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Energy loss fluctuations caused by Rutherford scattering
Nuclear Instruments North-Holland
and Methods
in Physics
Energy loss fluctuations
Research
349
B53 (1991) 349-351
caused by Rutherford
scattering
A.N. James Department
of Physics,
University of Liverpool,
P.O. Box 147, Liverpool L69 3BX, UK
K.A. Connell and R.A. Cunningham Science and Engineering Received
7 September
Research
Council, Daresbury
Laboratory,
1990 and in revised form 9 November
Warrington
WA4 4AD, UK
1990
An &butane filled ionisation chamber has been constructed with three anodes to provide atomic number assignments for heavy ions with energies greater than 1 MeV per nucleon. The energy loss section of the ion chamber is divided into two small sections to enable identification of events where abnormal energy loss profiles along the heavy ion track would cause false assignments. In an experiment with 49Cr ions such abnormal events were clearly identified and are shown to be due to Rutherford scattering of the 49Cr ions off the carbon nuclei of the isobutane ion chamber gas. The split energy loss section of the ion chamber enables rejection of these events to several orders of magnitude, with the rejection factor depending on the details of the heavy ion speed and type.
1. Introduction The energy loss of heavy ions moving through a gas begins to depend significantly on the atomic number (2) of the heavy ion once the speed is greater than 1 MeV/u (0.0463~). This dependence is exploited by experimentahsts who use gas ionisation chambers to assign atomic number to individual high energy heavy ions [l-3]. The resolution in Z depends on the rate of change of energy loss with Z and on the width of the the observed energy loss peak in the energy loss section of the ion chamber. The shape of the observed peak, to at least 0.01 of the peak height, is entirely consistent with single charge exchange statistics and can be modelled easily using the electron capture cross sections a, of Schlachter et al. [4] and a charge state distribution width characterised [5] by a width d, - 1.4 (the formulae of Betz [5], p. 470, have been used to relate the electron loss cross section oi to d, and a,). Empirically the atomic number dependence is reproduced by the formula of Schmidt-Backing and Homung [1,2,6]. In an experiment to search [7] for the isoto e “Zr the peak shape of the energy loss curves for “Sr and “Y, which were several thousand times more numerous, were found to have high energy shoulders on the energy loss peak for approximately 0.5% of heavy ions. This shoulder corresponded roughly to the intensity expected from Rutherford scattering from the carbon in the isobutane gas. If this is the correct explanation of the phenomenon then an ion chamber in which the energy loss section is split into two equal length parts should be able to distinguish heavy ion tracks which have been 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
subject to such a rare fluctuation and, consequently, to improve the selectivity of the ion chamber for rarely occurring higher atomic number ions. The ion chamber at the Daresbury recoil separator [l] has been modified in such a way and we report observations on 49Cr ions of 2.3 MeV/u which are in complete agreement with the pattern of energy loss expected from Rutherford scattering.
2. Three anode ion chamber signals from 49Cr The ion chamber used at the Daresbury recoil separator now has three anodes. The first two are each 50 mm long along the track of the ion and the third, to measure the total ion energy, is 200 mm long. In an experiment [S] measuring the y-ray spectroscopy of 49Mn the nucleus 49Cr was very strongly populated and a very high percentage of all in beam y-ray cascades proceeded through the 272 keV first excited state to ground state decay. The detection of a 272 keV y-ray established that a 49Cr nucleus of 2.3 MeV/u was causing the signal in the ion chamber. About one in a thousand ions selected in this fashion were either 49V or &Ti due to background in the window selecting the y-ray energy. Fig. 1 shows a density plot relating the pulse heights in the first two anode sections of the ion chamber for heavy ions selected in this way. The abnormal energy loss events, those occurring within the triangle with its apex at the 49Cr peak, clearly
B.V. (North-Holland)
A. N. James et al. / Energy loss fluctuations
350 5MeV
ANODE
ONE
40Me\
caused by Rutherford scattering
diagonal lines on fig. 1 while about 99% of the abnormal pulses can be rejected as outside the diagonal lines. Fig. 2 shows the pulse height distribution observed in the second anode section of the ion chamber for 49Cr ions which have normal energy loss in the first anode section.
40MeV
3. Rutherford scattering
15MeV
/ 0
1
30
1000
30,000
Fig. 1. Density plot relating the energy deposited in the first two anodes of the ion chamber with an isobutane filling at 30 Torr. Points within the area bounded by the two diagonal lines are the normal response for 49Cr, 49V and &Ti ions plus a streak due to pile up of pulses in the ion chamber electronics. The structure forming a triangle with its apex at the 49Cr peak is due to events where Rutherford scattering of 49Cr from 12C nuclei in the isobutane gas has caused a rare fluctuation in the energy loss profile of the heavy ions.
have the property that generally the abnormal energy loss is concentrated in one section of the ion chamber or the other. This feature is important experimentally since it allows 99% of normal events to be accepted within the
A simple Monte Carlo code modelling the energy loss process in the first two anode sections of the ion chamber has been written which uses ranges in (CH,), from the tables of Northcliffe and Schilling [9] to approximate the energy losses of 49Cr, “C, and ‘H ions in isobutane (2-methyl-propane C,H,,). Rutherford scattering for close collisions was introduced into the code by setting a mean free path of one metre corresponding at 30 Torr pressure to an impact parameter of about 0.5 pm, well inside all electron orbits. Heavy ion tracks encountering an interaction were chosen randomly to be either scattering from protons or carbon nuclei and with a properly weighted impact parameter distribution. Energy losses were then computed using the new ion energies and angles. The dimension of the ion chamber in the direction perpendicular to the original track was taken to be 60 mm and if any track passed through this side wall of the chamber it was rejected since it could not have registered a correct total energy signal. Scattering from protons produces a strongly forward peaked proton distribution whose energy loss rate is insignificant in relation to the energy loss rate for 49Cr ions (effective charge - 15). Central collisions on carbon
I”
loo7 20
25
Energy
(MeV)
30
35
Fig. 2. The pulse height observed in the second anode for those pulses which have a normal signal for 49Cr in the first anode. process producing the abnormal pulse heights must occur within the second anode zone of the ion chamber.
The
A. N. James et al. / Energy loss fluctuations
caused by Rutherford scattering
351
106 .
l
. . . 104-. v)
z :
.
0
. l
.
102--
l
*
l
* l
’ .
. . .
Energy Fig. 3. Simulated pulse height distribution for the second anode where the energy loss in the first anode had been normal. The resolution used in the simulation is that expected from single charge exchange processes. The energy scale does not match that of fig. 2 exactly due to systematic errors in the stopping powers. occurring with low probability also produce a strong forward distribution of ions which, almost fortuitously, mimics the energy loss profile of 49Cr and would be experimentally undetected. The abnormal energy loss events are caused by scattering at centre of mass angles from 0.1 to 1 rad for which the laboratory angle of the carbon recoils is nearly perpendicular to the 49Cr track. Fig. 3 shows a calculated pulse height distribution which has been folded with a resolution function to simulate the charge exchange statistics. It can be seen that the shape of the distribution matches the observed distribution. The cut off at large pulse heights in the simulation can be changed by changing the distance to the chamber side wall. The simulation reproduces the distribution observed in fig. 1 where for a small fraction of events both anode signals are abnormal but the combined excess signal has a limited size. These events are those where the carbon recoil, near perpendicular to the 49Cr track actually crosses the boundary between the two anodes. Some events will necessarily lie between the diagonal boundaries marked on fig. 1 and it will not be possible to discriminate these events from an event with larger atomic number using a simple three anode ion chamber. nuclei
4. Summary It has been shown that Rutherford scattering from heavy nuclei in an ion chamber gas causes a few events to have abnormal energy loss profiles with well localised, much increased, excess energy deposition. A simple three anode ion chamber is sufficient to discriminate against these events giving a greatly enhanced signal to
background ions.
sensitivity
for rare high atomic number
Acknowledgements This work was supported by grants from the United Kingdom Science and Engineering Research Council.
References PI A.N. James, T.P. Morrison,
K.L. Ying, K.A. Connell, H.G. Price and J. Simpson, Nucl. Instr. and Meth. A267 (1988) 144. PI J.P. Bouquet, R. Brissot and H.R. Faust, Nucl. Instr. and Meth. A267 (1988) 466. H.-H. Knitter, Ch. Straede, F.-J. 131 C. Budtz-Jorgensen, Hambsch and R. Vogt, Nucl. Instr. and Meth. A258 (1987) 209. 141 AS. Schlachter, J.W. Steams, W.G. Graham, K.H. Berkner, R.V. Pyle and J.A. Tams, Phys. Rev. A27 (1983) 3372. PI H.D. Betz, Rev. Mod. Phys. 44 (1972) 465. and H. Homung, 2. Phys. A286 (1978) WI H. Schmidt-Backing 253. [71 C.J. Lister, M. Campbell, A.A. Chishti, W. Gelletly, L. Goettig, R. Moscrop, B.J. Varley, A.N. James, T. Morrison, H.G. Price, J. Simpson, K.A. Connell and 0. Skeppstedt, Phys. Rev. Lett. 59 (1987) 1270. M.A. Bentley, A.M. Bruce, R.A. CunPI J.A. Cameron, ningham, W. Gelletly, H.G. Price, J. Simpson, D.D. Warner and A.N. James, Phys. Lett. 235B (1990) 239. 191 L.C. Northcliffe and R.F. Schilling, Nucl. Data Tables A7 (1970) 233.
and Methods
in Physics
Energy loss fluctuations
Research
349
B53 (1991) 349-351
caused by Rutherford
scattering
A.N. James Department
of Physics,
University of Liverpool,
P.O. Box 147, Liverpool L69 3BX, UK
K.A. Connell and R.A. Cunningham Science and Engineering Received
7 September
Research
Council, Daresbury
Laboratory,
1990 and in revised form 9 November
Warrington
WA4 4AD, UK
1990
An &butane filled ionisation chamber has been constructed with three anodes to provide atomic number assignments for heavy ions with energies greater than 1 MeV per nucleon. The energy loss section of the ion chamber is divided into two small sections to enable identification of events where abnormal energy loss profiles along the heavy ion track would cause false assignments. In an experiment with 49Cr ions such abnormal events were clearly identified and are shown to be due to Rutherford scattering of the 49Cr ions off the carbon nuclei of the isobutane ion chamber gas. The split energy loss section of the ion chamber enables rejection of these events to several orders of magnitude, with the rejection factor depending on the details of the heavy ion speed and type.
1. Introduction The energy loss of heavy ions moving through a gas begins to depend significantly on the atomic number (2) of the heavy ion once the speed is greater than 1 MeV/u (0.0463~). This dependence is exploited by experimentahsts who use gas ionisation chambers to assign atomic number to individual high energy heavy ions [l-3]. The resolution in Z depends on the rate of change of energy loss with Z and on the width of the the observed energy loss peak in the energy loss section of the ion chamber. The shape of the observed peak, to at least 0.01 of the peak height, is entirely consistent with single charge exchange statistics and can be modelled easily using the electron capture cross sections a, of Schlachter et al. [4] and a charge state distribution width characterised [5] by a width d, - 1.4 (the formulae of Betz [5], p. 470, have been used to relate the electron loss cross section oi to d, and a,). Empirically the atomic number dependence is reproduced by the formula of Schmidt-Backing and Homung [1,2,6]. In an experiment to search [7] for the isoto e “Zr the peak shape of the energy loss curves for “Sr and “Y, which were several thousand times more numerous, were found to have high energy shoulders on the energy loss peak for approximately 0.5% of heavy ions. This shoulder corresponded roughly to the intensity expected from Rutherford scattering from the carbon in the isobutane gas. If this is the correct explanation of the phenomenon then an ion chamber in which the energy loss section is split into two equal length parts should be able to distinguish heavy ion tracks which have been 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
subject to such a rare fluctuation and, consequently, to improve the selectivity of the ion chamber for rarely occurring higher atomic number ions. The ion chamber at the Daresbury recoil separator [l] has been modified in such a way and we report observations on 49Cr ions of 2.3 MeV/u which are in complete agreement with the pattern of energy loss expected from Rutherford scattering.
2. Three anode ion chamber signals from 49Cr The ion chamber used at the Daresbury recoil separator now has three anodes. The first two are each 50 mm long along the track of the ion and the third, to measure the total ion energy, is 200 mm long. In an experiment [S] measuring the y-ray spectroscopy of 49Mn the nucleus 49Cr was very strongly populated and a very high percentage of all in beam y-ray cascades proceeded through the 272 keV first excited state to ground state decay. The detection of a 272 keV y-ray established that a 49Cr nucleus of 2.3 MeV/u was causing the signal in the ion chamber. About one in a thousand ions selected in this fashion were either 49V or &Ti due to background in the window selecting the y-ray energy. Fig. 1 shows a density plot relating the pulse heights in the first two anode sections of the ion chamber for heavy ions selected in this way. The abnormal energy loss events, those occurring within the triangle with its apex at the 49Cr peak, clearly
B.V. (North-Holland)
A. N. James et al. / Energy loss fluctuations
350 5MeV
ANODE
ONE
40Me\
caused by Rutherford scattering
diagonal lines on fig. 1 while about 99% of the abnormal pulses can be rejected as outside the diagonal lines. Fig. 2 shows the pulse height distribution observed in the second anode section of the ion chamber for 49Cr ions which have normal energy loss in the first anode section.
40MeV
3. Rutherford scattering
15MeV
/ 0
1
30
1000
30,000
Fig. 1. Density plot relating the energy deposited in the first two anodes of the ion chamber with an isobutane filling at 30 Torr. Points within the area bounded by the two diagonal lines are the normal response for 49Cr, 49V and &Ti ions plus a streak due to pile up of pulses in the ion chamber electronics. The structure forming a triangle with its apex at the 49Cr peak is due to events where Rutherford scattering of 49Cr from 12C nuclei in the isobutane gas has caused a rare fluctuation in the energy loss profile of the heavy ions.
have the property that generally the abnormal energy loss is concentrated in one section of the ion chamber or the other. This feature is important experimentally since it allows 99% of normal events to be accepted within the
A simple Monte Carlo code modelling the energy loss process in the first two anode sections of the ion chamber has been written which uses ranges in (CH,), from the tables of Northcliffe and Schilling [9] to approximate the energy losses of 49Cr, “C, and ‘H ions in isobutane (2-methyl-propane C,H,,). Rutherford scattering for close collisions was introduced into the code by setting a mean free path of one metre corresponding at 30 Torr pressure to an impact parameter of about 0.5 pm, well inside all electron orbits. Heavy ion tracks encountering an interaction were chosen randomly to be either scattering from protons or carbon nuclei and with a properly weighted impact parameter distribution. Energy losses were then computed using the new ion energies and angles. The dimension of the ion chamber in the direction perpendicular to the original track was taken to be 60 mm and if any track passed through this side wall of the chamber it was rejected since it could not have registered a correct total energy signal. Scattering from protons produces a strongly forward peaked proton distribution whose energy loss rate is insignificant in relation to the energy loss rate for 49Cr ions (effective charge - 15). Central collisions on carbon
I”
loo7 20
25
Energy
(MeV)
30
35
Fig. 2. The pulse height observed in the second anode for those pulses which have a normal signal for 49Cr in the first anode. process producing the abnormal pulse heights must occur within the second anode zone of the ion chamber.
The
A. N. James et al. / Energy loss fluctuations
caused by Rutherford scattering
351
106 .
l
. . . 104-. v)
z :
.
0
. l
.
102--
l
*
l
* l
’ .
. . .
Energy Fig. 3. Simulated pulse height distribution for the second anode where the energy loss in the first anode had been normal. The resolution used in the simulation is that expected from single charge exchange processes. The energy scale does not match that of fig. 2 exactly due to systematic errors in the stopping powers. occurring with low probability also produce a strong forward distribution of ions which, almost fortuitously, mimics the energy loss profile of 49Cr and would be experimentally undetected. The abnormal energy loss events are caused by scattering at centre of mass angles from 0.1 to 1 rad for which the laboratory angle of the carbon recoils is nearly perpendicular to the 49Cr track. Fig. 3 shows a calculated pulse height distribution which has been folded with a resolution function to simulate the charge exchange statistics. It can be seen that the shape of the distribution matches the observed distribution. The cut off at large pulse heights in the simulation can be changed by changing the distance to the chamber side wall. The simulation reproduces the distribution observed in fig. 1 where for a small fraction of events both anode signals are abnormal but the combined excess signal has a limited size. These events are those where the carbon recoil, near perpendicular to the 49Cr track actually crosses the boundary between the two anodes. Some events will necessarily lie between the diagonal boundaries marked on fig. 1 and it will not be possible to discriminate these events from an event with larger atomic number using a simple three anode ion chamber. nuclei
4. Summary It has been shown that Rutherford scattering from heavy nuclei in an ion chamber gas causes a few events to have abnormal energy loss profiles with well localised, much increased, excess energy deposition. A simple three anode ion chamber is sufficient to discriminate against these events giving a greatly enhanced signal to
background ions.
sensitivity
for rare high atomic number
Acknowledgements This work was supported by grants from the United Kingdom Science and Engineering Research Council.
References PI A.N. James, T.P. Morrison,
K.L. Ying, K.A. Connell, H.G. Price and J. Simpson, Nucl. Instr. and Meth. A267 (1988) 144. PI J.P. Bouquet, R. Brissot and H.R. Faust, Nucl. Instr. and Meth. A267 (1988) 466. H.-H. Knitter, Ch. Straede, F.-J. 131 C. Budtz-Jorgensen, Hambsch and R. Vogt, Nucl. Instr. and Meth. A258 (1987) 209. 141 AS. Schlachter, J.W. Steams, W.G. Graham, K.H. Berkner, R.V. Pyle and J.A. Tams, Phys. Rev. A27 (1983) 3372. PI H.D. Betz, Rev. Mod. Phys. 44 (1972) 465. and H. Homung, 2. Phys. A286 (1978) WI H. Schmidt-Backing 253. [71 C.J. Lister, M. Campbell, A.A. Chishti, W. Gelletly, L. Goettig, R. Moscrop, B.J. Varley, A.N. James, T. Morrison, H.G. Price, J. Simpson, K.A. Connell and 0. Skeppstedt, Phys. Rev. Lett. 59 (1987) 1270. M.A. Bentley, A.M. Bruce, R.A. CunPI J.A. Cameron, ningham, W. Gelletly, H.G. Price, J. Simpson, D.D. Warner and A.N. James, Phys. Lett. 235B (1990) 239. 191 L.C. Northcliffe and R.F. Schilling, Nucl. Data Tables A7 (1970) 233.