An electrostatic separator for heavy recoil nuclei

Nuclear Instruments and Methods in Physics Research 219 (1984) 513-518 North-Holland, Amsterdam

513

AN E L E C T R O S T A T I C S E P A R A T O R F O R HEAVY R E C O I L NUCLEI M. D A H L I N G E R , W. B O N I N a n d E. K A N K E L E I T lnstitut fur Kernphysik der Technischen Hochschule Darmstadt, Schlossgartenstr. 9, D-6100 Darmstadt, West German); H. B A C K E lnstitut fur Physik tier Universitiit Mainz, Postfach 3980, D-6500 Mainz, West Germany Received 31 August 1983

An electrostatic separator is described which separates evaporation residues produced in heavy ion reactions with medium heavy projectiles (e.g. 160) from the beam particles and fission fragments. The high charge state of the recoil nuclei, resulting from electron stripping in the target and Auger cascades following internal conversion transitions, is taken advantage of for deflection in a simple electrical condensor with a potential difference of less than 65 kV. In combination with a conversion electron spectrometer and germanium detectors, the spectroscopy of evaporation residues becomes possible also for reactions with high fission competition.

1. Introduction It is well known that the investigation of nuclear structure by means of in-beam "/-ray or conversion electron spectroscopy after (HI, xn) reactions in nuclei with Z >/82 is made difficult by the large background resulting from fission reactions. Many different techniques have been applied in the past to overcome this problem. The background can be suppressed by anticoincidence measurements with the fission fragments [1] but coincidence measurements with evaporation residues have been proven to be much more efficient. The latter technique uses the fact that the residual nuclei are knocked out of a thin target due to the transferred momentum of the projectile. A triple focusing magnet has been described [2] to separate fission isomers from the beam. For the separation of evaporation residues produced by light particle reactions an ellipsoidal electrostatic mirror was developed [3]. A velocity filter with electric and magnetic fields [4] has been used with great success for the separation and fusion products after heavy ion reactions. Alternatively, the different ranges of recoil ions and beam particles or fission fragments can be utilized also' by means of gas scintillation counters [5]. In this paper we report on the performance of an electrostatic separator for evaporation residues produced after (HI, xn) reactions with medium heavy projectiles (e.g. 16'180). The electrostatic separation principle was chosen because the radius of curvature, p, is proportional to E / q in an electric field, whereas in a magnetic field p is proportional to p / q (here, E is the 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

kinetic energy, p the momentum and q the atomic charge state of the particle). Furthermore, the momentum p of the projectiles and evaporation residues in fusion reactions is approximately equal but the energy E of the recoil nuclei is much smaller than the energy of the beam particles or fission fragments. By way of example, for the reaction 2°spb(180,4n)222Th at a beam energy of 95 MeV the ratios of the radii of curvature for recoil ions 0r (qr = 19+), for projectiles pp (qp = 8 +) and for typical fission fragments Pf (qf = 21 +) is pe: p~: p~ = 1 : 31 : 10 in an electric field and pm : ppi n : p~ = 1 : 2.5 : 2.2 in a magnetic field. The separator uses the fact that high charge states occur due to stripping of electrons in the target in combination with Auger cascades following internal conversion transitions outside the target. In the above mentioned reaction a mean charge state of 19 + is attained already after one converted transition. Therefore, a fast and efficient separation of the recoiling nuclei from the beam and fission fragments can be achieved in an electrostatic field of a strength as small as = 20 kV/cm. Thus, the electrostatic separator can be designed in a very compact manner and can be coupled easily to various experimental set-ups.

2. Experimental In the following we describe an experimental configuration installed at the Max-Planck-Institut for Kernphysik in "Heidelberg, in which the separator was used in connection with a solenoidal electron transport sys-

514

M. Dahlinger et al. /" Electrostatic separator for heaqv recoil nuclet

~/~~Si

(L i)

[~Feedthrough

till i ll

_

~

{

i

U

v+lOcr ~i

Cooling

P

:~'o,~

Fig. 1. Cut through the experimental set-up (B = beam, T = target). Conversion electrons are detected in two Si(Li) counters and y-rays in two Ge detectors, one placed under the target and not displayed here. The recoiling nuclei enter the separator through the exit slit of the solenoid, are deflected between the high voltage electrode (H) and the grounded grid electrode (G) and are detected with a surface barrier detector (D) which is cooled by means of a cooling coil in the flange. The beam is stopped in a Faraday cup (FC) about three meters away (P = turbomolecular pump under the separator).

tem [6] for conversion electron spectroscopy. Fig. 1 shows a cut containing the solenoid and b e a m axis. The 180 b e a m o f the M P - t a n d e m accelerator with an energy of 95 MeV hits the enriched 2°spb target of 120 # g / c m 2

(sandwiched between carbon layers of 35 and 15 / x g / c m 2) positioned on the solenoid axis. The energy of the conversion electrons is measured with two LN 2cooled Si(Li) detectors at one end of the solenoid tube, to which the electrons move on helical paths along the

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.-~ 2 0 0

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200 I

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300

400

500

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Fig. 2. Gamma-ray spectra taken after the bombardment of a 120 # g / c m 2 2°spb target with a 180 beam of 95 MeV energy. The upper part shows the y-ray singles spectrum while the lower part is the y-ray spectrum recorded in coincidence with recoil nuclei. Transitions labeled by a star are assigneg! to 222Th (see ref. 12). A modified y - y experimental set-up was used for this measurement.

0.0 0

100

200

e"

-

Energy

300

400

500

(keV)

Fig. 3. Electron spectra taken after the bombardment of a 190 # g / c m 2 2°spb target with a 160 beam of 94 MeV energy. The experimental set-up of fig. 1 was used. The upper part shows the e--singles spectrum, the lower part the e--spectrum recorded in coincidence with recoil nuclei. The three dominant conversion electron transitions are assigned to 22°Th.

M. Dahlinger et al. / Electrostatic separator for heavy recoil nuclei

magnetic field lines. Two germanium detectors of 15% peak-efficiency are positioned at 90 ° with respect to the beam axis at a distance of 16 cm, one under the target and the other at the shorter end of the solenoid tube. With these detectors double and triple coincidences of "t-quanta, conversion electrons and recoil nuclei can be taken simultaneously. The recoil nuclei leave the solenoid through an exit slit, which limits the acceptance angle to + 3.8 ° in plane and +11.3 ° perpendicular to the plane of projection, and then enter the electrostatic separator. It consists of two 335 mm long electrodes inclined at 3.8 ° from the beam axis to avoid secondary scattering of beam particles. The high voltage electrode is made of polished aluminium, while the electrode at ground consists of longitudinal strained resistance wires of 0.35 mm diameter with a separation of 3 mm. The minimum separation distance of the electrodes is 30 mm. A positive high voltage of maximum 65 kV (limited by the power supply) is applied to the massive electrode via a high vacuum FRIALIT ® ceramic feedthrough (No. 552-0970-1 of Friedrichsfeld Mannheim, Germany). No breakdowns have been observed during many days of operation. The deflected evaporation residues leave the electric field through the grid electrode and can be detected in a rectangular surface barrier detector with an active area of 34 × 58 mm 2. For optimization of the detection efficiency, the detector can be displaced easily along a support parallel to the grid electrode. The optimum detector position is determined as described in the next section. Typical "y-ray and electron spectra are shown in figs. 2 and 3 together with the corresponding singles spectra. The strong suppression of the background originating from ,fission and transfer reactions is obvious.

I

20 A

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30

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l

l

40

l

l

50

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High Vottage (kV) Fig. 4. Measured detection efficiency e as a function of the supplied high voltage for the reaction 166Er(160,4n)178Oswith 88 MeV projectile energy and a 250/~g/cm 2 target. The results of the Monte Carlo calculation for this reaction are shown by the shaded curve. The parameters used were 01/2 = 7.1°, and Eer = 7.1 ( + 1.1) MeV, qlc + ~/s= 16.6 ( + 3.6) and ~/2¢+ ~/s= 24.6 (+ 6.9) (variances in parantheses).

515

Although the separator has no focusing elements, its detection efficiency c is rather high. It is defined by the ratio of evaporation residues leaving the target to detected recoil nuclei and was measured using the reaction 166Er(160, 4n)178Os at 88 MeV beam energy with a target of 250 # g / c m 2 areal density. This reaction has a large cross section for the production of evaporation residues. Electron singles spectra and spectra in coincidence with evaporation residues have been taken using the recoil shadow method [6]. The efficiency c was determined by calculating the intensity ratio of the Lplus M-conversion electron lines of the 132 keV transition in 178Os appearing in the coincidence and singles spectra. With this method the detection efficiency was measured for such evaporation residues which experienced at least one internal conversion transition. The results of these measurements are shown in fig. 4 as a function of the applied high voltage. At about 50 kV the detection efficiency is as high as 25% for this reaction. It should be stressed that ~ depends strongly on the target thickness because of multiple scattering processes in the target and acceptance limitation by the exit slit of the solenoid.

3. Calculation of trajectories, detection efficiency and discussion In principle, the optimum position of the recoil detector can be determined experimentally. However, since many parameters can be varied, such a procedure is very beam-time consuming. Therefore, the optimum detector position was calculated by a Monte Carlo simulation will be described in this section. The correctness of this simulation was tested by comparing the experimental and calculated detection efficiencies for the 166Er(160,4n)178Os reaction. At first, the electric potential and the electric field strength were evaluated by a method described in ref. 7. Thereafter, the trajectories in this field were calculated by solving the equations of motion by an extrapolation method with rational functions [8]. In fig. 5 the equipotential lines calculated in this way are shown together with some typical trajectories. For the Monte-Carlo calculations, the ionic charge state and the kinetic energy of the evaporation residue, its angle ~ with respect to the beam direction and its azimuthal angle ~ are generated randomly. An uniform distribution for 4, and Gaussian distributions for the other parameters are assumed. The calculation of the mean values and variances of the energy- and angular distribution is described in the appendix. The charge state of the evaporation residues results from electron stripping in the target and from Auger cascades after internal conversion behind the target. For the equilibrium charge distribution caused by stripping

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516

H

B lOcm

R"

D

G

Fig. 5. Calculated equipotential lines for a high voltage of 55 kV and a voltage difference between two lines of 5 kV. Trajectories are shown for recoil nuclei (R) with E = 7.2 MeV, A = 222, q = 19 + and three different starting angles (0 °, ± 3.8 ° with respect to the beam axis), an ]SO projectile (B) with E = 95 MeV and q = 8 + and fission fragments (F) with E = 83 MeV, A =111 and q = 21 * (starting angles 0 ° and -3.8°). For further abbreviations see fig. 1.

one gets according to ref. 9 a Gauss±an with the mean value

,~ = z{1 + [ v / ( z ° v , ) ] - ' ~ }

-~,

jectile energy. The n u m b e r of particles reaching the detector normalized to the number of nuclei starting from the target, gives the calculated detection efficiency

(a)

and a variance I

aq~ = do { £/s[ 1 - ( ? i s / Z ) t/~ ] }]/2

(2)

10

where Z denotes the nuclear charge and v the velocity o f the evaporation residue. The values ~ = 0.6, a = 0.45, v' = 0.36 c m / n s and d o = 0.5 have been chosen for the other parameters [9]. The mean charge state, ~/1c, after one internal conversion process is taken from fig. 3 of ref. 10, in which the charge state due to Auger cascades after a sudden creation of a hole in a specified atomic shell is displayed. A single converted E2-transition, affecting essentially the K-, L n- and Lin-shells, results for Th in a m e a n charge state of 11.3. The mean charge state is shifted by additional 8 and 5 charge units after a second and a third converted nuclear transition, respectively [11]. The full width at half m a x i m u m is 8 - 1 0 charge units per converted transition [11]. The n u m b e r of converted transitions (0, 1, 2 . . . . ) is determined r a n d o m l y from the known or expected nuclear level scheme and the corresponding conversion coefficients. The final charge state is assumed to be the sum of the charge states due to electron stripping and conversion. In order to calculate the detection efficiency of the separator, a M o n t e Carlo calculation with several h u n d r e d s of particles was performed. Fig. 6 shows as an example the calculated points of impact of recoil nuclei for the reaction 2 ° s p b ( t s o , 4n)222Th at 95 MeV pro-

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y(cm) Fig. 6. Scatterplot in the detector plane at a distance of 35 cm from the target. The calculated points of impact of 500 evaporation residues after the reaction 2°spb(lso,4n)~22Th with 95 MeV projectile energy and a 120 # g / c m 2 target are shown. A high voltage of 55 kV was assumed. The detector position is optimum for those particles which underwent at least one internal conversion transition. The parameters used are v~/2 = 5.1 ° and Ee~ = 7.2 (+0.8) MeV, ~h = 8.1 ( ± 3.3), ~/~¢+ qs = 19.4 ( ± 3.6) and ~/2c+ qs = 27.4 ( ± 6.9) (variances in parentheses). The points of impact of a projectile (B) and fission fragments (F) are displayed assuming parameters given in fig. 5. The target is located at the origin of the coordinate system.

M. Dahlinger et at / Electrostatic separatorfor heavy recoil nuclei of the separator. The detector position was optimized to obtain the maximum efficiency. The so calculated optimum detection efficiency for the above mentioned reaction, assuming a target of 120 # g / c m 2 areal density and a supplied high voltage of 55 kV turns out to be (51 + 3)% for evaporation residues which underwent at least one converted transition. For the 166Er(160, 4n)178Os reaction, these calculations reproduce the experimentally determined detection efficiency with good accuracy, see fig. 4. The reduced detection efficiency is mainly caused by losses due to multiple scattering in the comparatively thick Er-target of 250 # g / c m 2. In conclusion, it can be said that the electrostatic device described in this paper separates highly charged evaporation residues with o/c = 1% very effectively from the beam particles and fission fragments. In combination with "y-ray and electron detectors, in-beam spectroscopy becomes possible after reactions leading to very fissile compound nuclei. The separation of faster recoil ions is impeded due to their greater electric rigidity. To overcome this problem, it might be sufficient to slow down the recoil nuclei by means of an appropriate target backing. The separation of evaporation residues induced by reactions with light projectiles suffers from a decrease of the detection efficiency due to the broadening of the angular distribution and the problems concerning the detection of slow recoils with surface barrier counters. Perhaps, the use of channel plates or open photomultipliers may solve this problem. We gratefully acknowledge fruitful discussions with D. Habs. We thank W. Patzner for the production of the recoil detectors and H. Folger for target preparation. We are indebted also to S. Glienke, M. Kramer and B. Schwartz for their help during measurements. This work was supported in part by the Bundesministerium fiir Forschung und Technologic and GSI in Darmstadt.

Appendix

Energy- and angular distribution of the evaporation residues The mean energy,/~e~, of an evaporation residue can be calculated from the kinematics of the reaction as follows: Eer = ( A p A e J A 2 , ) " ( E p - 0.5~$Ep)- 0.5~$Eer,

517

was assumed to be half of the maximum energy loss, ~Eer. For numerical calculations 6Ep and SEer are taken from ref. 13. The full width at half-maximum of the energy distribution consists of three parts, which are added quadratically: (1) broadening due to the target thickness: AEtt = I~Eer- ~Ep'Ap/Aerl',

(4)

(2) broadening due to energy straggling [14]: AEstr = 2.89 × ] 0 - 2 [

~EpZpZt( Zip/3 q- Z ~ / 3 )

+SeorZe,.Z,(Z:? +

'/2,

-2

O)

with Zp, Z t and Ze~ the projectile, target and evaporation residue charge number, respectively; and (3) broadening after particle emission: AEem = (8 In 2)1/2(2/31/2)( EerxE,/Aer ) 1/2 ,

(6)

where x is the number of evaporated nucleons a n d / T , the mean energy of an emitted particle. For the angular distribution, two contributions have to be considered which are added also quadratically: (1) spreading after particle emission: 01/2 --1/2 e m _- [(2 In 2 ) / 3 ] 1/2 [xEn//(AerEer)] = A Eem/4Eer ;

(7)

and (2) spreading due to multiple scattering according to ref. 15:

0,/2 = 2 Zef Zte 2 6 , / 2 / ( a g e , ) ,

(8)

with the screening parameter a=O.885ao/[Z2t/3+ Zff/3] a/2 (Bohr radius a 0 = 5.29 × 104 fm) and e the elementary charge (e 2 = 1.44 MeV fro). In order to simplify the calculation of the reduced half width, 61/2, as a function of the reduced target thickness ~-= NL~ra2t/A ( N L = Avogadro s number and t = target thickness in g/cm2), the following formula was used: 61/2 = 0.3697"r 0"7377.

(9)

The numerical parameters were obtained by fitting data points given in ref. 16.

References

(3)

where (Ep- 0.5~Ep) is the mean energy of the projectile, 8Ep is the total energy loss of the projectile in the target and Ap, Aer, Acn are the mass numbers of the projectile, evaporation residue and compound nucleus, respectively. The second term in eq. (3) originates from the mean energy loss of the evaporation residue, which

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Meth. 188 (1981) 535.

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M. Dahlinger et al. / Electrostatic separator for hea~:v recoil nuclei

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