- Email: [email protected]
A cyclotron as a powerful tool for mass measurements of exotic nuclei
Nuclear Instruments
and Methods in Physics Research B 126 (1997) 334-339
NOMB
Beam Interactions with Materials 8 Atoms
ELSEVIER
A cyclotron as a powerful tool for mass measurements of exotic nuclei M. Chartier aT* , G. Auger a, W. Mittig a, A. tipine-Szily b, L.K. Fifield ‘, J.M. Casandjian a, M. Chabert a, J. FermC a, A. Gillibert d, M. Lewitowicz a, M. Mac Cormick a, M.H. Moscatello a, O.H. Odland e, N. Orr f, G. Politi g, C. Spitaels a, A.C.C. Villari a a GANIL, Bid Henri Becquerel, BP 5027, 14021 Caen Cedex. France b IFUSP-Uniuersidude de Silo Pa&, C.P. 66318.05389-970 S&I Paulo. Bruzil ’ Depurtment of Nuclear Physics, Australian National University, Cunberru ACT 0200, Austruliu d CEA/ DSM/ DAPNIA / SPhN. CEN Sucluy, 91191 Gif-sur-Yvette. France e Universitetet i Bergen, Fysivk Institutt. All&uten 55, 5007 Bergen, Norwuy f LPC-ISMRA, Bid du Murlchul Juin, 14050 Caen Cedex, France g Universitci di Cutunia, Dipurtimento di Fisicu. Corso Ituliu 57, 95125 Cataniu, Ituly
Abstract An original method using the second cyclotron of GANIL as a high resolution mass spectrometer was recently developed to measure the masses of exotic nuclei. This direct time of flight technique has been successfully used with A = 100 isobars in the vicinity of ‘@%n, produced via the fusion-evaporation reaction “Cr +58Ni at 25.5 MeV. The secondary ions were simultaneously accelerated and detected inside the cyclotron close to the extraction radius, using a silicon detector telescope mounted on a radially movable probe. The cyclotron was first tuned, using this detection system, with the 5oCr”+ primary beam degraded to the same velocity as the A = 100 secondary ions. This very critical and quite tricky preliminary tuning procedure, as well as the mass determination procedure, which are common to all the experiments using this method, are presented and discussed. PACS: 29.20.Hm; 21.10.Dr Keywords: lsochronous cyclotron;
Direct mass measurement
1. Introduction The method available for direct mass measurements of exotic nuclei produced in fragmentation reactions employs time-of-flight over a linear flight path of - 50-100 m, using high precision magnetic spectrometers (SPEG [I] at GANIL and TOFI [2] at Los Alamos). Nevertheless, with currently available count rates, the mass resolution that can be achieved with this well known technique (- 3 X IOp4) is not good enough to perform precise mass measurements of heavy ions such as ‘?Sn for instance. Given the much increased path length when the ions follow a spiral trajectory, we have developed a method
* Corresponding
author.
Fax:
+ I-5 17-353-5967;
email:
chartieQganac4.in2p3.fr. 0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SOl68-583X(96)01004-X
using the second cyclotron of GANIL (CSSZ) as a high precision spectrometer. The mass resolution obtained with the simultaneous acceleration of A/q = 3 light ions c6He, 9Li) was shown to be IOm6 [3]. Using this method the masses of ‘%d, IWIn and ‘%Sn were recently measured with respect to the reference mass of ‘O”Ag [4]. The mass resolution achieved with these heavy ions was of the order of 3 x 10-5.
2. Mass measurement
method
Details of the method used with light ions have already been published [3], so here we will concentrate on features specific to the experiment aimed at measuring the masses of A = 100 isobars in the vivinity of ‘OOSn which was performed recently [4,5]. The method consists in substituting the existing stripper located between the two cy-
M. Churrier et al./Nucl.
Instr. and Meth. in Phyr. Res. B 126 (1997) 334-339
CSSl cyclotron
33s
css2 cyclotron
Fig. I. Schematic diagram of the experimental set-up clotrons by a production target, in which the secondary nuclei are produced and are subsequently injected into and accelerated by CSS2. Fig. 1 shows a schematic diagram of the experimental set-up. Neutron-deficient A = 100 nuclei were produced by a
fusion-evaporation reactions between a s°Cr9c beam accelerated by the CSSl cyclotron and a 58Ni target. This reaction is known to be very favorable for the production of nuclei around ‘“Sn [6]. The highest cross-section is for the production of ‘O”Ag, followed by 1°0Cd and IWIn.
a 3500 z $3000 c z2500 9 2 2000 Y 1500 1000 500 0
1‘. L ’ ,_’
I,. l..LL-‘..b ’
L
’
’
2500 3000 3500 4000 4500 5000 5500 6000
3000
2500 Energy
3500
(channels)
Fig. 2. Energy versus time of flight spectrum obtained with the degraded primary beam “Cr ‘I+ injected in CSS2 and detected with the silicon detector telescope mounted on the radial probe when one varies the radial position of the probe. This spectrum was obtained before tuning correctly the isochronism and the injection.
SECTION X. IN-FLIGHT
SEPARATION
M. Charrier et al./ Nucl. Instr. and Meth. in Phys. Rex B 126 (1997) 334-339
336
Using the Monte Carlo codes PACE and HIVAP, the optimal energy for the production of ‘?Sn was estimated to be 255 MeV. In the fundamental cyclotron equation, the mean magnetic induction B, the radio-frequency applied to the cavities flo = 2af>, the harmonic h (number of radiofrequency periods per turn) are related to the mass/charge ratio m/q, the orbital radius p and the velocity u by: B
-==-=-=o/h
BP
nz
ymo
V
4
4’
where y is the usual relativistic factor. Considerations of matching between the two cyclotrons further require that the ratio of the velocities u, and u2 at extraction from CSSl and at injection into CSS2 respectively should satisfy: V2 -=“I
2
the lowest practicable harmonic is h, = 5. The incident beam energy of 265 MeV and the target thickness of 1.3 mg/cm* were therefore chosen to maximise the yield of ‘?Sn nuclei emerging from the target with u2 = 2/5 v,. In order to allow the use of beam currents between 300 and 500 nAe, the target was rotated and cooled. Accelerated ions were detected inside the cyclotron using a silicon detector telescope (AE 35 pm, E 300 pm) mounted on a radial probe which can be moved from the injection radius (u 1250 mm) up to the extraction radius (_ 3000 mm). The time of flight, or phase (one RF period being equal to 360”), of the detected ions was measured relative to the RF signal of the CSS2 cyclotron. For two nuclei, masses M and m + Sm, accelerated simultaneously in CSS2, the separation in time, 6 t, after N.,. turns is given to first order by:
6t
6m
t
m’
-=-
h, ’
where h, is the CSS2 harmonic. Since the velocity of the considered compound system is somewhat less than 0.5 u,,
(3)
where t is the total transit time, t = N,.h,/f, and f is the frequency of the accelerating voltage. This relation is the
;? 35002 5 3000a 2
2500-
Q 8
2000-
$
1500 1000 f500
t
0” ’ ’ ’ ’ ’ ’ L ’ ’ ’ I ’ ( ’ I 2500 3000 3500 4000 4500 5000 5500 6000 Phase (channels)
2500
3000
3500
Energy (channels) Fig. 3. Energy versus time of flight spectrum precession
has been eliminated,
the “bunches”
obtained after tuning the isochronism, the injection and the optimal of trajectories have disappeared and the individual orbits are visible.
initial phase. The
M. Chat-tier et ul./Nucl. basis
lnsrr. und Merh
of the calibration
procedure: the unknown mass from the well known reference mass m if the number of turns N, or the total time of flight I are known. If NT or t are not known, the calibration can still be achieved if we have more than one reference mass simultaneously accelerated with the unknown masses.
m + 6m can be determined
3. Tuning of the cyclotron Since the most probable charge state of the A = 100 secondary ions emerging from the target was 22+, it was possible to use “Cr” + to tune the cyclotron and therefore conserve the same settings in the transport line and in the CSS2. These ions were produced by degrading the primary “Cr beam to 2/5 of its initial velocity in a 22 mg/cm* Ta target. Their intensity was not, however, sufficient to tune the cyclotron in the standard way using the measurement of currents, and thus it was necessary to use the silicon detector telescope also to map the phase as a function of radius during the tuning procedure. We present in Fig. 2 the bidimensional spectrum energy versus time of flight and its projection on the energy axis obtained with the degraded primary beam “Cr”’ when the radial probe was moved from the injection to the extraction radii. The variable energy is related to the radial position of the ions. We observe a large variation of the phase (time-of-flight) with the radius, resulting from a non isochronism. and a superposition of trajectories seen as “bunches” in the figure, due to precession effects. Fig. 3 shows the same spectrum obtained after final corrections for the isochronism and optimisation of the injection parameters: the injection angle, the injection radius and the initial phase. These “bunches” have disappeared, the individual orbits are perfectly visible, and the phase is constant with the increasing radius. In two more recent experiments, which are currently analysed, we injected directly the secondary beams and tuned the cyclotron with them. This procedure is obviously more time-consuming because of very low beam intensities. but this is generally the only solution when no primary beam which has the same A/q ratio can be used for the tuning.
4. Acceleration
of secondary
in Phys. Rrs. B 126 f 1997’ 334-339
337
isochronism curve as achieved for s°Cr” ’ at the end of the tuning process. The precision that can be achieved with this mass measurement method was shown to be of the order of 10-.6, provided that ions of unknown masses are simultaneously accelerated with reference masses (31. Fig. 4 represents results of a simulation calculating the trajectories of different ions in the cyclotron. It shows that as the radius increases, the phase of the ions which have a relative mass/charge difference with respect to the isochronous ions larger than 4 X 10~-4 cannot be accelerated to the maximum radius. The cyclotron acceptance for the simultaneous acceleration of different ions is between IO -IIO-j, strongly dependent on target homogeneity and the specific reaction considered. The predicted fractional mass/charge difference between lmSn2”* and ‘ooAg’2+ [7] is 2.3 X IO -J. whence it is possible to accelerate looAg2’+. ‘OOCdz2’, ‘ooIn2’t and ‘ooSn2’+ simultaneously to maximum radius in CSS2. However, with increasing radius. the phase of ions other than looSn22 c moves further and further ahead of the isochronous phase as shown from calculations on Fig. 4. Fig. 5a shows a plot of total energy versus phase for secondary ions with A = 100 and c, = 22’ between ‘ooSn’2L and ‘O”Ag”+ which were simultaneously accel-
beams
Once isochronism, as indicated by constant phase with increasing radius and complete separation of individual orbits, had been established, the target was changed to 58Ni and the cyclotron’s magnetic field was increased by SH/B = 4.7 X 10e4. This equals the estimated fractional difference in the mass/charge ratio between “Cr”+ and m&22+ [7]. Hence, at this new field, ‘ooSn22+ ions would have been accelerated with the same phase and
Fig. 4. Simulation of the ion trajectories in the cyclotron. When the phase difference of the non-isochronous ions with respect to the initial phase reaches 90”. for relative mass/charge differences larger than 4~ IO-‘, the ions cannot be accelerated to the maximum radius.
SECTION X. IN-FLIGHT SEPARATION
338
M. Charrier er al. /Nucl.
Insrr. and Merh. in Phys. Res. B 126 (19971334-339
not high enough for them to be readily apparent in this
erated and intercepted in the silicon detector telescope with the radial probe at radii close to the extraction. Note that the increasing phase corresponds to smaller time of flight and thus to smaller mass. To aid the interpretation of this spectrum, a simulation of the ion trajectories in the cyclotron has been performed, and is shown in Fig. 5b. This simulation incorporates the observation that the tuning with 5oCr”+ resulted in an isochronous phase for ‘coIn22’ ions close to the extraction radius. Thus ‘@kr 22+ is expected around - 10” and lighter nuclei are detected at larger phases, proportional to their fractional mass/charge differences. For the 1coAg2z+ ions, which are farthest from isochronism, portions of several turns are intercepted by the detector at a given radius. For ions closer to isochronism, the spread in energies or radii for a single turn are less and fewer turns are intercepted at a fixed radius. These features are clearly visible in Fig. Sa. Several turns of the prolifically produced IwAg ions may be seen, and ?d are also evident. The intensities of ‘DOInand ‘?Sn ions are
‘0 260
250
plot. In order to determine the masses of the various isobars it was then necessary to determine their centroids in phase. The number of turns before detection, N-r, which was
required to turn the time differences between the various isobars and ‘O”Aginto mass differences, according to Eq. (3). was also determined in a separate measurement at the conclusion of the experiment. Using the mass of ‘O”Agas a reference, the masses of ‘@kd, 1°0In and ‘%r could be determined with precisions (depending on the statistics) of 2 x 10-6. 3 x 10-6 and lO-5 respectively [4].
5. Conclusion We have developped an original method which has allowed us to accelerate and separate for the first time
.
f-
I
I
I
-25.
-15.
-5.
. . . .. . ri.’
5.
15.
25.
35.
45.
Phase (degrees) 2
4.4
s z3
r
0
4.3
2 k
4.25
1
A In
4.35
5 $ L
Sn
0
Cd
*
Ag
detector
G 4.2
_-
4.15 -;25.
-15.
-5.
5.
15.
25.
3s.
45.
Phase (degrees) Fig. 5. Experimental and simulated energy versus phase spectra accumulated over different positions of the radial probe close to the extraction radius. (a) Experimental spectrum in which the several adjacent orbits intercepted for the most produced ions (tooAgZZ + and tooCd2*+) are clearly visible. (b) Result of a simulation for the simultaneous acceleration of ‘ooSn22+, ‘ooIn22t, “‘Cd2*+ and ‘ooAg2z’ (without any correction for relative production rates). The cut of the detector (located at a given radius) is indicated by horizontal lines.
M. Chartirr rt cd./ Nucl. Instr. und Mrth. in Phys. Rex R 126 f 1997) 334-339
exotic secondary beams in one of the GANIL cyclotrons. With this technique we have performed a series of mass measurement experiments. A mass resolution of - 3 X 10e5 was obtained for the simultaneous acceleration of A = 100 secondary ions and the masses of ‘“Cd, ‘“In and ‘?h were determined with a good precision using the mass of ‘O”Ag as a reference. Other experiments aimed at measuring the masses of A = 101 isobars in the vicinity of “‘Sn and the excitation energy of fission isomeric states of ‘J’Pu and 242Am were also very recently performed using this original method. In order to run such experiments huge experimental difficulties had to be overcome, and this required the development of new beam diagnosis for very low intensity radioactive beams, and the optimisation of a tuning procedure of the cyclotron. The experience that we have acquired in this field has already provided very useful infor-
mation for the future exploitation the SPIRAL project at GANIL.
339
of the CIME cyclotron in
References [l] L. Bianchi, B. Femandez, J. Gastebois, A. Gillibert. W. Mitt& et al., Nucl. Instr. and Meth. A 276 (1989) 509. [2] J.M. Wouters, D.J. Vieira. H. Wollnik. G.W. Butler, R.H. Kraus Jr. et al., Nucl. Instr. and Meth. B 26 (1987) 286. (31 G. Auger. W. Mittig, A. Ltpine-Szily, L.K. Fifield, M. Bajard et al., Nucl. Instr. and Meth. A 350 (1994) 235. [4] M. Chartier. G. Auger, W. Mittig. A. Lkpine-Szily. L.K. Fifteld et al., Phys. Rev. Lett. 77 (1996) 2400. [S] M. Chartier, G. Auger. W. Mittig, A. LCpine-Szily, D. Bibet et al., Act. Phys. Polon. B 27 (1996) 451. [6] E. Roe&l. private communication ( 1994). [7] G. Audi and A.H. Wapstra, Nucl. Phys. A 595 ( 1995) 409.
SECTION X. IN-FLIGHT SEPARATION
and Methods in Physics Research B 126 (1997) 334-339
NOMB
Beam Interactions with Materials 8 Atoms
ELSEVIER
A cyclotron as a powerful tool for mass measurements of exotic nuclei M. Chartier aT* , G. Auger a, W. Mittig a, A. tipine-Szily b, L.K. Fifield ‘, J.M. Casandjian a, M. Chabert a, J. FermC a, A. Gillibert d, M. Lewitowicz a, M. Mac Cormick a, M.H. Moscatello a, O.H. Odland e, N. Orr f, G. Politi g, C. Spitaels a, A.C.C. Villari a a GANIL, Bid Henri Becquerel, BP 5027, 14021 Caen Cedex. France b IFUSP-Uniuersidude de Silo Pa&, C.P. 66318.05389-970 S&I Paulo. Bruzil ’ Depurtment of Nuclear Physics, Australian National University, Cunberru ACT 0200, Austruliu d CEA/ DSM/ DAPNIA / SPhN. CEN Sucluy, 91191 Gif-sur-Yvette. France e Universitetet i Bergen, Fysivk Institutt. All&uten 55, 5007 Bergen, Norwuy f LPC-ISMRA, Bid du Murlchul Juin, 14050 Caen Cedex, France g Universitci di Cutunia, Dipurtimento di Fisicu. Corso Ituliu 57, 95125 Cataniu, Ituly
Abstract An original method using the second cyclotron of GANIL as a high resolution mass spectrometer was recently developed to measure the masses of exotic nuclei. This direct time of flight technique has been successfully used with A = 100 isobars in the vicinity of ‘@%n, produced via the fusion-evaporation reaction “Cr +58Ni at 25.5 MeV. The secondary ions were simultaneously accelerated and detected inside the cyclotron close to the extraction radius, using a silicon detector telescope mounted on a radially movable probe. The cyclotron was first tuned, using this detection system, with the 5oCr”+ primary beam degraded to the same velocity as the A = 100 secondary ions. This very critical and quite tricky preliminary tuning procedure, as well as the mass determination procedure, which are common to all the experiments using this method, are presented and discussed. PACS: 29.20.Hm; 21.10.Dr Keywords: lsochronous cyclotron;
Direct mass measurement
1. Introduction The method available for direct mass measurements of exotic nuclei produced in fragmentation reactions employs time-of-flight over a linear flight path of - 50-100 m, using high precision magnetic spectrometers (SPEG [I] at GANIL and TOFI [2] at Los Alamos). Nevertheless, with currently available count rates, the mass resolution that can be achieved with this well known technique (- 3 X IOp4) is not good enough to perform precise mass measurements of heavy ions such as ‘?Sn for instance. Given the much increased path length when the ions follow a spiral trajectory, we have developed a method
* Corresponding
author.
Fax:
+ I-5 17-353-5967;
email:
chartieQganac4.in2p3.fr. 0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SOl68-583X(96)01004-X
using the second cyclotron of GANIL (CSSZ) as a high precision spectrometer. The mass resolution obtained with the simultaneous acceleration of A/q = 3 light ions c6He, 9Li) was shown to be IOm6 [3]. Using this method the masses of ‘%d, IWIn and ‘%Sn were recently measured with respect to the reference mass of ‘O”Ag [4]. The mass resolution achieved with these heavy ions was of the order of 3 x 10-5.
2. Mass measurement
method
Details of the method used with light ions have already been published [3], so here we will concentrate on features specific to the experiment aimed at measuring the masses of A = 100 isobars in the vivinity of ‘OOSn which was performed recently [4,5]. The method consists in substituting the existing stripper located between the two cy-
M. Churrier et al./Nucl.
Instr. and Meth. in Phyr. Res. B 126 (1997) 334-339
CSSl cyclotron
33s
css2 cyclotron
Fig. I. Schematic diagram of the experimental set-up clotrons by a production target, in which the secondary nuclei are produced and are subsequently injected into and accelerated by CSS2. Fig. 1 shows a schematic diagram of the experimental set-up. Neutron-deficient A = 100 nuclei were produced by a
fusion-evaporation reactions between a s°Cr9c beam accelerated by the CSSl cyclotron and a 58Ni target. This reaction is known to be very favorable for the production of nuclei around ‘“Sn [6]. The highest cross-section is for the production of ‘O”Ag, followed by 1°0Cd and IWIn.
a 3500 z $3000 c z2500 9 2 2000 Y 1500 1000 500 0
1‘. L ’ ,_’
I,. l..LL-‘..b ’
L
’
’
2500 3000 3500 4000 4500 5000 5500 6000
3000
2500 Energy
3500
(channels)
Fig. 2. Energy versus time of flight spectrum obtained with the degraded primary beam “Cr ‘I+ injected in CSS2 and detected with the silicon detector telescope mounted on the radial probe when one varies the radial position of the probe. This spectrum was obtained before tuning correctly the isochronism and the injection.
SECTION X. IN-FLIGHT
SEPARATION
M. Charrier et al./ Nucl. Instr. and Meth. in Phys. Rex B 126 (1997) 334-339
336
Using the Monte Carlo codes PACE and HIVAP, the optimal energy for the production of ‘?Sn was estimated to be 255 MeV. In the fundamental cyclotron equation, the mean magnetic induction B, the radio-frequency applied to the cavities flo = 2af>, the harmonic h (number of radiofrequency periods per turn) are related to the mass/charge ratio m/q, the orbital radius p and the velocity u by: B
-==-=-=o/h
BP
nz
ymo
V
4
4’
where y is the usual relativistic factor. Considerations of matching between the two cyclotrons further require that the ratio of the velocities u, and u2 at extraction from CSSl and at injection into CSS2 respectively should satisfy: V2 -=“I
2
the lowest practicable harmonic is h, = 5. The incident beam energy of 265 MeV and the target thickness of 1.3 mg/cm* were therefore chosen to maximise the yield of ‘?Sn nuclei emerging from the target with u2 = 2/5 v,. In order to allow the use of beam currents between 300 and 500 nAe, the target was rotated and cooled. Accelerated ions were detected inside the cyclotron using a silicon detector telescope (AE 35 pm, E 300 pm) mounted on a radial probe which can be moved from the injection radius (u 1250 mm) up to the extraction radius (_ 3000 mm). The time of flight, or phase (one RF period being equal to 360”), of the detected ions was measured relative to the RF signal of the CSS2 cyclotron. For two nuclei, masses M and m + Sm, accelerated simultaneously in CSS2, the separation in time, 6 t, after N.,. turns is given to first order by:
6t
6m
t
m’
-=-
h, ’
where h, is the CSS2 harmonic. Since the velocity of the considered compound system is somewhat less than 0.5 u,,
(3)
where t is the total transit time, t = N,.h,/f, and f is the frequency of the accelerating voltage. This relation is the
;? 35002 5 3000a 2
2500-
Q 8
2000-
$
1500 1000 f500
t
0” ’ ’ ’ ’ ’ ’ L ’ ’ ’ I ’ ( ’ I 2500 3000 3500 4000 4500 5000 5500 6000 Phase (channels)
2500
3000
3500
Energy (channels) Fig. 3. Energy versus time of flight spectrum precession
has been eliminated,
the “bunches”
obtained after tuning the isochronism, the injection and the optimal of trajectories have disappeared and the individual orbits are visible.
initial phase. The
M. Chat-tier et ul./Nucl. basis
lnsrr. und Merh
of the calibration
procedure: the unknown mass from the well known reference mass m if the number of turns N, or the total time of flight I are known. If NT or t are not known, the calibration can still be achieved if we have more than one reference mass simultaneously accelerated with the unknown masses.
m + 6m can be determined
3. Tuning of the cyclotron Since the most probable charge state of the A = 100 secondary ions emerging from the target was 22+, it was possible to use “Cr” + to tune the cyclotron and therefore conserve the same settings in the transport line and in the CSS2. These ions were produced by degrading the primary “Cr beam to 2/5 of its initial velocity in a 22 mg/cm* Ta target. Their intensity was not, however, sufficient to tune the cyclotron in the standard way using the measurement of currents, and thus it was necessary to use the silicon detector telescope also to map the phase as a function of radius during the tuning procedure. We present in Fig. 2 the bidimensional spectrum energy versus time of flight and its projection on the energy axis obtained with the degraded primary beam “Cr”’ when the radial probe was moved from the injection to the extraction radii. The variable energy is related to the radial position of the ions. We observe a large variation of the phase (time-of-flight) with the radius, resulting from a non isochronism. and a superposition of trajectories seen as “bunches” in the figure, due to precession effects. Fig. 3 shows the same spectrum obtained after final corrections for the isochronism and optimisation of the injection parameters: the injection angle, the injection radius and the initial phase. These “bunches” have disappeared, the individual orbits are perfectly visible, and the phase is constant with the increasing radius. In two more recent experiments, which are currently analysed, we injected directly the secondary beams and tuned the cyclotron with them. This procedure is obviously more time-consuming because of very low beam intensities. but this is generally the only solution when no primary beam which has the same A/q ratio can be used for the tuning.
4. Acceleration
of secondary
in Phys. Rrs. B 126 f 1997’ 334-339
337
isochronism curve as achieved for s°Cr” ’ at the end of the tuning process. The precision that can be achieved with this mass measurement method was shown to be of the order of 10-.6, provided that ions of unknown masses are simultaneously accelerated with reference masses (31. Fig. 4 represents results of a simulation calculating the trajectories of different ions in the cyclotron. It shows that as the radius increases, the phase of the ions which have a relative mass/charge difference with respect to the isochronous ions larger than 4 X 10~-4 cannot be accelerated to the maximum radius. The cyclotron acceptance for the simultaneous acceleration of different ions is between IO -IIO-j, strongly dependent on target homogeneity and the specific reaction considered. The predicted fractional mass/charge difference between lmSn2”* and ‘ooAg’2+ [7] is 2.3 X IO -J. whence it is possible to accelerate looAg2’+. ‘OOCdz2’, ‘ooIn2’t and ‘ooSn2’+ simultaneously to maximum radius in CSS2. However, with increasing radius. the phase of ions other than looSn22 c moves further and further ahead of the isochronous phase as shown from calculations on Fig. 4. Fig. 5a shows a plot of total energy versus phase for secondary ions with A = 100 and c, = 22’ between ‘ooSn’2L and ‘O”Ag”+ which were simultaneously accel-
beams
Once isochronism, as indicated by constant phase with increasing radius and complete separation of individual orbits, had been established, the target was changed to 58Ni and the cyclotron’s magnetic field was increased by SH/B = 4.7 X 10e4. This equals the estimated fractional difference in the mass/charge ratio between “Cr”+ and m&22+ [7]. Hence, at this new field, ‘ooSn22+ ions would have been accelerated with the same phase and
Fig. 4. Simulation of the ion trajectories in the cyclotron. When the phase difference of the non-isochronous ions with respect to the initial phase reaches 90”. for relative mass/charge differences larger than 4~ IO-‘, the ions cannot be accelerated to the maximum radius.
SECTION X. IN-FLIGHT SEPARATION
338
M. Charrier er al. /Nucl.
Insrr. and Merh. in Phys. Res. B 126 (19971334-339
not high enough for them to be readily apparent in this
erated and intercepted in the silicon detector telescope with the radial probe at radii close to the extraction. Note that the increasing phase corresponds to smaller time of flight and thus to smaller mass. To aid the interpretation of this spectrum, a simulation of the ion trajectories in the cyclotron has been performed, and is shown in Fig. 5b. This simulation incorporates the observation that the tuning with 5oCr”+ resulted in an isochronous phase for ‘coIn22’ ions close to the extraction radius. Thus ‘@kr 22+ is expected around - 10” and lighter nuclei are detected at larger phases, proportional to their fractional mass/charge differences. For the 1coAg2z+ ions, which are farthest from isochronism, portions of several turns are intercepted by the detector at a given radius. For ions closer to isochronism, the spread in energies or radii for a single turn are less and fewer turns are intercepted at a fixed radius. These features are clearly visible in Fig. Sa. Several turns of the prolifically produced IwAg ions may be seen, and ?d are also evident. The intensities of ‘DOInand ‘?Sn ions are
‘0 260
250
plot. In order to determine the masses of the various isobars it was then necessary to determine their centroids in phase. The number of turns before detection, N-r, which was
required to turn the time differences between the various isobars and ‘O”Aginto mass differences, according to Eq. (3). was also determined in a separate measurement at the conclusion of the experiment. Using the mass of ‘O”Agas a reference, the masses of ‘@kd, 1°0In and ‘%r could be determined with precisions (depending on the statistics) of 2 x 10-6. 3 x 10-6 and lO-5 respectively [4].
5. Conclusion We have developped an original method which has allowed us to accelerate and separate for the first time
.
f-
I
I
I
-25.
-15.
-5.
. . . .. . ri.’
5.
15.
25.
35.
45.
Phase (degrees) 2
4.4
s z3
r
0
4.3
2 k
4.25
1
A In
4.35
5 $ L
Sn
0
Cd
*
Ag
detector
G 4.2
_-
4.15 -;25.
-15.
-5.
5.
15.
25.
3s.
45.
Phase (degrees) Fig. 5. Experimental and simulated energy versus phase spectra accumulated over different positions of the radial probe close to the extraction radius. (a) Experimental spectrum in which the several adjacent orbits intercepted for the most produced ions (tooAgZZ + and tooCd2*+) are clearly visible. (b) Result of a simulation for the simultaneous acceleration of ‘ooSn22+, ‘ooIn22t, “‘Cd2*+ and ‘ooAg2z’ (without any correction for relative production rates). The cut of the detector (located at a given radius) is indicated by horizontal lines.
M. Chartirr rt cd./ Nucl. Instr. und Mrth. in Phys. Rex R 126 f 1997) 334-339
exotic secondary beams in one of the GANIL cyclotrons. With this technique we have performed a series of mass measurement experiments. A mass resolution of - 3 X 10e5 was obtained for the simultaneous acceleration of A = 100 secondary ions and the masses of ‘“Cd, ‘“In and ‘?h were determined with a good precision using the mass of ‘O”Ag as a reference. Other experiments aimed at measuring the masses of A = 101 isobars in the vicinity of “‘Sn and the excitation energy of fission isomeric states of ‘J’Pu and 242Am were also very recently performed using this original method. In order to run such experiments huge experimental difficulties had to be overcome, and this required the development of new beam diagnosis for very low intensity radioactive beams, and the optimisation of a tuning procedure of the cyclotron. The experience that we have acquired in this field has already provided very useful infor-
mation for the future exploitation the SPIRAL project at GANIL.
339
of the CIME cyclotron in
References [l] L. Bianchi, B. Femandez, J. Gastebois, A. Gillibert. W. Mitt& et al., Nucl. Instr. and Meth. A 276 (1989) 509. [2] J.M. Wouters, D.J. Vieira. H. Wollnik. G.W. Butler, R.H. Kraus Jr. et al., Nucl. Instr. and Meth. B 26 (1987) 286. (31 G. Auger. W. Mittig, A. Ltpine-Szily, L.K. Fifield, M. Bajard et al., Nucl. Instr. and Meth. A 350 (1994) 235. [4] M. Chartier. G. Auger, W. Mittig. A. Lkpine-Szily. L.K. Fifteld et al., Phys. Rev. Lett. 77 (1996) 2400. [S] M. Chartier, G. Auger. W. Mittig, A. LCpine-Szily, D. Bibet et al., Act. Phys. Polon. B 27 (1996) 451. [6] E. Roe&l. private communication ( 1994). [7] G. Audi and A.H. Wapstra, Nucl. Phys. A 595 ( 1995) 409.
SECTION X. IN-FLIGHT SEPARATION