Low energy radioactive beams by inversion kinematics

NuclearInstruments and Methodsin Physics Research North-Holland

B70 (1992) 374-379 Beam Interactions with Materials 6Atoms

Law energy radioactive beams by inversion kinematics T. Yamaya, M. Saito, T. Miyamoto and T. Ishigaki Department of Physics, Tohoku University, Sendai, 980, Japan

K. Kotajima and K. Gotoh

Department of Nuclear Engineering, Tohoku University, Sendai, 980, Japan

T. Shinozuka and M. Fujioka

Cyclotron and Radioisotope Center, Tohoku University, Sendai, 980, Japan

Kinematically focussed radioactive beams of energies lower than 10 MeV/nucleon were produced by means of inversion kinematics in heavy ion induced reactions on 1 H. The characteristics of the produced radioactive beam were examined with the view of performing scattering and reaction experiments . On the basis of these results, a new beamline was constructed to collect and focus the radioactive beams. Using this beamline the elastically scattered spectra of 150 obtained from the 1H(15N, 150) reaction were observed with an energy resolution better than 0.6 MeV. 1. Introduction The properties of nuclear structures and reaction mechanisms in relation to different interactions between nuclei have so far been studied via stable nucleus-nucleus collisions. However, it is significant to extend the studies to nuclear scattering and reactions using radioactive beams. In particular, low energy radioactive beams provide a great scope for studies of the properties which enhance the effects of strong absorption, fusion, isospin-dependence, coupled channels transfer reactions andso on . Forthe production of radioactive beams, in many cases, heavy ions at energies higher than 100 MeV/nucleon have been applied, since it is useful to use the projectile fragmentation mechanism to convert the primary beam into a secondary radioactive beam . Such a technique has been pioneered at both LBL [1] and GANIL [2] . In an experiment which requires better energy resolution and lower energies, the produced secondarybeams are injected into a storage-cooler ring to cool down and decelerate . An example of such a storage ring is a project with aheavyion cooler and a storage ring being developed at GSI as an important part of the high energy heavy ion accelerator project, SIS18 [3]. However, this project is very large and expensive, and therefore it is very difficult as a general rule to attain one's aims. Nevertheless, low-energy radioactive beams with a small energy spread are indispensable for the studies r, a great variety of different phenomena in nucleus-nucleus collision ..

As one of the methods of radioactive beam production using secondary beams, low-energy beam products utilizing inversion kinematics are useful for scattering and reaction experiments. Although the types of radioactive beams produced by this reaction are limited, the short lifetimes of the produced beams do not prevent the experiments. Furthermore, the emitted particles of the secondary beam are focussed into a very small forward scattering cone according to the inversion kinematics condition In the present work, kinematically focussed radioactive heavy-ion beams of 150 at energies lower than 10 MeV/nucleon were produced from the reaction (15N, 150) on a 1 H target, and the qualities of the obtained secondary beam are experimentally examined . 2. Energy distributions of the secondary beam In the first step, a simple radioactive beam production system was used for examining the secondary beam qualities. This system is able to accommodate all installations in a large scattering chamber with an inside diameter of 90 cm . The radioactive beams are produced from a thick primary target of 1 H by the charge exchange reaction . In the present work, the secondary beams of 150 and 13 N were produced in the reactions (15N, 150) and (13 C, 13 N) on a 1 H target, respectively. In the present paper, the discussions are limited to the 15 0 secondary beam as a typical example. The primary beam of 15N5+ at E = 85 MeV was

0168-583X/9/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

T. Yamaya et al. / Radioactivebeamsby inversion kinematics 100

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large. Two peaks of 150 emitted at Oc .m.= 40° and 140° are shown in fig. lb. The energy difference between

(a) 0 L- =0 .75*

these peaks is smaller than that shown in fig. la. Furthermore, the 15 0 beam emitted at the angles Oc.m . L.e

=0 . 40°

= 80° and 100° shows spectra with a single peak in fig. lc. As seen in these figures, these energy distributions

of 15 0 beams show charactc-istic features of the beam qualities of 15 0 owing to the, inversion kinematics .

0 200

3. Energy resolution and intensity

0 -=1.55*

The effectiveness of the present reaction with re0 - =1 .30'

0 350

spect to the secondary beam's intensity and energy spread depends critically on the material of the primary target incorporating hydrogen atoms, and on the target thickness, the optimum of which is determined by balancing the effects leading to energy spread and yield of the produced secondary beam. Targets of LiH,

(c) 0 L-2.2'

CH 2 and HZ gas as the hydrogen target were tested for the energy resolution of the 15 0 beam . The energy loss of ions by the LiH target is much the same as by the polyethylene target, where the number of hydrogen atoms included in these materials are the same in thickness. However, the LiH target foil became crumpled after long-duration bombardment by the primary beam, resulting in an increase of the energy loss and Energy

Fig. 1. Kinematics dependence of 15 0 beam obtained from the (15N, 150) reaction on 1 H, where the target material is a polyethylene foil.

provided from the Tohoku AVF-cyclotron with a small PIG-type heavy ion source [4] . By an effect of inversion kinematics, all of the emitted 15 0 ions are scattered into a cone with small angles within 01ab = 2.2°. The

ts0 emitted in the 0° direction was secondary beam of tsN primary beam by a small beam separated from the separation magnet installed in the large scattering chamber . The maximum rigidity of the small magnet is

0.8 T m. The ' 50 beam was detected with a telescope system that consists of DE and E solid state detectors, and was identified by

the pulse height from these

detectors. To examine the qualities of the secondary beam by inversion kinematics, the kinematics dependence of the 150 spectra was measured using a polyethylene foil target of 1.9 mg/cm2 thickness. The energy distributions of the 15 0 beam are shown in fig. 1. These energy distributions depend on the angles of outgoing 15 0 in

the center of mass system. For example, the 15 0 beams of E = 77 and 68 MeV are emitted at e,... =0° and 180° (01ab = 0°), respectively. The spectrum in fig. la indicates two peaks of 15 0 emitted at Oc.m. = 16° and 164°, thus the energy difference between these peaks is

the energy spread in the secondary beam. The ratio of energy loss of the ions to the unit number of hydrogen atoms of the polyethylene (C'H Z) target is about three times greater than that of HZ gas. Therefore, if we are able to use a suitable window foil for the gas target cell, HZ gas will be better than polyethylene as the primary target. Aramid films of 0.6 mg/em2 with a coating of evaporated aluminum of 40 l1g/cm2 thick-

ness were used for the gas target window foils. The spectra of 15 0 secondary beams from the reaction on the targets of H Z gas and CH Z foil are compared in fig. 2. The thicknesses of the CH Z and HZ gas targets are 20 wm and 0.5 bar, respectively, and the number of hydrogen atoms in both target materials was almost the same, where the length of the H2 gas cell is 6 cm. In this case, note that the integrated total currents of the primary beam bombarded on both targets were not invariably of the same quantity . The target material dependence of the energy resolution for the secondary

beams is shown in fig . 2. One can see that the gas target is clearly useful for obtaining better energy resolution . The energy spread of 15 0 beam is mainly determined by the difference in energy loss arising from where the reaction takes place in the target. This situation is illustrated in fig . 3 as a function of the

angles of outgoing 15 0 in the laboratory system. As seen in fig. 3, this explains the narrow energy spread of the higher energy peak and the wide energy spread of V. RECOILS/FRAGMENTS

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T. Yamaya et al. / Radioactive beams by inversion kinematics 100 0 .50 bar.

0 .35 bar.

0

0~

0 100 0 .15 bar.

Energy of 150 beam (MeV) Fig . 2. Comparison of the energy spread between 150 beams on H Z gas and polyethylene targets . The number of hydrogen atoms included in both materials is the same. the lower energy peak of the spectra near ©lab = 0° in fig . 2. The energy spread of the secondary beam is also very sensitive to the H Z gas pressure in the gas target cell. The energy losses and energy spreads of the 15 0 spectra for various gas pressures are shown in fig . 4, Energy of "0 beam (MeV)

-3 "

-Z "

'°0

0' -1 . 1° Scattering angle 0

2' (deg)

0

Energy Fig . 4. Dependence of energy spread of 15 0 beam on H Z gas pressure . and the gas pressure dependence of the energy spread (FWHM) is plotted in fig . 5 . Assuming that the beam energy spread increases linearly with the gas pressure, an energy spread of 0.2 MeV at zero pressure is estimated . This energy spread originates in the energy straggling of the primary and secondary beams by the window foils at the entrance and exit of the gas target cell, adding to the intrinsic energy spread of the primary beam. The elastic scattering of 15 0 from Pb was measured at 01ab = 4° . The obtained energy spectrum is shown in fig. 6 . The energy resolution of this beam was about 0.6 MeV, and the production yield was about 104 /s for the 100 enA 15N5+ primary beam.

3°

Fig. 3. Energy spread of the secondary 15 0 beam by the kinematics condition . The solid line indicates the kinematics of the 1 1 .1( 15 N, 150) reaction at the target entrance, and the dashed line indicates the kinematics of the reaction at the target exit. The shadow shows the energy spread of 15 0 beam by the difference in energy loss arising from the reaction at the different places .

4. Optical focussing of radioactive beams In the first step of the production of radioactive beams using inversion kinematics, the produced secondary beam was not optically focussed at the secondary target position . Thus, the size of these beams on the secondary target is defined by a defining slit

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T. Yamaya et al. / Radioactivebeamsby inversion kinematics

(MeV) 2

Energy resolution of "0 beam

Energy

Fig. 6. Elastically scattered spectra of 15 0 at e heb = 4° from the Pb target.

0. 2 MeV 0

0.1

0.2

0.3

0.4

0.5

0.6

H, gas pressure (bar.)

Fig. 5. Dependence of secondary beam energy resolution on HZ gas pressure . One can take the value of 0.2 MeV to be the intercept on the ordinate in this figure . This value is understood as being the energy spread due to the window foils of the gas target cell and the intrinsic energy spread of the primary 15 Nbeam . placed in front of the target for the experiments. In this case, part of the useful beam is cut off by the slit, and furthermore, many fragments of the inelastically scattered primary beam also bombard the target. Therefore, it is very significant for the scattering and reaction experiments that the produced radioactive beam is focussed on the secondary target . All of the produced secondary 150 beam is emitted into a scattering cone of 2.2° in the laboratory system . 150 beam emitted at angles smaller than 0.6°, in the present case, were focussed at the secondary target position by the ion optical system. This system consists

of a quadrupole doublet and a 27° deflection dipole magnet. An outline of the ion optical system is shown in fig. 7. The maximum rigidity of the dipole magnet is 1.1 T m for the central trajectory . For the quadrupole doublet, the pole gaps are 4 cm and the maximum magnetic gradient is 4 kG/cm. A ray plot of the secondary beam is shown in fig. 8. The upper part of the plot is for the vertical direction and the lower part is for the horizontal direction. A primary 1sN beam collimated to a spot of 2 mm diameter bombarded the primary target. The acceptance angles of the quadrupole magnet are 0.6° for the horizontal direction and 1.4° for the vertical direction in the laboratory system. These acceptance angle regions in the center of mass system correspond to -16° ( -164°) 5 ec.m.5 + 16° (+ 164°) for the horizontal direction and -40° ( -140°) 5 ©c.m.5 +40° ( +140°) for the vertical direction. The secondary beam transmitted through the quadrupole magnets was tuned to a point-to-parallel ray, and was focussed at the secondary target position by the dipole magnet . The beam spot on the target was smaller than 2 mm in diameter . Then the resolving scattering chamber

large scattering chamber Fig. 7. Schematic layout of the optically focussed secondary beamline. V. RECOILS/FRAGMENTS

T. Yamaya et al /Radioactive beams by inversion kinematics

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Primary target

beam selector quadlupole doublet magnet dipole magnet

secondary target position 15

Vertical

10 5

-H

0

IlrImary

5

beam

10 15

Fig. 8. Rayplot for the ion optical system . The upper and lower plots are for the vertical and horizontal directions, respectively. power Plt1 P was about 100. In the present geometrical acceptance for the secondary beam, if one assumes an isotropic angular distribution (in the c.m . system) of 150 in the 'H(' 5N, 150) reaction, about 14% of the total cross section can be used as a good resolution secondary, beam for the experiments. At an incident energy of 85 MeV, considering that the angular distribution of emitted '50 shows large differential cross sections at forward and backward angles in the c.m. system, about 20% of the total cross section was useful for the experiments. As shown in fig . 9, the energy spread depends on the ratio of beam acceptance to the total cross section. In the present work, the energy spread due only to the kinematics condition of overall resolution is about 0.6 MeV.

ô ö d m

5. Conclusion The energy spectrum of the finally obsérved radioactive beam is shown in the upper part of fig. 10. The energy resolution of the beam is about 0.6 MeV (FWHM). Furthermore, the spectrum of the elastically scattered 150 at Olab = 10° from Pb is shown in the 300

a

Secondary beam spectra of ' ° 0

A ;

'N('°N,'°0)n at E=85 MeV

4

.-

0 .6

0

; r 10

MeV

Elastic scattering of "0 on Pb

Pb("0,"0) E=65 MeV

at 0-=10'

- 0 .6 MeV

acceptance ratio (%) Fig. 9. Energy spread dependence of the beam acceptance ratio to the total cross section. The angulardistribution of '50 was asûumed to be isotropic in thecenter of mass system.

Fig. 10. Energy spectra of the optically focussed secondary ' 50 beam (upper part) and the elastically scattered 150 from Pb (lower part). The thickness of the Pb target is about 1 Mg/c.2 .

T. Yamaya et al. / Radioactive beams by inversion kinematics lower part of fig. 10, compared to the secondary beam spectrum. The energy resolution of the scattered spectrum was as good as that of the secondary beam. It is suggested that the energy spread due to the emittance of the secondary beam is very small compared to the intrinsic energy spread of the secondary beam . The production yield of useful 110 beam for the scattering and the reaction experiments was about 10° /s for the 100 enA primary I5N5+ beam.

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References [1] I. Tanihata et al., Phys. Rev. Lett. 55 (1985) 2676 . [2] J .P. Dufour et al., Nucl . Instr. and Meth . A248 (1986) 267 ; R . Anne et al., Nucl . Instr. and Meth. A257 (1987) 215 . [3] G . Münzenberg et al ., Proc . 1st Int . Conf. on Radioactive Nuclear Beams, Berkeley, California, USA, 1991, eds. W.D . Myers, J .M. Nitschke and E.B. Norman, p . 91. [4] T. Yamaya et al ., Nucl . Instr. and Meth. 226 (1984) 219 .

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