Commissioning the A1900 projectile fragment separator

Nuclear Instruments and Methods in Physics Research B 204 (2003) 90–96 www.elsevier.com/locate/nimb

Commissioning the A1900 projectile fragment separator D.J. Morrissey b

a,b,*

, B.M. Sherrill

b,c

, M. Steiner b, A. Stolz b, I. Wiedenhoever

b,d

a Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA c Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA d Department of Physics, Florida State University, Tallahassee, FL 32306, USA

Abstract An important part of the recent upgrade of the NSCL facility is the replacement of the A1200 fragment separator with a new high acceptance device called the A1900. The design of the A1900 device represents a third generation projectile fragment separator (relative to the early work at LBL) as it is situated immediately after the primary accelerator, has a very large acceptance, a bending power significantly larger than that of the cyclotron and is constructed from large superconducting magnets (quadrupoles with 20 and 40 cm diameter warm bores). The A1900 can accept over 90% of a large range of projectile fragmentation products produced at the NSCL, leading to large gains in the intensity of the secondary beams. The results of initial tests of the system with a restricted momentum acceptance (
0.5%) indicate that the A1900 is performing up to specifications. Further large gains in the intensities of primary beams, typically two or three orders of magnitude, will be possible as the many facets of high current extraction from the ion sources, acceleration of intense, low charge-state ions in the K500 cyclotron, transfer and stripping injection in K1200 cyclotron are optimized. A liquid-lithium cooled beryllium target system is being constructed to use with the high power beams (up to 5 kW) that will be available from the coupled-cyclotron facility. An overview of the design, construction and commissioning studies of the A1900 device will be presented along with some of the results from the initial exotic isotope production studies. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 29.30)h; 07.75þh Keywords: Electromagnetic separator; Nuclear reaction products

1. Introduction The National Superconducting Cyclotron Laboratory (NSCL) has recently completed the

* Corresponding author. Address: National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA. Tel.: +1-517-333-6321; fax: +1-517-3535967. E-mail address: [email protected] (D.J. Morrissey).

conversion of the existing K500 and K1200 superconducting cyclotrons from independent operation into a coupled system [1]. The new facility will provide intense heavy ion beams of both commonly available and very unusual isotopes. The intense beams will be reacted a target system at the object point of a new, large acceptance, high resolution projectile fragment separator called the A1900 to provide a wide variety of fast exotic beams for nuclear physics studies. Construction of the facility was essentially complete at the end of

0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(02)01895-5

D.J. Morrissey et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 90–96 ion sources

RT-ECR

91

20 ft

SC-ECR

10 m

K500

coupling line K1200

A1900

image 1 image 2

stripping foil

focal plane image 3

production target

Fig. 1. A schematic diagram of the coupled cyclotron facility and the operation of the A1900 fragment separator, see the text.

2000 and the commissioning studies of the accelerators and fragment separator started in 2001. The general features of the new equipment are described below and the results of initial studies of the parameters of the separator are also presented. Several experiments with exotic secondary beams have already been completed. The heart of any nuclear physics program with exotic nuclear beams is the device that separates and analyzes the myriad of reaction products from the primary beam/target interaction and provides the secondary beams. The exotic beams created with the projectile fragmentation technique are produced from a primary beam in a transmission target, retaining most of their initial velocity. The mixture of unreacted primary and secondary ions are first filtered to select a single magnetic rigidity, Bq ¼ mv=q, by a dispersive beam line in conjunction with an aperture. Isotopic selection is completed by passing the ions through an energy degrading ‘‘wedge’’ from which ions entering with a single Bq but with different atomic numbers emerge with different momenta. A second dispersive beam line then provides, in most cases, isotopic separation. The nature and thickness of the production target and the energy degrader, as well as the sizes of momentum apertures, are parameters that are adjusted to control the secondary beam intensity and purity. The NSCL project uses a new large projectile-fragment separator whose design was based on experience with the previous A1200 device [3,5] and the properties of the FRS at GSI [4] and other similar devices operated at similar facilities around the world [2].

Fig. 1 shows a schematic layout of the A1900 separator connected to the K500 and K1200 cyclotrons. Intense beams of stable ions from the electron cyclotron-resonance ion sources are injected into the center of the K500 cyclotron and accelerated to an energy sufficient to strip most of the remaining electrons in a foil positioned near the center of the K1200 cyclotron. The highly stripped ions are then accelerated to high energies, extracted and focused onto the production target. As an example, the design of the facility calls for a 8 plA beam of 86 Kr14þ ions to be accelerated to 14 MeV/A in the K500 cyclotron, stripped to 34þ with a final beam current of 100 pnA beam at 155 MeV/A. For example, the A1900 has a collection efficiency approaching 100% for fragmentation products with A  50 at 150 MeV/A compared to a value of 2–4% previously possible with the A1200. Some of the considerations that went into the design of the A1900 were summarized in a previous report [3].

2. The A1900 separator The A1900 separator relies on superconducting iron-dominated quadrupole magnets. The general specifications of the magnets used in the A1900 are given in Table 1. The device is made up from 24 quadrupole magnets (in eight cryostats) and four 45° dipoles. All of the quadrupole magnets have a warm bore with a radius of at least 10 cm. The superconducting coils are attached to iron pole and yoke pieces submerged in liquid helium.

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Table 1 Specifications for magnets in the A1900 Magnet type

Maximum B

Length (m)

Pole radius (m)

Multipolesa

Quantity

Quad Quad Quad Quad Quad

2.3 2.0 2.0 2.3 2.3

0.65 0.325 0.715 0.381 0.625

0.13 0.15 0.15 0.21 0.15

No Yes Yes No No

4 12 4 2 2

(q) 3.1

(gap) 0.09

No

4

ÔaÕ ÔbÕ ÔcÕ ÔdÕ ÔeÕ

Dipole

T/m T/m T/m T/m T/m

2.0 T

a

Coaxial superconducting octupole and sextuple package. The pattern of magnets in the A1900 is: aab-D1-aba.aba-D2-bed.deb-D3aba.aba-D4-baa.

Sixteen of the quadrupoles have a coaxial set of superconducting hexapole and octupole coils for aberration correction. Five different quadrupole magnet designs were necessary to accommodate the large solid angle and momentum acceptance of the A1900. The dipole magnets have a 45° bend with a full vertical gap of 9.0 cm. A high degree of symmetry has been incorporated to minimize higher order geometrical aberrations and simplify the correction process for chromatic aberrations. The magnetic field from each set of coils in each magnet has been mapped and the required current

in each power supply is set according to the desired magnetic rigidity (Bq) by automatically scaling the calculated ion-optical setting. Table 2 contains some of the parameters of the A1200 [5], of the new A1900, and for comparison, of RIPS an existing separator with a large acceptance, [6], and of the high resolution separator proposed for RIA [7]. The values of the relative acceptances and the figures of merit in Table 2 have been calculated assuming that the distribution of reaction products is approximately constant in momentum and in angle. These assumptions are

Table 2 Ion-optical parameters of fragment separators Parameter

A1200a

RIPSb

A1900

RIA-HRc

Solid angle (msr) Momentum acceptance (%) Resolving powerd

0.8 3 2400

5.0 6 1500

8 5.5 2915

10 6 3000

Intermediate imagee Magnification Mx Magnification My Dispersion (cm/%) Length (m) Quadrupole gradient (kG/cm) Dipole full vertical gap (cm) Rigidity (T m)

0.7 3.0 1.67 22 3.5 5 5.4

)1.6 )5.7 2.4 21 1.41 14 5.76

2.04 0.75 5.95 35 2.3 9 6

8

Acceptance (rel. to A1200) Figure of meritf

1 0.5

12.5 3.6

18.3 10.2

25 14.3

a

Symmetric medium acceptance optical mode [5]. As described in [6]. c As described in these Proceedings [7]. d Resolving power ¼ Dispersion=X0 Mx , where X0 is the beam spot size at the target (assumed to be 1 mm) and Mx is the magnification. e First order, at the wedge position. f Figure of merit ¼ DX=4P  Dp=p  Resolving power. b

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θ [mrad]

40

z = -100

z = - 50

20

20

40

0

0

0

-20

-20

-40

-40

-40

0 x [mm]

50

z = 50

40

-50

0 x [mm]

50

z = 100

40

-50

20

20

0

0

0

-20

-20

-20

-40 -50

0 x [mm]

50

0 x [mm]

50

z = 150

40

20

-40

z =0

20

-20

-50

θ [mrad]

40

93

-40 -50

0 x [mm]

50

-50

0 x [mm]

50

Fig. 2. The positions in ÔxÕ in mm versus the x-angle h in mrad for fragments detected in the focal plane PPACÕs are shown for six values of the ÔzÕ coordinate separated by 50 mm.

reasonable for the limited angular range of these devices and the production of fragments in thick targets by primary beams with 100 MeV/A. The real gain is, however, very isotope-dependent. The parameters of a high resolution separator under consideration for the rare isotope accelerator facility (RIA-HR) are also shown in Table 2 for comparison. The design of projectile separators has progressed to the point that further large gains in the production of rare isotopes by the projectilefragmentation mechanism will have to come from accelerators providing higher power primary beams whereas the production of neutron-rich nuclei by the projectile-fission mechanism will require new developments such as those proposed for the Super-FRS described by Geissel et al., in these proceedings [8]. The A1900 has three intermediate images. Access to the first image is limited by a thick steel shielding wall and only contains a beam stop and viewer at present. The wedge is placed at the second image along with high rate position sensitive detectors to track the particles, aperture plates to define the momentum acceptance, and a scintillator for time-of-flight measurements. The third image occurs in a large vacuum box fitted with

continuously adjustable slits to define the acceptance and other auxiliary equipment. The focal plane chamber houses a pair of position sensitive detectors (parallel-plate avalanche counters), a simple silicon PIN detector, a stack of silicon PINs and a thick plastic scintillator, all of which can be moved independently. An example of the tracking of projectile fragments can be seen in Fig. 2. The horizontal position, x and the horizontal angle, h, calculated from the positions measured in a pair of traditional parallel plate avalanche counters (10  10 cm2 , each) [9] are shown at six different positions along the beam axis separated by 50 mm (z ¼ 0 corresponds to the focal position in the ion optics).

3. Initial results The ion optical modes of the A1900 and higherorder aberrations were studied prior to construction with the ray tracing program MOTER [10] and with the infinite-order analytical code COSYinfinity [11]. The separator was initially operated with small adjustments of the calculated first-order optics; more recently refinements have been made

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as higher order effects were quantified using particle tracking, see below. A feature of the fragment separator optical mode is that the horizontal and vertical magnifications at the position of the degrader wedge have been set so that emittance growth from the energy loss process is minimized. A magnification of 2.0 in the dispersive (wedge) coordinate at intermediate image was chosen to reduce the effects of energy loss straggling in the wedge. In the NSCL energy range, energy loss straggling can be of the order of 0.2% in momentum. Thus, if the image-2 magnification would be near 1.0, this straggling would contribute 4 mm to the final spot size. When the magnification in the first half is increased to 2.0, the contribution of straggling to the final spot size is reduced by a factor of 2. This choice has the disadvantage that the transverse emittance growth is directly proportional to the magnification. However, the large angular acceptance of the A1900 and the small initial production spot size will minimize this problem. Another feature incorporated in the design is that the (hjd) term is zero at the intermediate image which gives a more uniform acceptance as a function of momentum and can significantly improve the measurement of angular distributions when running the device in an energy loss mode. This and other features were studied in the development runs using particle tracking discussed below. The preliminary results of the initial operation of the A1900 were reported by Wiedenh€ over [12]. These studies were performed with fragments that were emitted within
0.5% of the central momentum using nominal settings of the sextupoles and with the octupoles turned off. The yields of a variety of fragments from 18 O, 40 Ar and 86 Kr beams were measured and found to be in reasonable agreement with the yield predictions from the LISE code [13]. For example, the measured production rate for 11 Li fragments with
0.5% of the most probable momentum from a 18 O beam at 120 MeV/A was 270 particles/(sec-pnA), a rate that is 70% of the predicted rate. In another case the observed production rate of 8 B from 16 O in a thick beryllium target was 5.5k particles/(sec-pnA) compared to a predicted rate of 6.2k at the peak of the momentum distribution. These and several

other measurements of the yields and momenta of various fragments indicated that the basic operation of the separator was sufficient to provide secondary beams for an initial experimental program. Several nuclear physics experiments were carried out with 33 Al and similar neutron-rich ions. These experiments essentially demonstrated the successful operation of the entire facility during the latter part of 2001. A short series of experiments was carried out with a 48 Ca beam in the January 2002 to test operation with a metal feedstock in the ion source. Further development tests of the A1900 separator were performed in the spring of 2002 to study the optical properties with an aim towards operation with the full acceptance of the device. A pair of new high rate particle tracking detectors were placed at the intermediate image (two large area PPACs with individual strip readout and software-centroid calculation). These detectors have been used to track individual particles at the rate of a few thousand/s in a total rate on the order of a million/s. The new detectors along with the focal plane (two analogue PPACs) were used to determine the dispersion, angular acceptance, magnifications and other interesting properties. For example, some of the measurements of the optical properties of the separator during the test run can be inferred from the correlated positions and angles of projectile fragments shown in Fig. 3. The data were obtained by fragmenting a 16 O beam with 155 MeV/A in a 1900 mg/cm2 beryllium target and then tracking fragments within the full momentum acceptance (Dp=p ¼
2:5%) at Bq ¼ 3:562 T m. The fragments are predominantly 15 7þ N ions. The top panels in Fig. 3 display the spatial positions of particles at dispersive image-2 (I2) and at the focal plane (FP). The angular magnifications in the second half of the device can be inferred from the slopes apparent in the panels in the second row. The horizontal, x, coordinate at image-2 is linearly proportional to the momentum deviation, d of the particle. Thus, the slight curvature of the data for hFP versus xI2 in the bottom left panel indicates a small aberration remains to be corrected with the multipole coils whereas the data for hI2 versus xI2 in the bottom right panel indicates the uniform angular acceptance of the

D.J. Morrissey et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 90–96

95

40 40

FP y [mm]

I2 y [mm]

20 0 -20

20 0 -20 -40

-40

-200

-100

0

100

200

-40

-20

100

100

50

50

0 -50

-100

-100

-50

0

50

-100

-50

0

50

100

100

200

100

200

I2 φ [mrad] 40

FP y [mm]

FP x [mm]

40

-50 -100

100

40 20 0 -20

20 0 -20

-40

-40 -200

-100

0

100

200

-200

-100

I2 x [mm] 100

100

50

50

0 -50

-200

-100

0

0

I2 x [mm]

I2 θ [mrad]

FP θ [mrad]

20

0

I2 θ [mrad]

-100

0

FP x [mm]

FP φ [mrad]

FP θ [mrad]

I2 x [mm]

100

200

I2 x [mm]

0 -50 -100

-200

-100

0

I2 x [mm]

Fig. 3. An example of the measurement of ion-optical parameters of the fragments from an 16 O beam during a test run of the A1900 separator is shown. The usual coordinates are used with x and h in the horizontal plane perpendicular to the beam motion, y and / in the vertical plane. The labels I2 and FP refer to image-2 and focal plane, see the text for details.

first half of the device. Further studies and tuning of the separator are necessary and will be performed as time permits. The design goals for the highest beam-power from the coupled cyclotron facility is 5 kW in the

A  40 region. This beam power is significantly higher than can be tolerated in traditional metal targets. In collaboration with Argonne National Laboratory we are developing a new hybrid target system using liquid lithium to cool a beryllium

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metal target that can sustain power-losses by the beam of up to 1 kW (20% of beam power). Another novel feature of the new target system is that the lithium will flow through a channel in the center of a tapered beryllium block connected to a rigid stainless steel loop. The thickness of the target can be varied by translating the entire loop with the tapered beryllium piece up or down relative to the beam axis. A description of this work is contained in the report by Nolen et al., in these proceedings [14].

separator. The magnet design and construction is the work of H. Blosser, F. Marti, D. Johnson, J. DeKamp, R. Zink and A.F. Zeller. Engineering and installation relied on D. Lawton, J. Ottarson, D. Sanderson and C. Snow. The operation and testing of the device was carried out with much assistance from D. Bazin and J. Stetson. J. Yurkon designed and built the large area position-sensitive PPACs. This work was supported by the National Science Foundation under grants PHY95-28844 and PHY01-10253.

4. Conclusion

References

The A1900 projectile fragment separator has been constructed with large aperture iron-dominated superconducting magnets. All of the magnets were constructed at the NSCL and installed in the region between the K1200 cyclotron and the existing experimental vaults. Maps of the magnetic fields of all of the magnets were used to recalculate the ion-optical settings of the separator. Preliminary tests of the device as a fragment separator were carried out during the summer of 2001. A few initial experiments were performed using beams within the central 1% of the momentum acceptance. Subsequent studies have been aimed at measuring the particle correlations and improving the ion-optical tune. The successful operation of the separator with a limited acceptance and more recently with the full acceptance is an important preliminary result of this work. These successes will help assure a rich program of nuclear physics with exotic beams at the NSCL and indicate that nearly ‘‘full’’ acceptance devices for intermediate energy projectile fragments are possible. Further gains in the yields of projectile fragments will have to be obtained from accelerators and target systems with higher power primary beams.

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Acknowledgements Many people contributed to the design and construction of the components of the A1900