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Thin film scintillation detectors for evaporation residue detection: angular distributions and coincidence trigger
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH
Nuclear Instruments and Methods in Physics Research A316 (1992)446-449 North-Holland
Sect,on A
Letter to the Editor
Thin film scintillation detectors for evaporation residue detection: angular distributions and coincidence trigger N.N. Ajitanand, M. Beuhler, C. Gelderloos and J.M. Alexander Departments of Chemistry and Physics, State Unicersity of New York at Stony Brook, NY 11794, USA Received 11 December 1991
A thin film scintillation detector has been used to measure the angular distribution of evaporation residues from the reaction 142 MeV 32S + ZTAI. Measurements were carried out from 5° to 30° with respect to the beam. Good separation of heavy residues from elastic scattering was demonstrated by two dimensional scatter plots of time-of-flight versus light output, even for positions well inside the grazing angle. The angular distributions compare very well with published data obtained with silicon semiconductor detectors. The ability to withstand radiation and give good separation of residues from elastics, even at high count rates, makes the thin film scintillation detector an excellent trigger for coincidence experiments.
In the study of heavy ion reactions it is often required to detect evaporation residues formed after the decay of a hot intermediate nucleus. These residues are contained within a small cone around the beam direction. The predominance of projectile elastic scattering within this cone makes it necessary to devise a method to separate the two classes of events. A standard way to avoid this problem is to make measurements outside the grazing angle where elastic events are removed by the nuclear reactions [1]. This, however, reduces the efficiency and flexibility with which residues can be detected. The use of deflectors to remove the elastic scattering has been very successful at the cost of experimental complexity [2]. Another way is to use a time-of-flight technique to separate the much slower residues from the elastics. Thin film scintillation detectors (TFSD) have a fast response and can withstand very high counting rates without deterioration of performance. They are therefore very suitable for detection of evaporation residues by the time-offlight technique, even within the grazing angle. In the present study we have used a thin film scintillation detector to measure the angular distribution of evaporation residues from the reaction 142 MeV a2S + 27A1. A thin film scintillator of thickness 10 ~m was prepared directly on the photomultiplier surface by applying a solution of the scintillator (NE102A) dissolved in toluene followed by drying under controlled evacuation [3]. After collimation the exposed central area was 2 cm in diameter. The response of such a film to heavy ions has been described by a semi-empirical expression [4] as shown in fig. 1. In general it can be seen that the response is essentially
J
"~" 6O
O
4O
..~ Z20
J
/ / /
Z=I0
1.5
2,5
3.5
E/A (MeV/nucleon) Fig. I. Response of I0 p.m thick TFSD to heavy ions of different Z and E / M .
linear in Z of the ion and has a power law dependence on ion velocity. The experimental arrangement is shown schematically in fig. 2; a flight distance of 54.5 cm from target to scintillator resulted in a solid angle of = l msr. Beam currents of 3 particle nA (from the Stony Brook linear
0168-9002/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
N.N. Ajitanand ctal. / Thin film scintillation detectors
\
447
US + =AI -, ER's
. . ~ ~ ~
td
TFSD
Silicon TFSD
~ Beam
target
Fig. 2. Experimental arrangement for the measurement of residue angular distribution (142 MeV 32S+ 27AI).
1o' accelerator) were used on a 100 i~g/cm 2 aluminum target giving counting rates in excess of 104/s at the smallest angle. Measurements were made well within the grazing angle of 16°, i.e. angles from 5 ° to 32 °. To determine absolute angles, the elastic scattering intensities were measured at each position from a 265 i~g/cm z Au target. Angles were then determined by comparing the data to calculations for Rutherford scattering. A start signal for the time-of-flight measurement was obtained from the accelerator (150 ps FWHM). For elastically scattered 32S the observed FWHM of the time-of-flight peak was 420 ps and that of the light output was 14%. Fig. 3 shows scatter plots for time-of-flight versus TFSD light output for the aluminum target. It is seen that the residue region (bounded by the "banana" gate for the 10° case) is distinguishable from the scattered projectiles in every case. The angular distribution is obtained by integrating the counts in the residue region and normalizing to the counts from the Faraday 142 MeV ~S ÷ a'AI -' ER's(0) |
|
i
|
O:lO"
0--5" 600
:.....-.~'..-; 400
.. :
,.o~-~..
'
I
0
' :
I
'
'
I
19 "
'
0:
I 28.5
' "
~D .ml .,,,,.q q,..q ~-. 0
800
i 4O0
"' "
.'2; .... "
"~..:
. ~-i
i
400
800
.
i 400
,
l 800
Light Output (channel number) Fig. 3. Scatter plots of time-of-flight vs light output for different angular positions.
Io-'
.
l 5
,
I 10
,
I
i
!
!
15
20
25
30
O (degrees) Fig. 4. Residue angular distributions obtained with TFSD and silicon semi-conductor detector [3].
cup corrected for dead time effects. Fig. 4 shows the angular distribution compared to results for the same system at the same energy using a semi-conductor detector for residue detection [5]. The rather close agreement demonstrates the success of the TFSD method. The final direction for an individual evaporation residue is decided by the sum of the recoil kicks due to the evaporation of particles in that evem. These recoil effects are particularly large for heavy particle emission, so that the overall width of the angular distribution reflects the relative numbers of heavier particles versus the evaporated nucleons. In fig. 5 we compare the experimental results to calculated angular distributions obtained from a statistical model calculation [6] for two different assumptions. For the first (dashed curve) only neutron and proton emission is considered, while in the second (solid curve) deuteron, triton and alpha-particle emission is also included. It can be seen that the emission of multi-nucleon clusters is very important in order to account for the width of the experimental angular distribution. The velocity distribution of residues at a particular lab angle is also indicative of the amount of linear momentum transferred (LMT) to the composite system [7]. In fig. 6 the velocity distribution of residue events at 10° is compared with the results of statistical model
448
N.N. Ajitanand et al. / Thin film scintillation detectors
142 MeV US + a'Al -* ER's(IO')
=S + =AI -, ER's •
Experiment
frFSV)
"°
tO0 Y. LMT
Calculations n.p.d.La ...... n.p only
tO00
.....
80 % LIlT
•
Experiment (Ti~3D)
800
10-'
•
800
z
//
/
/ ,i
J I ',
",
.° •
400
I
#' /
tO-'
/
/
200
/
°°
5
10
15
2O
25
30
0 (degrees) Fig. 5. Comparison of experimental residue angular distributions with statistical model calculations for neutron and proton emission only (dashed curve) and for neutron, proton, deuteron, triton, and alpha emission (solid curve). calculations for differing amounts of linear momentum transfer. The calculations consider neutron, proton, deuteron, triton and alpha emission. The curve shown for 80% LMT is a sum of two equally weighted contributions. For the first contribution it is assumed that 80% of the projectile fuses with the target and, for the second, that the projectile fuses with 80% of the target. The width for 80% L M T is substantially larger than the experimental width, whereas both the peak position and width are reasonably well reproduced for 100% LMT. For near symmetric systems, at higher energies where the percentage LMT is expected to be lower, an increase in the width of the velocity distribution has indeed been observed [8]. Finally in fig. 7 we show contours of time-of-flight versus TFSD light output for the data compared to calculations from a statistical model calculation coupled with the semi-empirical TFSD response expression [4]. The elastic scattering peaks at two different energies were used for the light output calibrations. The general positions of the calculated and experimental residue contours are close but the spread in light output is larger for the calculated contours. In summary, it has been shown that thin film scintillation detectors are quite useful for the detection of evaporation residues even at angles well within the
•
L0
s~ L4 S.e t.S 2.0 Velocity, V (cm/ns) Fig. 6. Experimental residue velocity distribution at 10° compared with statistical model calculations for 100% momentum transfer (solid curve) and 80% momentum transfer (dashed curve). Simulations are area normalized to the experimental data.
142 MeV aS + hAl -, ER's(O) Flight path = 54.6 cm '
"
O-tO° TFSD = t mg cm "ffi
' /i
'
25
20
0
~
Elastic
../"
4,J
/ / /' / i"" .., .:/
.~: :.
........ ." /
5
~" " /
i
Experiment .... Calculation *
40
I
,
30
I
20
,
i
,
I0
Time of flight (ns) Fig. 7. Contours of time-of-flight vs light output from experiment (solid curves) and statistical model calculations using the semi-empirical TFSD response function from ref. [2] (dashed curves).
N.;,'. Ajitanand el al. / Thin film scintillation detectors grazing angle. For one reaction we have shown that the residue angular distribution ab. . . . . . . . . . .~. . . . . ." wilh resuits from semi-conductor detector measurements [5], and that the velocity distributions are in agreement with statistical model calculations. This study demonstrates that TFSDs are well suited for ER experiments even if high counting rates are encountered from elastic scattering.
Acknowledgements We thank the accelerator staff as well as Professor Linwood Lee, Felix Liang, Jack Mahon and Brian Fineman of the Physics Department at Stony 3rook for help at various stages of the experiment. Financial support for this work was provided by the US D O E and by US NSF. One of us (Michael Beuhler) is grateful for an NSF traineeship in the program "Research Experience for Undergraduates".
449
References [1] See for exataple: A. Budzanowski, H. Dabrowski, Y. Chan, R.G. Stokstad, !. Tserruya and S. Wald, Phys. Rev. (232 (1985) 1534. [2] See for example: S. Gil, R. Vandenbosch, A. Charlop, A. Garcia, D.D. Leach, S.J. Luke and S. Kailas, Phys. Rev. C43 (1991) 701. [3] N.N. Ajitanand and ICN. lyengar, Nucl. Instr. and Meth. 133 (1976) 71. [4] N.N. Ajitanand, Nucl. Instr. and Meth. 143 (1977) 345. [5] G. Doukellis, G. Hlawatsch, B. Kolb, A. Miczaika, G. Rosner and B. Sedelmeyer, Nucl. Phys. A485 (1988) 369. [6] N.N. Ajitanand, J.M. Alexander and M.T. Magda, MODGAN - an efficient statistical model code for multistep evaporation of particles from a hot high spin composite (unpublished report). [7] H. Morgenstern, W. Bohne, W. Galster, K. Grabish and A. Kyanowski, Phys. Rev. Lett. 52 (1984) 1104. [8] L. Pienkowski, A. Lleres, H. Nifencker, J. Blachot, J. Crancon, A. Gizon, M. Maurel and C. Ristori, Z. Phys A334 (1989) 315.
Nuclear Instruments and Methods in Physics Research A316 (1992)446-449 North-Holland
Sect,on A
Letter to the Editor
Thin film scintillation detectors for evaporation residue detection: angular distributions and coincidence trigger N.N. Ajitanand, M. Beuhler, C. Gelderloos and J.M. Alexander Departments of Chemistry and Physics, State Unicersity of New York at Stony Brook, NY 11794, USA Received 11 December 1991
A thin film scintillation detector has been used to measure the angular distribution of evaporation residues from the reaction 142 MeV 32S + ZTAI. Measurements were carried out from 5° to 30° with respect to the beam. Good separation of heavy residues from elastic scattering was demonstrated by two dimensional scatter plots of time-of-flight versus light output, even for positions well inside the grazing angle. The angular distributions compare very well with published data obtained with silicon semiconductor detectors. The ability to withstand radiation and give good separation of residues from elastics, even at high count rates, makes the thin film scintillation detector an excellent trigger for coincidence experiments.
In the study of heavy ion reactions it is often required to detect evaporation residues formed after the decay of a hot intermediate nucleus. These residues are contained within a small cone around the beam direction. The predominance of projectile elastic scattering within this cone makes it necessary to devise a method to separate the two classes of events. A standard way to avoid this problem is to make measurements outside the grazing angle where elastic events are removed by the nuclear reactions [1]. This, however, reduces the efficiency and flexibility with which residues can be detected. The use of deflectors to remove the elastic scattering has been very successful at the cost of experimental complexity [2]. Another way is to use a time-of-flight technique to separate the much slower residues from the elastics. Thin film scintillation detectors (TFSD) have a fast response and can withstand very high counting rates without deterioration of performance. They are therefore very suitable for detection of evaporation residues by the time-offlight technique, even within the grazing angle. In the present study we have used a thin film scintillation detector to measure the angular distribution of evaporation residues from the reaction 142 MeV a2S + 27A1. A thin film scintillator of thickness 10 ~m was prepared directly on the photomultiplier surface by applying a solution of the scintillator (NE102A) dissolved in toluene followed by drying under controlled evacuation [3]. After collimation the exposed central area was 2 cm in diameter. The response of such a film to heavy ions has been described by a semi-empirical expression [4] as shown in fig. 1. In general it can be seen that the response is essentially
J
"~" 6O
O
4O
..~ Z20
J
/ / /
Z=I0
1.5
2,5
3.5
E/A (MeV/nucleon) Fig. I. Response of I0 p.m thick TFSD to heavy ions of different Z and E / M .
linear in Z of the ion and has a power law dependence on ion velocity. The experimental arrangement is shown schematically in fig. 2; a flight distance of 54.5 cm from target to scintillator resulted in a solid angle of = l msr. Beam currents of 3 particle nA (from the Stony Brook linear
0168-9002/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
N.N. Ajitanand ctal. / Thin film scintillation detectors
\
447
US + =AI -, ER's
. . ~ ~ ~
td
TFSD
Silicon TFSD
~ Beam
target
Fig. 2. Experimental arrangement for the measurement of residue angular distribution (142 MeV 32S+ 27AI).
1o' accelerator) were used on a 100 i~g/cm 2 aluminum target giving counting rates in excess of 104/s at the smallest angle. Measurements were made well within the grazing angle of 16°, i.e. angles from 5 ° to 32 °. To determine absolute angles, the elastic scattering intensities were measured at each position from a 265 i~g/cm z Au target. Angles were then determined by comparing the data to calculations for Rutherford scattering. A start signal for the time-of-flight measurement was obtained from the accelerator (150 ps FWHM). For elastically scattered 32S the observed FWHM of the time-of-flight peak was 420 ps and that of the light output was 14%. Fig. 3 shows scatter plots for time-of-flight versus TFSD light output for the aluminum target. It is seen that the residue region (bounded by the "banana" gate for the 10° case) is distinguishable from the scattered projectiles in every case. The angular distribution is obtained by integrating the counts in the residue region and normalizing to the counts from the Faraday 142 MeV ~S ÷ a'AI -' ER's(0) |
|
i
|
O:lO"
0--5" 600
:.....-.~'..-; 400
.. :
,.o~-~..
'
I
0
' :
I
'
'
I
19 "
'
0:
I 28.5
' "
~D .ml .,,,,.q q,..q ~-. 0
800
i 4O0
"' "
.'2; .... "
"~..:
. ~-i
i
400
800
.
i 400
,
l 800
Light Output (channel number) Fig. 3. Scatter plots of time-of-flight vs light output for different angular positions.
Io-'
.
l 5
,
I 10
,
I
i
!
!
15
20
25
30
O (degrees) Fig. 4. Residue angular distributions obtained with TFSD and silicon semi-conductor detector [3].
cup corrected for dead time effects. Fig. 4 shows the angular distribution compared to results for the same system at the same energy using a semi-conductor detector for residue detection [5]. The rather close agreement demonstrates the success of the TFSD method. The final direction for an individual evaporation residue is decided by the sum of the recoil kicks due to the evaporation of particles in that evem. These recoil effects are particularly large for heavy particle emission, so that the overall width of the angular distribution reflects the relative numbers of heavier particles versus the evaporated nucleons. In fig. 5 we compare the experimental results to calculated angular distributions obtained from a statistical model calculation [6] for two different assumptions. For the first (dashed curve) only neutron and proton emission is considered, while in the second (solid curve) deuteron, triton and alpha-particle emission is also included. It can be seen that the emission of multi-nucleon clusters is very important in order to account for the width of the experimental angular distribution. The velocity distribution of residues at a particular lab angle is also indicative of the amount of linear momentum transferred (LMT) to the composite system [7]. In fig. 6 the velocity distribution of residue events at 10° is compared with the results of statistical model
448
N.N. Ajitanand et al. / Thin film scintillation detectors
142 MeV US + a'Al -* ER's(IO')
=S + =AI -, ER's •
Experiment
frFSV)
"°
tO0 Y. LMT
Calculations n.p.d.La ...... n.p only
tO00
.....
80 % LIlT
•
Experiment (Ti~3D)
800
10-'
•
800
z
//
/
/ ,i
J I ',
",
.° •
400
I
#' /
tO-'
/
/
200
/
°°
5
10
15
2O
25
30
0 (degrees) Fig. 5. Comparison of experimental residue angular distributions with statistical model calculations for neutron and proton emission only (dashed curve) and for neutron, proton, deuteron, triton, and alpha emission (solid curve). calculations for differing amounts of linear momentum transfer. The calculations consider neutron, proton, deuteron, triton and alpha emission. The curve shown for 80% LMT is a sum of two equally weighted contributions. For the first contribution it is assumed that 80% of the projectile fuses with the target and, for the second, that the projectile fuses with 80% of the target. The width for 80% L M T is substantially larger than the experimental width, whereas both the peak position and width are reasonably well reproduced for 100% LMT. For near symmetric systems, at higher energies where the percentage LMT is expected to be lower, an increase in the width of the velocity distribution has indeed been observed [8]. Finally in fig. 7 we show contours of time-of-flight versus TFSD light output for the data compared to calculations from a statistical model calculation coupled with the semi-empirical TFSD response expression [4]. The elastic scattering peaks at two different energies were used for the light output calibrations. The general positions of the calculated and experimental residue contours are close but the spread in light output is larger for the calculated contours. In summary, it has been shown that thin film scintillation detectors are quite useful for the detection of evaporation residues even at angles well within the
•
L0
s~ L4 S.e t.S 2.0 Velocity, V (cm/ns) Fig. 6. Experimental residue velocity distribution at 10° compared with statistical model calculations for 100% momentum transfer (solid curve) and 80% momentum transfer (dashed curve). Simulations are area normalized to the experimental data.
142 MeV aS + hAl -, ER's(O) Flight path = 54.6 cm '
"
O-tO° TFSD = t mg cm "ffi
' /i
'
25
20
0
~
Elastic
../"
4,J
/ / /' / i"" .., .:/
.~: :.
........ ." /
5
~" " /
i
Experiment .... Calculation *
40
I
,
30
I
20
,
i
,
I0
Time of flight (ns) Fig. 7. Contours of time-of-flight vs light output from experiment (solid curves) and statistical model calculations using the semi-empirical TFSD response function from ref. [2] (dashed curves).
N.;,'. Ajitanand el al. / Thin film scintillation detectors grazing angle. For one reaction we have shown that the residue angular distribution ab. . . . . . . . . . .~. . . . . ." wilh resuits from semi-conductor detector measurements [5], and that the velocity distributions are in agreement with statistical model calculations. This study demonstrates that TFSDs are well suited for ER experiments even if high counting rates are encountered from elastic scattering.
Acknowledgements We thank the accelerator staff as well as Professor Linwood Lee, Felix Liang, Jack Mahon and Brian Fineman of the Physics Department at Stony 3rook for help at various stages of the experiment. Financial support for this work was provided by the US D O E and by US NSF. One of us (Michael Beuhler) is grateful for an NSF traineeship in the program "Research Experience for Undergraduates".
449
References [1] See for exataple: A. Budzanowski, H. Dabrowski, Y. Chan, R.G. Stokstad, !. Tserruya and S. Wald, Phys. Rev. (232 (1985) 1534. [2] See for example: S. Gil, R. Vandenbosch, A. Charlop, A. Garcia, D.D. Leach, S.J. Luke and S. Kailas, Phys. Rev. C43 (1991) 701. [3] N.N. Ajitanand and ICN. lyengar, Nucl. Instr. and Meth. 133 (1976) 71. [4] N.N. Ajitanand, Nucl. Instr. and Meth. 143 (1977) 345. [5] G. Doukellis, G. Hlawatsch, B. Kolb, A. Miczaika, G. Rosner and B. Sedelmeyer, Nucl. Phys. A485 (1988) 369. [6] N.N. Ajitanand, J.M. Alexander and M.T. Magda, MODGAN - an efficient statistical model code for multistep evaporation of particles from a hot high spin composite (unpublished report). [7] H. Morgenstern, W. Bohne, W. Galster, K. Grabish and A. Kyanowski, Phys. Rev. Lett. 52 (1984) 1104. [8] L. Pienkowski, A. Lleres, H. Nifencker, J. Blachot, J. Crancon, A. Gizon, M. Maurel and C. Ristori, Z. Phys A334 (1989) 315.