Performance of the LNL recoil mass spectrometer

Nuclear Instruments and Methods in Physics Research A 339 (1994) 531-542 North-Holland

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A

Performance of the LNL recoil mass spectrometer C. Signorini, S . Beghini, A. Dal Bello, G. Montagnoli, F. Scarlassara, G.F. Segato, F. Soramel Dipartimento di Fisica dell'Universita and INFN, I-35131 Padova, Italy

D. Ackermann t , L. Corradi, A. Facco, H. Moreno INFN, Laboratori Nazionah di Legnaro, I-35020 Legnaro, Padova, Italy

2,

L. Willer, D.R. Napoli, G.F. Prete

(Received 19 October 1992 ; revised form received 30 June 1993)

The LNL recoil mass spectrometer, in operation for some years on line with the LNL XTU Tandem accelerator has been used primarily for the study of reaction products emitted at 0° to the beam direction . In agreement with the design goal this instrument has a good mass resolution, in the range of 1/300, even with large energy (±20%) and solid angle ( > 7 .5 msr) acceptances; the mass dynamic range is around ±6% of the central mass. The beam rejection factor at 0° ranges from 10+6 to 10 +11 according to various experimental parameters . Advantages and limitations of the spectrometer are discussed.

1. Introduction Spectrometers for experimental research in nuclear physics in the energy range available to Tandem Van de Graaff accelerators have made a continuous evolution . In a first period efforts were placed in the development and utilization of high resolution momentum spectrometers culminating in the highly evoluted Q3D family [1]. With the availability of accelerated beams of increasingly heavier masses, and consequently higher recoil velocities of the reaction products, an increasing need of mass spectrometers "on-line", i.e . for fast reaction products became evident, their main features being good mass resolution and the capability of operating at 0° to the beam direction particularly in connection with fusion reactions. These instruments need energy dispersion with rather high electric fields in addition to the momentum dispersion with magnetic fields. Schematically speaking the mass spectrometers have evolved following two main lines. In the first one particular attention was payed to the beam separation aspect and the mass identification

1 European Community fellow. 2 On leave from the University of Sevilla, Spain. Elsevier Science B.V . SSDI0168-9002(93)E0620-8

was done essentially with conventional nuclear physics detectors located downstream the spectrometer . This is the case of the "beam filters" built at GSI [2], Munich [3], Oak Ridge [4] and Dubna [5]. Successively real "mass spectrometers" have been built and the "beam filtering" was somehow an automatic "by-product" of the mass identification procedure . To this type of instruments belong the pioneer spectrometer of the BNL-MIT group [6], no more in operation, the Rochester [7], Daresbury [8], and Osaka [9] ones . The main guideline at LNL was to build, on the basis of the experience collected with the instruments mentioned above and of the experimental needs of the 15 .5 MV Tandem accelerator, an on-line "mass spectrometer" with substantial improvements, with respect to the previous ones, in solid angle, energy and mass acceptance without sacrificing mass resolution . Following this approach similar instruments have been built successively at ANL [10,11] and New Delhi [12] and are under construction at Oak Ridge [13] . A recent review of on-line mass spectrometers can be found in ref. [14] . The LNL recoil mass spectrometer (RMS) [15] now in full operation has been utilized in the last two years in various experiments . Progress reports have already been presented in International Conferences [16-20] and papers [21] on HI--y coincidences have already been published.

532

C. Signorcni et al. /Nucl. Instr. and Meth in Phys. Res. A 339 (1994) 531-542

The aim of the present article is to present a collection of the results most relevant to illustrate the qualities of the instrument .

Prior to the final spectrometer construction exten-

sive and successful HV tests [22] have been done . Following the results of these tests the electric dipole

The main goals of the spectrometer were the follow-

vessels and the HV electrodes have been manufactured

a) Good mass resolution : in the range 1/300.

roughness of 15

ing:

b) Large dynamic ranges : in m/q (±7%), in the

energy spread of a specific mass accepted (± 20%) and in the geometrical solid angle (> 10 msr) .

c) Possibility of operation at 0° to the beam direc-

tion and consequently a good suppression of the unwanted beamlike particles.

All these boundary conditions have dictated a rather

respectively in stainless steel, with a maximum surface ~Lm, and titanium, with

roughness of 3 wm . The 15 tolerance of 0.1%.

cm

a surface

dipole gap has a

The optics of the spectrometer has been described

in detail in ref. [15], therefore we will remind here only the main points .

The fundamental feature of the mass spectrometer

is given by the combined action of ED1 and MD which

sophisticated instrument with several high quality opti-

disperse according to the mass-to-charge state ratio

in the magnetic fields to reduce the various aberrations

ED1, and since the particles are already mass dis-

cal elements and need of higher multipole components

which, as a general rule, increase quadratically with the

size of the accepted parameters . We have soon realized, after the first tests, that a detailed calibration with the mapping of all the spectrometer characteristics would have been rather time

consuming at the cost of a busy experimental research program. Therefore after accomplishing a program of basic calibrations, obviously necessary,

priority was

given to the experiments. For this reason this paper describes mainly day-to-day performances rather than

optimum characteristics ; anyhow it is the opinion of

the authors that these types of data are not at all less important because they are the ones that really matter .

The present paper is organized as follows: section 2 will describe the instrument components and the most relevant technical solutions adopted. Section 3 will present the results of the off beam calibrations with an a source . Section 4 gives the results obtained in beam in some representative experiments, and finally in section 5 some conclusive remarks will be done in relation to the use of the spectrometer for experiments.

2. Spectrometer hardware The main components of the instrument are: the reaction chamber with the beam monitoring and diag-

nostics system, the optical elements, magnets and electrostatic deflectors, the focal plane detector, the vacuum components and the computer control system . A general layout of the spectrometer is shown in fig. l . The sequence of the various components starting

from the target is the following: a magnetic quadrupole doublet (01, Q2), a first large electrostatic cylindrical

deflector (ED1), a first correction sextupole (SI), a large magnetic dipole (MD), a second correction sex-

tupole (S2) and finally a second electrostatic deflector (ED2) identical to ED1.

m/q. ED2 tends to cancel the energy dispersion of persed at its entrance its effect is to produce the large

energy acceptance of the instrument . The large solid angle acceptance is granted by the entrance quadrupole

doublet. The spatial focusing in the dispersion (X) plane is due to Q1 + MD and in the perpendicular (Y) plane to Q2 + MD fringing field (produced by 7° shim angles). The sextupole S2 corrects the mass-dependent

focal plane tilt ; it essentially improves the mass resolu-

tion at the boundaries of the focal plane which would naturally be worse than at the centre . S1 and the MD boundary curvature correct for the energy-dependent

"walk" and "wobble" [15] effects, essentially keeping "optimum" mass resolution at least in the centre of the focal plane for a large energy range of the ions .

The focal plane position can vary from 40 to 110 cm

from the ED2 effective field boundary, and the corresponding magnification from

-4 .0 to

-1 .0 respec-

tively ; this means that in the "far" position there is the best mass resolution but, as we will see, unfortunately the aberrations are more severe .

For the electric dipoles we have developed a com-

pact HV power supply based on a Cockcroft-Walton type multiplier which is described in detail in ref. [23] .

The spectrometer has been utilized in test experi- 200 kV per plate and in long run experiments, up to - 180 kV per plate. The ED dements up to

flector plates have been conditioned up to ± 270 kV .

The reaction chamber has an inner diameter of 30

cm and has a sliding seal on the spectrometer side

which allows to rotate the whole instrument from -5° to +48° . "Strip" targets (1 .0 to 1.5 mm large) have been used in most experiments to minimize mass resolution worsening due to beam instability.

Between this chamber and the entrance quadrupole, slits can be inserted to define the entrance solid angle of the spectrometer . Vacuum inside RMS is better than 10 -7 Torr .

The focal plane detector [24] consists of two 14 X 14 cm z anode grids in the X and Y directions, with 1 mm wire spacing, and a common cathode made of alumi-

C. Signorini et al. I Nucl. Instr. and Meth . in Phys. Res. A 339 (1994) 531-542

533

E

E

0 Û U C1

O U U N

.c

w O O

T ro

r.i

bp LT.

534

C Signorint et al.INucl. Instr. and Meth . in Phys . Res. A 339 (1994) 531-542

nated mylar, 200 pgcm -2 thick, giving time information ; the entrance window is also a 200 pgcm -2 thick mylar foil . This parallel plate avalanche counter (PPAC) is followed by a 43 cm long Bragg chamber with an inner diamater of 150 mm : both detectors work in the same gas (isobuthane) volume, i.e . at the same gas pressure, to minimize the dead layers . The total geometrical transparency of the system is 87% . The Bragg chamber yields energy and range information that are normally used to discriminate between beam scattered particles and the recoils, which are typically evaporation residues (ER) in the range of 0.5-1 .0 MeV/amu. Alternatively, discrimination of the fusion-like from the beam-like particles is achieved with the time-offlight (TOF) through the RMS . This method is far better when the Bragg chamber counting rate exceeds - 1 kHz, as is the case e.g . for the inverse kinematic reactions. 3. Calibration and tests off beam The spectrometer has been calibrated with a monoenergetic (Y.-beam, E. = 5.805 MeV, from a 244Cm source [25] . The source had an intensity of 4 pCi and dimensions of 1 .5 x 1 .5 mm 2 and was positioned at the target site, object point of the spectrometer ; the particle trajectories were forced through the centre of the dipole magnet and adjusted to exit out parallel to the central axis of the spectrometer by closing its central slits and by properly balancing the three main fields . The outgoing trajectories were determined by subsequently observing them in two position sensitive Si detectors (PSSD) installed at the two extreme focal plane locations, 40 cm and 110 cm from the ED2 effective field boundary. In the "far" position, where the magnification is -1 .0, we observed the best mass resolution (FWHM) but severe geometrical aberrations (i .e. large FWl/ 10M) for a large solid angle acceptance ; in the "near" position, with -4 .0 magnification, there is a worse mass resolution but negligible geometrical aberrations. The results are shown in fig. 2: in the "near" location (d = 40 cm) the peak has a FWHM of 3.5 mm corresponding to a mass resolution (for the dispersion measured of 10 .5 mm/%) of 1/300, in the "far" position (d = 110 cm) a FWHM of 2.5 mm and mass resolution 1/420, but the large aberrations, clearly evident from the FWl/10M, make this location not well suited for most of the on-line measurements where several contiguous masses have to be detected . In an "intermediate" focal plane location of 80 cm, magnification -1 .3, we have compared the resolution using a 5 msr and a 0.5 msr solid angle. The data

150

Si detector d = 40 cm

100 V O U

E-- FWHM : 3 .5 mm 50

0

_, 0

10

20

40

Position (mm)

1

50

Si detector d = 110 cm

300

C

30

200 Q

0 U

FWHM . 2 .5 Calibration wire

100

0

10

20

30

Position (mm)

40

50

Fig. 2. Resolution tests with u-source and a silicon position sensitive detector in two extreme focal plane positions.

shown in fig. 3 indicate that aberrations totally disappear with the small solid angle in agreement with the optics calculations performed with the computer code GIGS [26] . Based on these results all measurements in beam have been done with the X-Y detector in a "near" position of 60 cm . 4. RMS experimental performances The intrinsic spectrometer performances like mass resolution and beam rejection capability cannot be given with just one number since it is known [15] that they depend on various parameters which in real experiments cannot be fixed a priori and sometimes unfortunately are not totally under control. For example the resolution depends on the entrance solid angle (which cannot be reduced arbitrarily

535

C. Segnonni et al. I Nucl. Instr. and Meth. in Phys. Res. A 339 (1994) 531-542

the beam direction . Therefore a tail of the particles scattered in forward directions can reach the focal plane directly or via multiple scattering particularly if the beam rigidity is not too different from that of the recoils as in the case of inverse reactions ; this point will be discussed later in detail . In the following, three typical examples will be presented in detail . In the first two cases ER from fusion reactions were detected at 0° . The cross sections were rather large (300 mb in the first example) and relatively small (7 mb in the second one). In the third case target-like recoils following transfer reactions were detected at very forward angles . a) 58Ni (212 MeV) + 64Ni The spectrometer was operated at 0° to the beam direction in order to detect the evaporation residues . At this energy the fusion cross section is known [27] to be 300 mb . In order to get an optimum separation between ER and beam-like particles the beam was pulsed with 800 ns time interval and also the (TOF) between the target and the focal plane was recorded in addition to the usual four detector signals (X, Y, Energy, Bragg Peak) . Two representative scatter plots

since it costs statistics) and on the energy spread of the particles which depends on many parameters such as target and kinematics . The beam rejection capability depends also on the beam tuning which is somehow operator dependent . For these reasons we are giving here results obtained in representative experiments performed during the last two years . The results presented here have been obtained with the detector previously described with an entrance window of 12 X 7 CM Z. Typically during the experimental runs the following parameters are recorded on tape for successive off-line analysis: X, Y, Energy and Bragg Peak. In some experiments also the TOF between target and focal plane has been accumulated with the start given by the pulsed beam or gamma detector located near the target and the stop by the cathode fast signal . The additional signals beside X and Y are very important in most cases to discriminate from background particles especially for 0° operation . In fact in this situation the beam is stopped on the surface of the first anode which obviously cannot be perpendicular to

m c a ro i s

Calculated . = 5 msr

300

200

100

3 .5 mm

FWHM

11 .6 mR FW1/LOM

Calculated . 0 .5 msr

c

200

FWHM

2 .n

10

20

Position

30 (mm)

40

50

1 .8 mm

100

10

20 30 40 Position (mm)

So

Fig. 3 . Resolution tests with a-source and different solid angle acceptances 3(a) and 3(c) . Comparison with optics calculations : 3(b) and 3(d).

536

C. Signorins et al. /Nucl. Instr. and Meth . i n Phys . Res. A 339 (1994) 531-542

of the data are shown in fig. 4. In the TOF vs E scatter plot the ER island is clearly distinguishable from the background originated by the beam particles scattered from the EDI anode . A gate on these events cleans up very well the position data as can be seen comparing fig. 4b and fig. 5a . In this reaction the signals from the Bragg chamber alone could not give a good background discrimination . Fig. 5 shows the X, Y, and E vs X events gated on the ER "island" mentioned above . The blank parts in the extreme right hand side of the X and in the centre of the Y spectrum are due to missing calibration wires, as in fig. 6b . From the X spectrum the resolution on the central mass peaks is M/OM= 300 (FWHM = 3.3 mm), for an energy spread of the ER estimated around ± 17%; three charge states, 25+, 26+, 27+, are fully observable, the mass dispersion being 10 .5 mm/% . Consequently the 12 em focal plane m/q dynamic range is ±6% in good agreement with the design goal (10 mm/% dispersion, i.e . ±7% dynamic range for 14 cm). The Y spectrum has the expected Lorentzian shape: it spans over 7 cm with a FWHM of 9 mm in agreement with the calculations (8 mm). The E vs X scatter plot shows that the energy dependent aberrations are not fully corrected within the present calibration of the optical elements (mainly the sextupoles) and indicates that the mass resolution on the sides of the focal plane can be improved by appropriate software corrections in sorting the data . In this experiment also the total efficiency of the

spectrometer has been obtained from the ratio of the gamma-ray peaks of the 2+- 0+, 337 keV, transition in 11 'Xe, (2p2n evaporation channel), observed directly and in coincidence with the focal plane detector . The -y-rays were observed with a 30% efficiency HP Ge detector located at 9 cm from the target at 90° to the beam . A global system efficiency of - 20% has been deduced for 118Xe mass spectrum shown in fig. 5 where three charge states are observed . Since these charge states represent around 40% of the total yield and within the 7.5 msr entrance angle we accept > 95% of the experimentally known ER angular distribution, we conclude that the "intrinsic" RMS transmission for particles with a specific mlq value is around 60%, as expected . b) 64Ni (220 MeV) + 9ZZr In this case the experimental setup was similar to the previous one: the ERs were detected at 0°, at this energy the cross section was 7 mb [28]. The four focal plane detector signals (X, Y, Energy and Bragg Peak) were recorded as well as the TOF. Representative scatter plots of the data are shown in fig. 6. Also for this reaction the Bragg chamber signals alone could not well separate the fusion products from the beamlike particles: the TOF signal was essential in this respect. Fig. 7 shows several spectra gated by the signals corresponding to ER : the X spectrum has a resolution for the central masses M/OM=_ 200, and three charge states 26 +, 27 +, 28 + are fully detected in the focal plane over a range of 12 cm . The Y spectrum

800F-

Z

600r

a 5001CO

ER

4001-

Q 300~

200

ENERGY (ARBITRARY UNITS)

10

20

30

RELATIVE X -POSITION (mm)

Fig. 4. (a) Scatter plots TOF vs E for the reaction 58 Ni(212 MeV)+ 64 Ni ; the ER are separated from the beam. (b) Scatter plot TOF vs X for the same reaction . Three main masses for three charge states are accepted in the focal plane.

C. Signorini et al. /NucL Instr. and Meth in Phys. Res. A 339 (1994) 531-542 has a Lorentzian shape with 10 mm FWHM . The E vs X scatter plot has energy dependent aberrations on the two focal plane edges . c) 32S (88 MeV) + 64 Ni (target like recoils) In this experiment the Nickel-like particles were detected in forward directions in order to investigate the quasi-elastic transfer processes around the Coulomb barrier . Due to the high background at 0° the data were collected starting from 5° with a solid angle of 5 msr ( ± 2° in X and Y direction), which means that the

537

spectrometer was accepting ions from 3° to 7° in the dispersion plane . The beam was dumped on a tantalum plate inside the reaction chamber covering a total angle of ±2° in the horizontal (dispersion) plane . Only the usual four focal plane detector signals were taken . Fig . 8 shows some significant experimental results . From the E vs Bragg Peak (BP) scatter plot we clearly see that in this case the BP signal is sufficient to discriminate the beam-like particles . Fig . 9 shows the mass spectra with a gate on the energy loss signal of

400

tn

360

Z Q

320

m Q 280

} O

W Z 240 W

200 10

0

20

30

40

50

60

70

80

RELATIVE X- POSITION (mm)

400

90

100

110

120

I

q=26`

350

b

M=119

300 q=25

250

q=27`

tn

z 200 O U

150 100 50 I

_eJ'11 , 20

Lu

40

120

l~ 60

80

-

i Ir 100

120

-Ut,

l 140

RELATIVE X - POSITION (m rn)

Fig. 5 . Spectra from the reaction 58 Ni(212 MeV)+ 64 Ni obtained with a gate on the ER "island" : E vs X (a), X-position (mass) (b), Y-position (c).

538

C. Stgnorini et al. INucl. Instr. and Meth. in Phys. Res. A 339 (1994) 531-542 2400

2000

1600

In Z 1200

O U

800

400

" 11 .i IÁ .i .l .l ~.( .U . .~I~'nI~1101ÎI1I o 10 20 30

0

III II111 ~~~ IIIIIIIIIIIIIIII~IiJaJu .mL~.~ LI I 40

50

60

ÎI1 . I1

n o

70

RELATIVE Y- POSITION (m rn)

Fig. 5 (continued)

the nickel-like ions . The X-position spectrum obtained with this gate shows a resolution M/OM-225 . Also in this case energy dependent aberrations are visible in the E vs X scatter plot . 4.1 . Beam rejection capability to day-to-day operation This is a very important characteristic of on-line mass spectrometers: these devices are mainly suited for 0° operation with fusion reactions since the electric rigidity available for the electrostatic deflectors severely

a

800

z 600

a

m

limits the capability of investigating other kinds of reactions. The beam rejection capability of such instruments must be very high since in a fusion reaction with a typical cross section of 100 mb and A = 100 target with a thickness of 200 Wg/cm2 the ratio between beam particles and fusion events is _ 10+7. From the experience gathered up to now with the RMS it has no sense to speak about "the rejection factor" since we have observed that it depends on several factors some of which may be dictated by the experiment, namely : a) ratio between the electric

ER

400

a 0 F

200

200

300

300

500

600

ENERGY (ARBITRARY UNITS)

700

0

20

40

60

80

100

RELATIVE X - POSITION (mm)

Fig. 6. Scatter plots TOF vs E (a) and TOF vs X (b) in the reaction 14 Ni(220 MeV)+ 92 Zr

120

C Signorim et al.INucl. Instr. and Meth, in Phys . Res. A 339 (1994) 531-542

rigidities of the particles under investigation and the primary beam, b) specific target characteristics, c) beam tuning during the specific run.

539

In fig. 10 a collection of experimental rejection factors measured in different experiments is presented. This factor has been calculated as the ratio between the number of ions incident on the target and all the background counts observed in the focal plane. The beam intensity has been evaluated from the target thickness and the numbers of the Rutherford-scattered particles detected in a silicon monitor in a forward direction inside the scattering chamber. The data have been extracted from day-to-day operation without any effort to optimize and fine tune this factor ; we believe that the "intrinsic" rejection of the spectrometer might be better . The values reported for each experiment are the extreme ones (maximum and minimum) . The large variations are mainly due to the different selected charge states . From fig. 10 we can observe that within the experimental errors all data, except one (where we had dead time problems with the electronics), follow a specific trend; the rejection factor anyhow ranges over many orders of magnitude from 10" to 10 +11 depending on various experimental conditions. A recent beam rejection test performed by our group in collaboration with C.N . Davids at the Argonne FMA spectrometer gave similar results.

5. Concluding remarks

b 1600

Co

z O U

1200

800

400

~~dulMIfIUdIN1Y~M"~b ".,,wV+N~,IUUülltl~i~__~ 0 10 20 30 40 50 60 70 RELATIVE Y -POSITION (mm)

Fig. 7. Spectra from the reaction 61 Ni(220 MeV)+ 92 Zr ob tained with a gate on the ER "island" : E vs X, X-position (mass) (a), Y-position (b).

The RMS characteristics agree quite well with the design goal . General comparisons with the most advanced operating spectrometers mentioned in the introduction bring the following considerations . SHIP at GST Darmstadt [2], which has many more optical elements, has a higher beam rejection (it was conceived primarily as a beam filter) but very limited mass resolution in its original structure . CARP at Osaka [9] has intrinsically lower beam rejection built-in capability (only 2 dipoles) but is not seriously handicapped from this because the RCNP AVF cyclotron used in connection with this instrument is suited mainly for light heavy-ion beams. ROCHESTER [7] has a higher mass resolution but with limited solid angle and energy acceptance . DARESBURY [8] has intermediate characteristics partly because some existing hardware (Wien filters) had to be used for the construction . FMA at Argonne [111, the last instrument that came into operation, tries as our RMS to overcome many of these disadvantages, it accepts higher electric rigidity ions at the cost of a smaller acceptance solid angle. Its performances are very similar to the RMS ones . In a variety of experiments the RMS on-line mass spectrometer has proved a very useful tool for nuclear physics research with a good mass resolution and a very large acceptance in energy, solid angle and mass;

540

C. Signonni et al. /Nucl. Instr. and Meth . in Phys. Res. A 339 (1994) 531-542

32S + 64 NI

to z

200

a

150

88 MeV

50

m

a

100

cD

w Z W

50

0

11111-like recoils

200

150

100

50

0

10

20

30

40

50

60

70

80

RELATIVE X - POSITION (mm)

BRAGG PEAK (ARBITRARY UNITS)

32S +64 Fig. 8 Scatter plots E vs Bragg peak and E vs X from the reaction (88 MeV) Ni . In the E vs Bragg Peak plot the N1-like recoils are clearly separated.

250

the spectrometer has also a good beam rejection factor

32S + 64NI

88 MeV

at 0° .

50

All these characteristics make RMS a very powerful instrument unique for 0° operation. It is a rather sophisticated spectrometer which however is not easy to use for all applications .

"Easy experiments" are all the measurements [21]

which require only the relative intensities of the masses

detected or simply a clean separation of certain reac-

tion products to study. This is the case of heavy-ion y

r

rr w z w

coincidences which benefit [29] of the large instrument 50-

200

FZ

10'

M= 64

O 10 , U Q 10 , z

150

0

F 10 , U w w 10'

p 100 U 50F-

66

65

10

20

63

N

30

40

62

10, 50

60

70

80

2

RELATIVE X -POSITION (mm) Fig. 9. Mass spectra from the reaction 32 S(88 MeV)+ 64 Ni obtained with a gate on the Ni target-like recoils.

Fig

4 Ep (beam) / Ep (ions)

6

8

0 10

10. Experimental beam rejection factor in day-to-day operation.

C. Signorini et al. /NucL Instr. and Meth . in Phys. Res. A 339 (1994) 531-542 acceptance and do not require the knowledge of the absolute efficiency . The experiments that need the determination of absolute cross sections require a considerably larger effort connected to the measure of the absolute system efficiency . This depends on many factors like charge state distribution, energy and angular distribution and/or acceptance, and focal plane detector setting. Moreover some of these parameters are cross correlated, and not easily controllable since very often imposed by the specific experiment . These types of experiment are "more difficult" as in all spectrometers with magnetic and electric elements ; they should be planned carefully and take up a large fraction of the available beam time for efficiency determination . In our opinion, they should not constitute routine measurements but rather be planned for few selected cases. A serious basic limitation of this type of spectrometers is given by the maximum electric field which can be produced under vacuum between large (> 10 cm) gaps; according to the experience with our separators and similar ones a realistic upper limit of - 60 kV/cm should be presently accepted . Acknowledgements The authors would like to thank G. Manente for the production of all the strip targets necessary for the experiments and the Tandem operators particularly for the continuous skillful assistance in focusing the beam. We also wish to acknowledge the help in various part of this work of several LNL foreign guests : E. Adamides, Zhi-Chang Li, B. Million, Y. Nagashima, M. Narayanasamy and E. Togun. A particular thanks to J.D . Larson for his continuous assistance . References [1] H.A . Enge, Nucl . Instr. and Meth . 162 (1979) 161. [2] G. Miinzenberg, W. Faust, S. Hofmann, P. Armbruster, K. Giittner and H. Ewald, Nucl . Instr. and Meth . 161 (1979) 65 . [3] K. Rudolph, D. Evers, P. Konrad, K.E .G . L66ner, U. Quade, S.J . Skorka and I. Weidl, Nucl . Instr. and Meth . 204 (1983) 407. [4] H.J . Kim, K.S . Toth, M.N. Rao and J.W . McConnell, Nucl . Instr. and Meth A 249 (1986) 386. [51 A.V . Yeremin, A.N . Andreyev, P.D . Bogdanov, V.I . Chepigin, V .A. Gorshkov, A.I . Ivanenko, A.P . Kabachenko, L.A. Rubinskaya, E.M . Smiznova, S.V . Stepanstov, E.N . Voronkov and G.M . Ter-Akopian, Nucl . Instr. and Meth . A 274 (1989) 528.

541

[6] H.A . Enge and D. Dorn, Nucl . Instr. and Meth . 145 (1977) 271. [7] T.M . Cormier, M.G . Hermann, B.S . Lin and P.M . Swertka, Nucl . Instr. and Meth . 212 (1983) 185. [8] A.N. James, T.P . Morrison, K.L. Ying, K.A . Connell, H.G . Price and J. Simpson, Nucl . Instr. and Meth . A 267 (1988) 144. [91 S. Morinobu, I. Katayama, M. Fuliwara, S. Hotori, N. Ikeda, H. Miyatake, H. Nakabushi and K. Katori, Nucl . Instr. and Meth . B 70 (1992) 331 . [10] C.N . Davids and J.D . Larson, Nucl . Instr. and Meth . B 40/41 (1989) 1224 . [111 C.N . Davids, B.B . Back, K. Bindra, D.J. Henderson, W. Kutchera, T. Lauritsen, Y. Nagame, P. Sugathan, A.V . Ramayya and W.B . Walters, Nucl . Instr. and Meth . B 70 (1992) 358. [12] G.K . Metha, Nuclear Science Centre (New Delhi, India) private communication ; A.K . Sinha et al ., Proc . Symp . Nuclear Physics of our Time, Sanibel Island, Florida, Nov . 18-24, 1992 . [131 J.D . Cole, T.M . Cormier, J.D . Hamilton and A.V . Ramayya, Nucl. Instr. and Meth . B 70 (1992) 343. [14] C. Signorini, S.J . Skorka, P. Spolaore and A. Vitturi (eds .), Proc . Symp . on Heavy Ion Interaction around the Coulomb Barrier, Legnaro 1988, Lecture Notes to Physics, vol. 317 (Springer, Berlin, Heidelberg, New York, 1988). [15] P. Spolaore, J.D . Larson, C. Signorini, S. Beghim, X.K. Zhu and H.Z . Si, Nucl . Instr. and Meth . A 238 (1985) 381. [16] P. Spolaore, Proc . Symp . on Heavy Ion Interaction around the Coulomb Barrier, Legnaro, 1988, eds. C. Signorini, S.J . Skorka, P. Spolaore and A. Vitturi, Lecture Notes in Physics, vol. 317 (Springer, Berlin, Heidelberg, New York, 1988) p. 305. [17] C. Signorini, Recent Advances in Nuclear Physics, Poiana Brasov, 1988 Summer School, eds. M. Petrovici and N.V. Zamfir (World Scientific, Singapore, London, 1989) p. 285. [181 C. Signorini, Proc . Int. Nucl . Physics Conference, S5o Paulo, ed . H. Hussem (World Scientific, Singapore, London, 1990) p. 591. [19] P. Spolaore et al ., Proc . Workshop on Nuclear Structure in Heavy-ion Reaction Dynamics, Notre Dame, 1990, eds. R.R . Betts and J.J . Kolata (Institute of Physics Conference Series no . 109, Bristol, Philadelphia, 1991) p. 63 . [20] A.M . Stefanini, Proc . Int. Conf. on Nucleus-Nucleus Collisions, Kanazawa, 1991, Nucl . Phys. A 538 (1992) 195c. [21] L. Muller et al ., Z. Phys . 341 (1992) 341 ; G. de Angelis et al ., Z. Phys . A 343 (1992) 121; A.M . Stefanini et al ., Nucl . Phys . A 548 (1992) 543. [22] P. Spolaore, G. Bisoffi, X.L . Guan, S. Beghini and C. Signorini, Nucl . Instr. and Meth . A 268 (1988) 397. [23] S. Beghini, G. Bovo and A. Dal Bello, Nucl . Instr . and Meth . A 300 (1991) 328. [241 A. Guerrieri, G. Maron, G. Montagnoli, D.R . Napoli and G. Prete, Nucl . Instr. and Meth . A 299 (1990) 133. [25] We thank H.J . Kim from Oak Ridge Nat. Lab. (USA) for providing the a-source .

542

C. Signorini et al /Nucl. Instr. and Meth. sn Phys. Res. A 339 (1994) 531-542

[26] H . Wollnik . J. Brezina, and C . Geisse, University of Giessen (Germany) unpublished . [27] M . Beckermann, M. Salomaa, A. Sperduto, H . Enge, J . Ball, A. Di Rienzo, S . Gazes, Y. Chen, J .D. Molitoris and Mao Nai-feng, Phys . Rev. Lett . 45 (1980) 1472 . [28] A.M . Stefanini, L. Corradi, H . Moreno, L . Muller, D .R .

Napoli, P . Spolaore, E . Adamides, S . Beghini, G .F . Segato, F . Soramel and C. Signorini, Phys. Lett. B 252 (1990) 43 . [29] P .J . Ennis and C.J . Lister, Nucl . Instr . and Meth . A 313 (1992) 413 .