Separator for Heavy ELement Spectroscopy – velocity filter SHELS

Nuclear Instruments and Methods in Physics Research B 376 (2016) 140–143

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Separator for Heavy ELement Spectroscopy – velocity filter SHELS A.G. Popeko a,⇑, A.V. Yeremin a, O.N. Malyshev a, V.I. Chepigin a, A.V. Isaev a, Yu.A. Popov a, A.I. Svirikhin a, K. Haushild b, A. Lopez-Martens b, K. Rezynkina b, O. Dorvaux c a

Flerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, Russia Centre de Sciences Nucleaires et de Sciences de la Matiere, Univ. Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, 91405 Orsay, France c Institut Pluridisciplinaire Hubert Curien, IN2P3-CNRS, 67037 Strasbourg, France b

a r t i c l e

i n f o

Article history: Received 31 August 2015 Received in revised form 17 March 2016 Accepted 20 March 2016 Available online 15 April 2016 Keywords: Electromagnetic separators Velocity filters Nuclear reactions a-, b-, c-spectroscopy Spontaneous fission

a b s t r a c t The SHELS velocity filter originated upon reconstruction of the VASSILISSA electrostatic separator used for investigations of heavy nuclei produced in complete fusion reactions. The goals of this modernization were to increase the transmission of products of asymmetric reactions and to extend the region of reactions to be investigated up to symmetric combinations. The first tests of the set-up were performed with the beams of accelerated 22Ne, 40Ar, 48Ca, and 50Ti ions. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction Over the past 20 years, the VASSILISSA electrostatic recoil separator [1,2] has been used for investigations of evaporation residues (ERs) produced in heavy-ion fusion reactions. In the course of the experimental work a wealth of data on the formation and properties of ERs has been collected [3]. Using the GABRIELA (Gamma Alpha Beta Recoil Investigations with the ELectromagnetic Analyzer) detector system [4] installed behind VASSILISSA a-, b-internal conversion and c-decays of isotopes of Fm, Md, No, and Lr have been studied in detail [5]. Electrostatic separators accomplish spatial filtering of recoil nuclei, transfer reaction products and beam particles based on differences in the ratios of their energy to ionic charge. The transmission of ERs originating in asymmetric, like 22Ne+238U!260 No* reactions, amounted to 2% due to their broad angular, energy and charge distributions. The maximal electric rigidity for VASSILISSA was limited to 2 MV, and the study of reactions more symmetric than 48Ca+208Pb was complicated using the existing separator. 2. Choice of the ion optical scheme for the new separator The most efficient separation of fusion reaction products can be performed by charge independent velocity filters. Thus, for the new ⇑ Corresponding author. E-mail address: [email protected] (A.G. Popeko). http://dx.doi.org/10.1016/j.nimb.2016.03.045 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.

separator we have chosen the scheme composed of two identical velocity filters with static spatially separated electric and magnetic fields. The ion optical scheme of the new separator, which is called ‘‘SHELS” (Separator for Heavy ELement Spectroscopy), can be described as QQQ-E-D-D-E-QQQ-D, where Q denote magnetic Quadrupole lenses, E are Electrostatic deflectors, D stand for magnetic Dipoles. The ion optical scheme of SHELS is shown in Fig. 1. In designing a new separator we have used as a model pattern the SHIP [6,7] velocity filter, which has a more complex optical scheme and has been successfully used in investigations of heavy ion induced reactions since 1975.

3. Ion optics of SHELS During the reconstruction the dispersive part of VASSILISSA consisting of 3 electrostatic deflectors was replaced by two mirror symmetric velocity filters. The diaphragm determining the velocity window is placed at the symmetry plane. Each filter consists of a parallel, flat plate condenser (deflector) and a dipole magnet. The nominal deflection angle of ERs in the electric field is aE =8°. The central trajectory of ions has a shape of the tilde character (). The construction of the condensers allows the distance between the plates to be adjusted to optimally match the angular distribution and electric rigidity of particles and makes accessible ERs with an electric rigidity up to 10 MV.

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12

Focal plane Detector

11 10 9 8

s

ter

c

an

t Dis

e nM ei

Time of Flight

7

Magnetic Dipole D8

6

5

Quadrupole Triplet 2 Electric Deflector 2

4 3

Magnetic Dipole D22-2

2

Magnetic Dipole D22-1

1

Beam Stop

0

Electric Deflector 1 Beam Target Wheel

Quadrupole Triplet 1

Fig. 1. Ion optical scheme of the SHELS velocity filter.

Under conditions of mirror symmetry, the deflection angle aM in the magnetic field is linearly related to aE , and for the given geometry aM ¼ 2:725  aE  22 . Varying these angles one can control the velocity dispersion and position of the velocity window. An example of the application of this technique to the study of transfer reactions is presented in [8]. The focusing system of SHELS consists of two magnetic quadrupole triplets. The first triplet forms a slightly converging beam of ERs emerging from the target. The second triplet focuses the ERs onto the focal plane detector. The last dipole magnet with the deflection angle of  8 removes the focal plane detectors from the direct view of the target. The principal components of SHELS are shown in Figs. 2–4 and their characteristics are listed in Table 1.

Monte-Carlo computer code [9] has been used for the separator transmission optimization.

4. Technical testing The field configurations in the quadrupole and dipole magnets have been measured by means of Hall-probes. The measured field maps and deduced excitation functions are used for setting up the separator. The currents in the coils of the magnetic dipoles and the high voltages on the electrostatic deflectors are determined according to the chosen deflection angles and the data on the mass, energy and ionic charge of ERs to be transported. A specially-designed

Fig. 2. Quadrupole triplet.

Fig. 3. Electrostatic deflector.

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5. First tests at the accelerator

Fig. 4. Magnetic dipole (22°).

Table 1 Principal components of SHELS. Quadrupole lenses Maximum field gradient Effective length Aperture radius

13 T/m 38 cm 10 cm

Electrostatic deflectors Maximum field gradient Effective length Distance between plates Nominal deflection angle

40 kV/cm 65.7 cm 10–20 cm 8°

Magnetic dipoles D22-1,2 Maximum field strength Effective length Gap height Nominal deflection angle

0.8 T 59.7 cm 13.5 cm 22°

Magnetic dipole D8 Maximum field strength Effective length Gap height Nominal deflection angle

0.2 T 58.8 cm 14 cm 8°

The ion optical calculations for the complete system have been checked by placing 226Ra and 233U sources at the target position and transporting the emitted a-particles to a position sensitive detector at the focal plane. The results of the measurements agree well with the expected ones.

For the first commissioning runs with beam from the U400 cyclotron, the target chamber, Time-of-Flight measurement, diagnostic, control and data taking systems have been transferred from the old VASSILISSA to the new SHELS. Tests have been performed with stationary and rotating targets. Since the U400 accelerator is running in the direct current (DC) mode an electrostatic chopper was installed behind the ECR-ion source to interrupt the beam when the target frames cross it. In May 2013 the first 22Ne beam was delivered to SHELS. In these experiments, a stationary 197Au and rotating 198Pt targets were used (see Table 2). At the maximal beam current of 1013 s1, no increase of leakage currents or breakdowns in the electrostatic deflectors were observed. For the detection of particles, a 58  58 mm2 Double-sided Si Strip Detector surrounded by backward Si strip and an array of Ge detectors was installed at the focal plane of the separator. The reactions studied and some experimental conditions are presented in Table 2. The suppression factors of the primary full-energy beam were better than 1015 and were determined mostly by the accelerator and beam line tuning. The determination of transmission has been performed in two ways: comparing the counting rates at the new separator with those from the reactions studied at VASSILISSA, and comparing the measured and calculated yields for reactions with wellknown cross-sections. These methods are not precise due to problems with: setting of correct beam energy, beam current measurement, identity of targets, scattering in timing foils etc. Thus, at this stage of testing, one can state only, that the transportation efficiency of SHELS is higher than that of VASSILISSA. 6. First results Despite the low number of runs, several interesting measurements have been performed during the commissioning reactions. In the 22Ne(206Pb,4n) reaction, a new 8095(11)-keV a-line in the decay of 224U has been observed [10]. Also a more precise half-life of 224U T1/2 = 396(17) ls and an excitation energy of 386.5(1) keV instead of 373.3(1) keV of the first excited 2+ state in 220Th have been determined. In several tests, a 50Ti beam accelerated at the U400 cyclotron for the first time from Metal Ions from Volatile Compounds (MIVOC) [12] method has been used. In the 50Ti(164Dy,4n) reaction decay cascades of the isomeric 210Ra state with a T1/2 = 2.1 ls were observed and the conversion coefficients for the main isomeric transitions have been deduced from the gamma and conversion electron spectra [13]. Using the 50Ti beam, the spontaneous fission of 256Rf, produced in the reaction 50Ti(208Pb,2n) has been studied. For the detection of fission fragments in coincidence with prompt neutrons, a Si Strip Detector (SSD) box was placed inside a 3He-based neutron

Table 2 Test reactions studied at the SHELS velocity filter. Reaction

22

Ne(238U,4–5n)255,256No Ne(208Pb,4n)226U 22 Ne(206Pb,4n)224U 22 Ne(198Pt,5-7n)213215Ra 22 Ne(197Au,4-6n)213215Ac 50 Ti(164Dy,3-5n)209211Ra 40 Ar(208Pb,2-3n)245,246Fm 50 Ti(208Pb,2n)256Rf 22

Beam energy

115 117 117 115–125 120 240 237 237

Target thickness (mg/cm2)

0.35 (U3O8) 0.36 (PbS) 0.23 (metal) 0.30 (metal) 0.35 (metal) 0.3 (Dy2O3) 0.36 (PbS) 0.36 (PbS)

ERs transmission Old

New

0.01 0.02 0.02 0.03 0.03 0.3 0.20 0.20

– – – 0.040 ± 0.015 0.065 ± 0.030 0.4 0.4 0.3 ± 0.1

Ref.

[10] [11] [11] [12,13] [13] [14]

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reactions has been increased, but the exact determination of gain factors, optimization and characterization of the new separator require further experiments. Acknowledgements The transformation of the VASSILISSA electrostatic separator to the SHELS velocity filter was financed by JINR and the French funding agency ANR (Contracts No. ANR-06-BLAN-0034–01 and ANR12-BS05-0013) and partially under the financial support of the Russian Foundation for Basic Research, Contracts No. 13–0212003, 14–02-91051. References

Fig. 5. Design of the new detector arrangement at the focal plane of SHELS.

multiplicity counter. The average multiplicity of prompt neutrons was found to be hmi = 4.47 ± 0.09. The TKE distribution has also been measured, but its evaluation needs additional calibrations [14]. 7. Future plans The next step in the development of SHELS will be the installation and commissioning of the new focal plane detector arrangement. The old ‘‘small” 58  58 mm2 DSSD will be replaced by a bigger 100  100 mm2 DSSD surrounded by a box of SSD. The new detector has 128  128 strips and requires 128 + 128 analog amplification channels. In addition, 128 channels are needed for the backward detectors. In the first tests [10], the DSSD showed excellent energy resolution of about 17 keV without cooling. The development of digital electronics for pulse shape analysis is now under discussion. The Fig. 5 shows the new detector arrangement at the focal plane of SHELS. The DSSD will be surrounded by 5 germanium detectors in order to register c- and X-rays from the decays of implanted ERs. The new arrangement will be more compact and more efficient than the previous GABRIELA setup. 8. Conclusion Commissioning of the new SHELS separator has shown satisfactory agreement between projected and experimental values. The achieved primary beam suppression factor of > 5  1015 allows one to accept beams with the intensity up to 10 plA. As a result of the reconstruction the transmission of ERs from asymmetric

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