ON-LINE AT THE ARGONNE SUPERCONDUCTING LINAC

C. N. Davids* Argonne National Laboratory, Argonne, IL 60439

G. W. Greenlees,** M. A. Finn* University of Minnesota, Minneapolis, MN 55455

D. A. Lewis,"'" R. M. Evans^ Iowa State University, Ames, IA 50011

This project, a collaboration between Argonne National Laboratory, Iowa State University, and the University of Minnesota, will use on-line laser spectroscopy to study the optical hyperfine structure of radioactive atoms. The objective is to extract information on spins, moments, and the variation of charge radii for ground states and isomers. The species under investigation will be produced by heavy- beams from the Argonne superconducting linac. The radioactive atoms recoil from the production target, become thermalized in a helium atmosphere, and then are transported by a liquid-nitrogen-cooled helium jet to the laser interaction region. Resonance fluorescence spectroscopy will be employed to observe the optical transitions. Essentially Doppler-free linewidths will be obtained by collimating the atoms into an atomic beam as they emerge from the helium jet. Two cooled photomultiplier tubes will detect the fluorescent light.

Figure 1 shows an outline drawing of the target chamber, including the cryogenic helium jet and the laser interaction volume. The target cylinder, sealed by 8 mg/cm^ Ta entrance and exit windows, has space for 5 or more foil targets. It is coupled to the 0.8 mm I.D., 50 cm long helium jet capillary tube by a funnel- shaped section. Both the capillary and target chamber are mounted on a large copper heat sink which is kept at 78°K by a continuous flow of liquid nitrogen. TO

The capillary discharges into a volume pumped by a 120 i/s Roots blower, and is positioned over a skimmer with a 0.63 mm dia. hole. Below the skimmer, the atomic beam is TO DIFFUSION intersected at 90° by laser light at the common line focus of a double elliptical cylinder. Fig. 1. Outline drawing of target chamber, Since the interaction region is 2 cm long, cryogenic helium jet, and laser several hundred photons will be scattered on interaction region. resonance by a single atom possessing an appropriate allowed transition. Pressure in this region is kept at <3 x 10 torr by a 6" chamber, a distance of about 3 meters. diffusion pump directly below the cylinder. Currently we have a CW ring , Spectra-Physics Model 380A, which is pumped by a To study the transport of radioactivities 5 watt Ar+ laser. The laser frequency is through the system, provision has been made for controlled using an offset locking technique, ' the insertion of catcher foils directly under based on a 6328 Â stabilized He-Ne laser. Light the capillary exit and under the skimmer. In from the dye laser has a frequency width of less the first measurement, a beam of 59 MeV C than 5 MHz, and the centroid can be controlled was used to bombard a natural Ni foil of to an accuracy of several hundred kHz by the thickness 1.1 mg/cm . Based on cross sections stabilization system. A separate atomic beam calculated with the program "Alice", the chamber for stable isotopes provides a efficiency of the helium jet transport was convenient frequency reference. To set the estimated to be £ 35%. Skimmer transmission was laser frequency approximately within ~0.5 GHz, a determined to be 2.6 ± 0.2 x 10 under the digital wavemeter based on a Michelson present conditions. It is hoped that this interferometer has been constructed. figure will be improved in future tests. Initial experiments are planned for atomic Figure 2 shows a block diagram of the laser transitions in which the photon burst technique system. These components are located in a can be used. In this method, bursts of photons shielded room immediately adjacent to the linac originating from multiple excitation of the same beam line. The laser beam will be conducted atom in the laser beam are recorded in separate through a hole in the wall to the interaction multiplicity spectra as a function of laser

42 - SLOW NOISE REDUCING PD2 SERVO FAST

8 GHZ PDI ETALON

ATT. t TUNABLE AR+ PUMP TO A.B. --*DYE JE LASER _^ s*-^— * CHAMBERS & LASER WAVEMETER

TUNABLE HENE LASER I I 0.5 GHZ I ETALON STABILIZED POL HENE LASER T ^ f--* PD4

HENE 633 _L PD5 SERVO NM • ETALON SERVO PD3 PDP 1 11/03 i fc DYE SERVO

BURST )PMT I 1 SORTING ELECTRONICS {PMT 2

Fig. 2. Block diagram of laser system.

wavelength. The resulting suppression of random DISCUSSION background noise leads to the ability to detect The photon-burst method, though sen• resonance-fluorescence peaks with as few as 10 D.E. Murnick: sitive, may have several potential problems - bran• atoms/sec crossing the laser beam. ching to metastable states, sensitivity to optical pumping to inaccessible levels and ineffectiveness Barium will be the first element studied. for low oscillator strengths, for example. I believe It has an appropriate photon burst transition, that it will only be useful for a few atomic species. and measurements of the isotope shift exist for How are the recoil atoms neutralized? And what is many short-lived isotopes, thus providing a the velocity profile of the final atomic beam? convenient testing ground for the techniques and apparatus. C.N. Davids: The photon-burst technique is not uni• versally applicable to all atoms. The requirements *Work supported by the U.S. Department of Energy are a recyclable transition with a lifetime in the under Contract W-31-109-Eng-38. tens of nanoseconds. Where the atomic spin J of the ground state is 0, no optical pumping limitations **Work supported by the National Science are present if a depolariser is used. This depola• Foundation. rization can be done either spatially or temporally, at rates comparable to the optical pumping rate. ^Work supported by the U.S. Department of Energy Optical pumping problems are not present if the to• under Contract DE-AC02-79ER10496. tal spin F of the lower level is less than that for the upper level of the transition. In cases where ^M. Blann, University of Rochester, Nuclear the ground state atomic spin J is >0, techniques Structure Laboratory, Report No. COO-3494-29 like RF double resonance can be brought to bear to (unpublished). eliminate optical pumping problems. All the atoms may not be neutral. We are counting

2)H. Gerhardt and A. Timmerman, Opt. Commun. 21, on the large number of collisions in the capillary 343 (1977). and the free electrons in the target chamber due to the bombardment to maximize the neutral atom frac• 3^F. V. Kowalski et al., J. Opt. Soc. Am. 66, tion. The atomic beam exists from the capillary at 465 (1976). the local speed of sound.

4 W.-D. Schmidt-Ott: Have you thought about using a ^D. A. Lewis et al., Phys. Rev. A _19_, 1580 laval-nozzle at the exit of your capillary in order (1979). to obtain a focused jet and to increase the Skimmer efficiency? C.N. Davids: Yes, we have prepared these nozzles and will be trying them in our next beamtime.

- 43 - A DYE LASER-CRYOGENIC HELIUM JET SYSTEM AND RECOIL-MASS-SEPARATOR FOR STUDIES OF NUCLEI FAR FROM STABILITY*

D.L. Clark, A.G. Martin, N.G. Nicolis, S. Banerjee,

T.M. Cormier, P.M. Stwertka, B.S. Lin and M.G. Herman

Nuclear Structure Research Laboratory, University of Rochester, Rochester, New York, USA.

Most of the recent applications of production of neutron deficient nuclei by to on-line measurements of hyperfine means of fusion-evaporation reactions that interactions and isotope shifts have been is limited only by the onset of charge par• made using high energy proton beams to pro• ticle evaporation, not beam energy. duce large fluxes of a wide range of unsta• ble atoms1). On-line mass separators The laser system shown schematically select the atomic species of interest. At in figure 1 is based on a Spectra-Physics NSRL we have in the final stages of devel• 380D stabilized ring dye laser which is opment an on-line laser spectroscopy system pumped by an 18 watt -ion laser. The for the upgraded MP tandem. The system is dye laser has been operated with bandwidth based on the use of more selective heavy- as narrow as 0.2 MHz and output power at ion reactions so that mass separation is 580 nm of greater than 1 wat. Long term not usually required, and very sensitive stability and computer control of the laser detection techniques so that measurements is provided by a "Sigmameter" which is are possible with small fluxes of atoms. similar in design to the device described A recoil mass separator is also near com• by Juncar and Pinard2). The Sigmameter is pletion and will be used for a variety of in turn electro-optically locked to a studies of nuclei far from stability, in• stabilized He-Ne laser that has a long term cluding providing, when necessary, mass stability better than 0.5 MHz. The Sigma• separated reaction products for study using meter has been designed to allow computer the laser system. The heavy ion beam ener• control of the laser frequency in discrete gy available from the upgraded MP allows steps as small as 1 to 2 MHz.

PDP-IO MICHELSON

WAVEMETER PDP-8 \—V X COMPUTER SIGMAMETER INTERFACE SIGMAMETER ELECTRONICS

STABILIZED He - Ne STABILIZATION LASER ELECTRONICS

ARGON ION LASER RING DYE LASERL STABILIZATION SP 171 SP 380D f CAVITIES TO H COMPUTER

HELIUM JET CAPILLARY

HEAVY-ION PARABOLIC REFLECTOR BEAM

TO TARGET COMPUTER

Figure 1. Schematic diagram of the on-line laser spectroscopy system.

- 44 - Unstable atoms are transported from beam tests5). A schematic layout of the the target to the laser beam by means of instrument is shown in figure 2. Six qua• a cryogenic helium jet system. The typi• drupole magnets, two high voltage electro• cally helium jet transport efficiency has static deflectors (0=15°, E/q=6 MeV/e) and been improved by means of aerosols to which one dipole magnet are the major components the nuclei become attached. This is an of the system. These components are ar• unsatisfactory technique for our studies ranged to produce a velocity dispersionless since the hyperfine spectra would be radi• focus just beyond the last quadruople. Re• cally altered. Aystö, et al.3) have shown action products recoiling from the target that cooling the helium also improves the with a given mass/charge are focused onto transport efficiency to useable levels. a spot with diameter less than 3 mm inde• In our system, the helium jet capillary is pendent of velocity. The primary beam is less than 1 meter long and transport effi• stopped inside the first electrostatic ciency of about 50% is expected. The deflector. Scattered beam particles are greatest losses are suffered in collimating efficiently rejected by the subsequent the atoms into an atomic beam to eliminate system components. Reaction products of Doppler broadening. The helium jet skimmer varying mass are dispersed along the focal is jumped by an 800 i/sec blower and the plane by ^13 mm/%. This is adequate to main chamber is evacuated by a 260 i¿/sec provide complete separation in any mass turbomolecular pump. range.

The radioactive atoms can be detected The useful solid angle of the spectro• by the previously reported multi-photon meter is limited by the chromatic aberra• detection scheme^) for atoms such as barium tions of the focusing elements. These or strontium or by standard resonance fluo• aberrations are partially corrected by a rescence and optical pumping techniques for weak sextupole field at the entrance to the other cases. The photon detection chamber dipole magnet. In actual applications we uses two parabolic reflectors in a clam expect useful solid angles of ^2 msr at shell configuration, as shown in figure 1, M=250, 4 msr at M=100 and 8 msr for M<_50 to collect photons from about 90% of 4TT AMU. steradians and image the interaction region onto two high quantum efficiency photomul- In typical heavy ion fusion evapora• tipliers. Two PMT's allow discrimination tion reactions, the recoil velocities of against afterpulsing which is a serious the ions of interest can be quite low background for higher multiplicities. The resulting in transit times of >5 usee. parabolic clam shells have been designed This corresponds to the shortest lifetime to allow an interaction region of ^2.5 cm that will be observable at the focal plane to maximize the number of photons produced for M=250 AMU. For lighter evaporation by atoms when photon scattering is not residues the recoil velocity is higher and limited by optical pumping. the minimum lifetime correspondingly short• er. In order to minimize atomic charge The laser system will be used in con• exchange during transit through the spec• junction with a recoil-mass - spectrometer trometer, an overall system vacuum of (RMS), which is in the initial stages of 10"8 torr is maintained.

Figure 2. Schematic layout of the recoil mass spectrometer.

- 45 - The spectrometer will be applied as a products transported to the various detec• fast separator of unstable species whose tion apparatus described above. In addi• decay can be studied in a wide variety of tion, since the RMS separates the reaction ways at the focal plane. Alpha decay and products from the beam it is possible to beta-gamma coincidence experiments of short catch the reaction products in a cooled lived products are in the planning stage. helium gas cell and directly observe opti• cal pumping or resonance fluorescence. The The study of prompt y decay of primary inhomogeneous Doppler broadening can be evaporation residues will be a major appli• greatly reduced by cooling the cell to tem• cation of the spectrometer. In these ex• peratures in the range of 15-20° K. This periments gammas detected at the target can be relatively easily accomplished using will be observed in coincidence with mass a cryopump cooling element. Although the selected products at the focal plane. This resonances are much broader in a gas cell procedure will provide in most cases a and there are relaxation processes that factor of 10 to 100 improvement in effi• must be considered in optical pumping, the ciency compared to the usual yy coincidence elimination of losses in the helium jet techniques. It is planned to use this raay result in an overall improvement. improved efficiency to extend (HI, xny) This is, in particular, true for very short studies well into the region where charged lived nuclei. particle emission competes strongly.

The spectrometer can also be used as References an angular momentum filter in the study of continuum gamma ray spectra produced during the prompt cascade following heavy ion This work is supported by the National fusion. By choosing channels with few Science Foundation under grants evaporated particles very high spin evap• PHY-79-23308 and PHY-80-17485 oration residues are selected with an effi• ciency which exceeds that achievable with 1) H.J. Kluge, Progress in Atomic Nal multiplicity filters or sum spectrome• Spectrocopy, ed. W. Hanle, H. ters. In this mode of operation the spec• Kleinpopen, New York, Plenum (1980) . trometer becomes a very effective tool in the study of nuclear properties at very 2) P. Juncar and J. Pinard, Optics Comm. high spin. 14, 438 (1975).

The RMS will be important for the on• 3) J. Äystö, S. Hillebrand, K. Hellmuth line laser spectroscopy studies in cases and K. Valli, Nucl. Instr. and Meth. where the production mechanism and/or the 120, 163 (1974). detection is not selective (e.g. beta asymmetry detection of optical pumping) 4) G.W. Greenlees, D.L. Clark, S.L. to eliminate background and consequently Kaufman, D.A. Lewis, J.F. Tonn and improve the sensitivity. The RMS provides J.H. Broadhurst, Optics Comm. 23, a very rapid method of selecting a particu• 236 (1977). lar reaction product which is unaffected by the chemical properties of the element. 5) T.M. Cormier and P.M. Stwertka, Nucl. The helium jet chamber can be installed at Instr. and Meth. (in press). the focal plane of the RMS and reaction

- 46 -