Qo/,, 4.10/r, 4.5%, lO.S%, 17.0%, 21.5%, 16.5%, lO.S%, 5.70/c, 2.70/c, 1.50/c, 0.90/c, 0.8% for Zl = 0, 1, 2, etc. , respectively. The N(E) shown in fig. 2, curve b, was Calculated by assuming P(Z1,22) = PCd(Zl+l) for Z2=landP(Z~,Z2)=OforZ2f, i.e., the Auger cascade proceeds uninfluenced by the halogen atom and thereafter one charge is transferred to the halogen atom. Curve c represents the maximum broadening obtainable from the initial charge distribution Pcd(Z1) by assuming that half of the charges are transferred to the halogen atom. Finally, to obtain curve d the width of ~2 was further increased by assuming that the presence of the halogen atom raises the total number of charges created by 50%. The experimental results seem to favour the assumptions of curve c or d indicating that the halogen atom becomes highly charged. Probably, more charges are created in the Cd-halide than in the Cd-atom. A detailed investigation of the
*****
62
ll
0
-1
ll
2
4
6
cos# 9
Fig. 2. Energy distribution, N(E), quanta.
IO E[eVl
of the 558 keV
charge spectrum is necessary for a more quantitative interpretation of the present results. The authors are indebted to Professor Dr. A. Flammersfeid for his interest and support of this investigation.
References 1. -F. R. Metzger, Phys. Rev. Letters 18(1967) 434; Phys. Ref. 171 (1968) 1257. 2. M. Berman and &. B.‘Beard, Phys. Rev. Letters 22 (1969) 753. 3. W. T. Milner et al., Nucl. Phys. A129 (1969) 687. 4. A. H. Snell, in Alpha, beta-, and gamma-ray spectroscopy, ed. K. Siegbahn (North-Holland, Amsterdam, 1965). F. K. MC Gowan, R. L. Robinson, P. H. Stelson and J. L. C. Ford, Nucl. Phys. 66 (1965) 97. A. H. Barrett and M. Mandel, Phys. Rev. 109 (1958) 1572. A. H. Snell, F. Pleasonton and S. L. Need, Phys. Rev. 116 (1959)1548. F. Pleasonton and A. H. Snell, Proc. Roy. Sot. A241 (1957) 141.