- Email: [email protected]
SORCERER: A novel particle-detection system for transfer-reaction experiments at ROSPHERE
Nuclear Inst. and Methods in Physics Research, A 951 (2020) 163090
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
SORCERER: A novel particle-detection system for transfer-reaction experiments at ROSPHERE T. Beck a ,∗, C. Costache b , R. Lică b , N. Mărginean b , C. Mihai b , R.E. Mihai b , O. Papst a , S. Pascu b , N. Pietralla a , C. Sotty b , L. Stan b , A.E. Turturică b , V. Werner a , J. Wiederhold a , W. Witt a a b
Institut für Kernphysik, TU Darmstadt, Schlossgartenstr. 9, D-64289 Darmstadt, Germany Horia Hulubei National Institute of Physics and Nuclear Engineering - IFIN-HH, R-077125 Bucharest, Romania
ARTICLE
INFO
Keywords: Gamma-ray spectroscopy Nuclear structure of exotic isotopes Doppler-shift lifetime methods
ABSTRACT A customizable and cost-efficient particle-detection system for transfer experiments at ROSPHERE has been designed and tested. Results from the commissioning experiment employing the 2𝑛-transfer reaction 94 Zr(18 O,16 O)96 Zr are presented and discussed. The device allows for an efficient suppression of contributions from fusion-evaporation reactions and an improvement of the peak-to-background ratio by about one order of magnitude. The construction, characteristics, and performance are presented and further developments are outlined.
1. Introduction The precise knowledge of nuclear level lifetimes provides an essential insight into the structure of nuclei. It sheds light on the underlying processes of nuclear dynamics and serves as a test case for predictions of theoretical models. For low-abundant or radioactive nuclei, knowledge on electromagnetic transition rates can be especially scarce, but is of utmost importance in order to access nuclear shape information. The lifetimes of low-energy nuclear states are typically in the range of femto- to a few picoseconds. In this domain, the Doppler-Shift Attenuation Method [1] (DSAM) and the Recoil-Distance Doppler Shift [2] (RDDS) method are applicable. Both are based on the measurement of photons emitted in the deexcitation of excited nuclei in flight. In many cases, for the population of off-yrast nuclear states of lowabundant or unstable nuclei close to the valley of stability (𝑝, 𝑛, 2𝑝, 2𝑛, 𝛼, etc.)-transfer reactions have proven to be suitable. In order to exploit such a transfer reaction with reasonable cross-section a (light) donor nucleus is accelerated to a kinetic energy closely below or above the Coulomb barrier of the projectile-target system. Naturally, even below the Coulomb barrier transfer reactions are in competition with subbarrier fusion-evaporation reactions. The latter’s cross-section exceeds the former’s by about one order of magnitude [3,4]. In order to discriminate between these two reaction mechanisms particle-𝛾 coincidences can be employed. The variety of experimental techniques, reaction processes, and kinematical properties fuels the need for a cost-efficient and customizable particle-detection setup. For this purpose, the SORCERER array
(SOlaR CElls for Reaction Experiments at ROSPHERE), based on commercial photodiodes, has been constructed, implemented and commissioned at the ROmanian array for SPectroscopy in HEavy ion REactions [5] (ROSPHERE). The following Section outlines the technical layout of the SORCERER array. Its performance in a first experimental application to investigate nuclear level lifetimes of 96 Zr is described in Section 3. Naturally, SORCERER’s range of application is not limited to DSAM and RDDS measurements. In fact, all transfer and scattering experiments where the necessity for using more sophisticated particle detectors is absent (cf. Section 4) might profit from the array introduced in the following. 2. Technical layout ROSPHERE is a versatile detector array installed at the 9 MV Tandem accelerator [6] at the ‘‘Horia Hulubei’’ National Institute of Physics and Nuclear Engineering (IFIN-HH) in Bucharest, Romania. It allows for the installation of up to 25 Compton-suppressed high-purity Germanium (HPGe) detectors or fast LaBr3 :Ce scintillators and can be equipped with a conventional reaction chamber for DSAM experiments and the Köln-Bucharest plunger device [5] for RDDS measurements. At energies around the Coulomb barrier the recoiling particles after the transfer process are scattered backwards with respect to the direction of the incoming ions if the mass of the projectile is small compared to the target. For the detection of the ejectiles a dedicated array of silicon photodiodes has been constructed. The mount was designed to best take
∗ Corresponding author. E-mail address: [email protected] (T. Beck).
https://doi.org/10.1016/j.nima.2019.163090 Received 21 August 2019; Received in revised form 31 October 2019; Accepted 5 November 2019 Available online 11 November 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.
T. Beck, C. Costache, R. Lică et al.
Nuclear Inst. and Methods in Physics Research, A 951 (2020) 163090
Fig. 1. Schematic drawings of the mounting structures for DSAM (left ) and RDDS (right ) experiments. Apart from the mounting lugs both versions of SORCERER are identical. The inner cone, housing the six photodiodes, has a maximum diameter of 19 mm and a height of 11.5 mm. The characteristic kinematic of the 2𝑛-transfer reaction is depicted schematically in the middle part. The initial 18 O ion beam propagates from below. Recoiling 16 O ions are detected by the photodiodes at backward angles with respect to the beam axis. The fusion-evaporation (FE) residuals conserve the momentum of the projectile and travel in the opposite direction with respect to the particle detectors.
advantage of the characteristic reaction kinematics (cf. middle part of Fig. 1). The holding structure itself is manufactured by the selective fusing of polyamide PA 2200 in a granular bed of the same material. This procedure is often addressed as selective laser sintering. Components produced by this technique have a significant advantage over fused deposition modeling as the occurrence of sealed holes is prevented. Thus, the mount can be exposed to pressures down to 10−6 mbar, which is crucial in the environment of accelerated ion beams. Additionally, PA 2200 does not fumigate in vacuum. The CAD (Computer-Aided Design) drawing of the individual versions of SORCERER for DSAM and RDDS experiments are shown in Fig. 1. The constructed mounts are tailored to the existing scattering chamber and the Köln-Bucharest plunger device. They house six Advanced Photonics PDB-C613-2-ND [7] diodes, which are equally spaced and oriented around a central beam hole with a diameter of 5 mm. Each photodiode has an active area of 8.8 × 9.8 mm2 which is about 89% of its surface area. Due to the specific design of the holding structure, the cells are pairwise electrically isolated and kept in place without the usage of adhesive. Hence, an easy exchange of degraded photodiodes is ensured. In operation, each diode is read out individually. The signals are delivered to Canberra 2003BT preamplifiers and mesytec MSCF-16-LN modules for amplification and discrimination. Afterwards, the signals are fed into mesytec MADC-32 and MTDC-32 digitizers for energy and time information, respectively. For each event in the photodiodes it is validated whether a trigger signal from the HPGe detectors is received. In order to cope with the different response times of the HPGe detectors of ROSPHERE and SORCERER’s photodiodes (cf. Table 1), the latter’s signals are stored by the digitizers until the trigger is received [8]. Subsequently, events in the photodiodes are only recorded if they lie within a 256 ns wide acceptance window. Due to the geometry of the device, the individual readout offers a position sensitivity of ±30◦ in the polar angle of the recoiling ion. With respect to the impinging ion beam polar angles between 121.7◦ and 163.5◦ for DSAM and 130.4◦ and 165.5◦ for RDDS experiments are covered by SORCERER. In total, a solid angle coverage of about 15.4% for DSAM and 11.0% for RDDS experiments is achieved. A compilation of characteristic parameters is given in Table 1.
Fig. 2. Gamma-ray energy spectra of ROSPHERE without (upper panel) and with (lower panel) time and energy conditions on ejected 16 O ions in the photodiodes of SORCERER. Without the application of particle conditions the spectrum is dominated by transitions from the fusion-evaporation products 108 Cd (▾) and 109 Cd (▴). In the properly gated spectrum, however, these reactions are effectively suppressed. Instead, transitions stemming from 96 Zr (★) and 98 Mo (⧫) produced in 2𝑛 and 𝛼 transfer reactions, respectively, are clearly visible besides inelastic excitations of the target (■). The inlays focusing on the 2+1 → 0+1 transition of 96 Zr highlight the effect of the energy and time conditions on the recoiling 16 O. Clearly, the peak-to-background ratio is improved (cf. Fig. 6) and peaks which are not originating from nuclei produced after transfer reactions are removed.
Fig. 3. (color online) Summed spectrum of SORCERER’s six photodiodes. The ejectiles after the transfer reactions, mainly 14 C and 16 O, are found in the plateau of the spectrum. The similar mass of 14 C and 16 O ions along with the large angular coverage of each individual photodiode results in a wide energy distribution of the detected ejectiles. Hence, different transfer processes are not distinguished but jointly selected by application of an energy condition (cf. Fig. 5). The energy range selected in the lower part of Fig. 2 is indicated in green.
excited states of 96 Zr were populated. The beam energy was chosen slightly below the projectile-target system’s Coulomb barrier of about 51 MeV. In this energy regime the fusion-evaporation channel, which is highly favored above the Coulomb barrier, is severely hindered. Nevertheless, the cross-section is still about one order of magnitude larger than for the desired transfer reaction [4]. In order to remove events stemming from electromagnetic decays of nuclei produced by fusion evaporation, particle-𝛾 coincidences are employed. The 94 Zr target thickness was 7.22 mg/cm2 . This ensures that target-like particles are stopped completely within the target, while 16 O ejectiles can leave the target and are detected with SORCERER for DSAM experiments (cf. Fig. 1 left).
3. Performance In the first DSAM experiment using SORCERER as described in Section 2, the intruder band of the nucleus 96 Zr was investigated. The structure of 96 Zr has recently attracted a great deal of attention [9–11]. At the IFIN Horia Hulubei 18 O ions were accelerated to 49 MeV by the Bucharest 9 MV Tandem and delivered to the 94 Zr target at the center of the reaction chamber. By the 2𝑛-transfer reaction 94 Zr(18 O, 16 O)96 Zr 2
T. Beck, C. Costache, R. Lică et al.
Nuclear Inst. and Methods in Physics Research, A 951 (2020) 163090
Table 1 Characteristic parameters of SORCERER and ROSPHERE. Specifications of the photodiodes are taken from the data sheets [7]. Details on ROSPHERE are found in Ref. [5]. Quantity
Unit
Value SORCERER for
Mean distance to target Active area Polar-angle coverage wrt beam axis Solid-angle coveragea Bias voltage Junction capacitance Response time
mm mm2
15.7
◦
msr V pF ns
121.7 − 163.5 1933+191 −173
Quantity
Unit
DSAM
Value ROSPHERE for
◦ ◦
Solid-angle coverageb a Assuming b Assuming
msr
130.4 − 165.5 1366+126 −117 −5 350 50
◦
1, 3 & 5 2 & 4
18.9 518
HPGe Number of detectors Polar angles of rings Azimuthal angles of rings
RDDS
2143+1 −101
BGO 25 in 5 rings 37, 70, 90, 110, 143 0, 72, 144, 216, 288 36, 108, 180, 252, 324 6836+25 −138
a displacement of ±1 mm parallel to the beam axis for uncertainty estimation. a displacement of +5 mm for HPGe detectors and +2 mm for BGO scintillators for uncertainty estimation.
Fig. 4. (color online) Time difference spectrum of HPGe and particle detectors with arbitrarily chosen zero. The readout of the detectors is triggered by the detection of a photon in one of the 25 HPGe detectors. A condition on the time difference in order to select events from 2𝑛-transfer reaction is indicated in green. Random coincident events from the region highlighted in red are subtracted.
Photons emitted in the decay of excited nuclear states were recorded by the 25 Compton-suppressed HPGe detectors of ROSPHERE. The 𝛾ray spectrum produced in the reaction of a 18 O beam on a 94 Zr target at 49 MeV recorded in singles mode is shown in the upper panel of Fig. 2. The most prominent peaks in the spectrum at 633.0, 875.5 and 203.4, 259.4, 522.4, 835.9 keV stem from 108 Cd and 109 Cd, respectively. Based on a calculation using the PACE4 [12,13] computer code, these nuclei are the most frequent products of fusion evaporation of 18 O with 94 Zr. For each photon detected in a single germanium detector, all events of the HPGe and particle detectors are collected. Typical singles particle and time difference spectra are shown in Figs. 3 and 4, respectively. The 𝛾-ray spectrum gated on charged-particle ejectiles is shown at the bottom of Fig. 2. Clearly, the 𝛾-rays of 96 Zr from the 94 Zr(18 O, 16 O) two-neutron transfer reaction are enhanced with respect to fusion-evaporation products. Based on this data, particle-𝛾 matrices are sorted applying a coincidence time-window. A matrix built from the 96 Zr data is shown in Fig. 5 along with the individual projections on each of the two axes. Applying coincidence conditions on energy and timing of the particle detectors results in an effective suppression of fusion-evaporation events (cf. lower panel of Fig. 2), as it places a constraint on the two-body scattering kinematics. In the present example, gates on the timing of the photodiodes were placed as shown in Fig. 4 and contributions from random coincidences (red band) were subtracted from true coincidences (green band). The energy condition was set to the interval between 750 and 2250 keV as determined from the relation of particle and 𝛾 energies (cf. upper panel of Fig. 5). In the 𝛾-ray spectrum with particle conditions peaks arising from the 3− → 2+ and 2+ → 0+ transitions of 96 Zr are identified at 146.7 and 1 1 1 1
Fig. 5. (color online) The particle-𝛾 coincidence matrix for all photodiodes of SORCERER and all HPGe detectors of ROSPHERE. The projections on the particle and HPGe detectors are shown in blue in the upper and right panel, respectively. In addition, the energy spectrum of the particle detectors in coincidence to 𝛾-rays from the 2+1 → 0+1 transition of 96 Zr and the 15∕2− → 11∕2− transition of 109 Cd are shown in the upper panel in green and red color, respectively.
1750.5 keV, respectively. In addition, peaks assigned to the nucleus 98 Mo prove the occurrence of 𝛼-transfer reactions with cross-section comparable to 2𝑛-transfer. Almost exclusively due to the removal of non-resonant background and fusion-evaporation events the statistics is reduced by a factor of roughly 50. The peak-to-background ratio for transitions of nuclei produced in transfer reactions is greatly enhanced due to the kinematical focus on backward angles. This improvement of the peak-to-background ratio is more than one order of magnitude, independent of the 𝛾-transition energy as shown in Fig. 6. Naturally, this is surpassed by spectra obtained after application of an energy condition on the 𝛾-ray energy of the 2+ → 0+ transition. Though, parti1 1 cle detection offers decisive advantages as it enables the selection of a specific reaction channel. Thus, the resulting spectra are cleaned from strongly perturbing contributions of competitive reaction mechanisms, mainly fusion evaporation. In the case of plain energy conditions on 𝛾-ray energies, these contributions can still dominate the 𝛾-ray spectra. In addition, the spectra after application of conditions on the particle 3
T. Beck, C. Costache, R. Lică et al.
Nuclear Inst. and Methods in Physics Research, A 951 (2020) 163090
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank the technical staff of the precision mechanical workshop of the Institute for Fluid Mechanics and Aerodynamics, TU Darmstadt for production of the mounting structures and the staff of the Bucharest Tandem accelerator for providing excellent experimental conditions. Additionally, the help of the electrical and mechanical workshop of the Institut für Kernphysik, TU Darmstadt is greatly acknowledged. One of the authors (TB) would like to thank C. Fransen for valuable advice during the commissioning of the photodiodes, P.R. John and M. Schilling for fruitful discussions, and J. Kleemann for providing the picture shown in Fig. 7. This work was supported by the BMBF under grant Nos. 05P18RDEN9 and 05P18RDFN1/9, the Romanian Ministry of Research and Innovation under contract No. PN 19060102, and the European Union within the Horizon 2020 research and innovation programme (ENSAR-2).
Fig. 6. Comparison of peak-to-background ratios from 𝛾-ray energy spectra without (▶) and with (◀) time and energy conditions on16 O ejectiles in the photodiodes (upper panel). The enhancement of the peak-to-background ratios is more than one order of magnitude and independent of the transition energy (lower panel). Uncertainties are within marker size.
References [1] T.K. Alexander, J.S. Forster, Lifetime measurements of excited nuclear levels by doppler-shift methods, in: M. Baranger, E. Vogt (Eds.), Advances in Nuclear Physics: Vol. 10, Springer US, Boston, MA, 1978, p. 197. [2] A. Dewald, S. Harissopulos, P. von Brentano, The differential plunger and the differential decay curve method for the analysis of recoil distance Doppler-shift data, Z. Phys. A 334 (2) (1989) 163–175. [3] F. Pühlhofer, On the interpretation of evaporation residue mass distributions in heavy-ion induced fusion reactions, Nuclear Phys. A 280 (1) (1977) 267–284. [4] W. Witt, Type-II Shell Evolution in 98 Zr (Ph.D. thesis), TU Darmstadt, 2019, https://tuprints.ulb.tu-darmstadt.de/8536/. [5] D. Bucurescu, I. Căta-Danil, G. Ciocan, C. Costache, D. Deleanu, R. Dima, D. Filipescu, N. Florea, D. Ghiţă, T. Glodariu, M. Ivaşcu, R. Lică, N. Mărginean, R. Mărginean, C. Mihai, A. Negret, C. Niţă, A. Olăcel, S. Pascu, T. Sava, L. Stroe, A. Şerban, R. Şuvăilă, S. Toma, N. Zamfir, G. Căta-Danil, I. Gheorghe, I. Mitu, G. Suliman, C. Ur, T. Braunroth, A. Dewald, C. Fransen, A. Bruce, Z. Podolyák, P. Regan, O. Roberts, The ROSPHERE 𝛾-ray spectroscopy array, Nucl. Instrum. Methods Phys. Res. A 837 (2016) 1–10. [6] S. Dobrescu, D.V. Mosu, D. Moisa, S. Papureanu, The bucharest FN tandem accelerator: Modernization and development, AIP Conf. Proc. 1099 (1) (2009) 51–54. [7] Luna Optoelectronics, Solderable Silicon Photodiodes PDB–C613–2, Technical Report, 2019, URL https://lunainc.com/wp-content/uploads/2016/06/PDB_C613_2. pdf. [8] mesytec GmbH & Co. KG, MTDC-32 (Data sheet V2.6_05), Technical Report, 2019, URL https://www.mesytec.com/products/datasheets/MTDC-32.pdf. [9] T. Togashi, Y. Tsunoda, T. Otsuka, N. Shimizu, Quantum phase transition in the shape of Zr isotopes, Phys. Rev. Lett. 117 (2016) 172502. [10] C. Kremer, S. Aslanidou, S. Bassauer, M. Hilcker, A. Krugmann, P. von NeumannCosel, T. Otsuka, N. Pietralla, V.Y. Ponomarev, N. Shimizu, M. Singer, G. Steinhilber, T. Togashi, Y. Tsunoda, V. Werner, M. Zweidinger, First measurement of collectivity of coexisting shapes based on type II shell evolution: The case of 96 Zr, Phys. Rev. Lett. 117 (2016) 172503. [11] W. Witt, N. Pietralla, V. Werner, T. Beck, Data on the structural coexistence in the 96 Zr nucleus, Eur. Phys. J. A 55 (5) (2019) 79. [12] A. Gavron, Statistical model calculations in heavy ion reactions, Phys. Rev. C 21 (1980) 230–236. [13] O. Tarasov, D. Bazin, LISE++: Radioactive beam production with in-flight separators, Nucl. Instrum. Methods Phys. Res. A 266 (19) (2008) 4657–4664, Proceedings of the XVth International Conference on Electromagnetic Isotope Separators and Techniques Related to their Applications. [14] T. Klug, A. Dewald, V. Werner, P. von Brentano, R. Casten, The 𝐵(𝐸2 ∶ 4+2 → 2+2 ) value in 152 Sm and 𝛽-softness in phase coexisting structures, Phys. Lett. B 495 (1) (2000) 55–62. [15] D. Kocheva, G. Rainovski, J. Jolie, N. Pietralla, A. Blazhev, R. Altenkirch, S. Ansari, A. Astier, M. Bast, M. Beckers, T. Braunroth, M. Cappellazzo, A. Dewald, F. Diel, M. Djongolov, C. Fransen, K. Gladnishki, A. Goldkuhle, A. Hennig, V. Karayonchev, J.M. Keatings, E. Kluge, T. Kröll, J. Litzinger, K. Moschner, C. Müller-Gatermann, P. Petkov, M. Scheck, P. Scholz, T. Schmidt, P. Spagnoletti, C. Stahl, R. Stegmann, A. Stolz, A. Vogt, N. Warr, V. Werner, D. Wölk, J.C. Zamora, K.O. Zell, V.Y. Ponomarev, P. Van Isacker, Low collectivity of the 2+1 state of 212 Po, Phys. Rev. C 96 (2017) 044305.
Fig. 7. Photodiodes mounted (cf. right-hand side of Fig. 1) in the plunger device with the target removed. The incident ion beam is directed from the rear right to the front left.
energy contain the complete structural information and not only on the states which are connected to the selected transition, i.e. 𝛾-cascades. 4. Conclusion The presented particle detectors based on silicon photodiodes are a valuable extension of the ROSPHERE detector array for DSAM and RDDS measurements after transfer reactions. Due to the simple production by laser sintering, SORCERER is cost-efficient and customizable to different experimental setups and requirements, e.g. reactions with different kinematical properties. For instance, the implementation into the Köln-Bucharest plunger device [14,15] is shown in Fig. 7. The presented measurement has highlighted the improved sensitivity for 𝛾ray spectroscopy experiments using the device. Another potential area of application for SORCERER in combination with fast scintillators, such as LaBr3 :Ce, are fast-timing experiments [16] induced by transfer reactions. An upgraded version may contain an additional ring of photodiodes [17] simultaneously increasing the solid-angle coverage and the sensitivity to different reaction processes based on their kinematical characteristics. A systematic continuation of this development might result in an upgraded version of SORCERER for reaction studies. In this case, a combination of a collimating system and a larger amount of miniaturized photodiodes with individual readout electronics is desirable. It enables the determination of angular distributions of the ejectiles produced in the studied reaction of projectile and target. In the present configuration SORCERER bridges the gap in available particle detectors if there is no necessity for more sophisticated devices. 4
T. Beck, C. Costache, R. Lică et al.
Nuclear Inst. and Methods in Physics Research, A 951 (2020) 163090
[16] N. Mărginean, D.L. Balabanski, D. Bucurescu, S. Lalkovski, L. Atanasova, G. Căta-Danil, I. Căta-Danil, J.M. Daugas, D. Deleanu, P. Detistov, G. Deyanova, D. Filipescu, G. Georgiev, D. Ghiţă, K.A. Gladnishki, R. Lozeva, T. Glodariu, M. Ivaşcu, S. Kisyov, C. Mihai, R. Mărginean, A. Negret, S. Pascu, D. Radulov, T. Sava, L. Stroe, G. Suliman, N.V. Zamfir, In-beam measurements of subnanosecond nuclear lifetimes with a mixed array of HPGe and LaBr3:Ce detectors, Eur. Phys. J. A 46 (3) (2010) 329–336.
[17] P. Voss, R. Henderson, C. Andreoiu, R. Ashley, R. Austin, G. Ball, P. Bender, A. Bey, A. Cheeseman, A. Chester, D. Cross, T. Drake, A. Garnsworthy, G. Hackman, R. Holland, S. Ketelhut, P. Kowalski, R. Krücken, A. Laffoley, K. Leach, D. Miller, W. Mills, M. Moukaddam, C. Pearson, J. Pore, E. Rand, M. Rajabali, U. Rizwan, J. Shoults, K. Starosta, C. Svensson, E. Tardiff, C. Unsworth, K. Van Wieren, Z.-M. Wang, J. Williams, The TIGRESS integrated plunger ancillary systems for electromagnetic transition rate studies at TRIUMF, Nucl. Instrum. Methods Phys. Res. A 746 (2014) 87–97.
5
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
SORCERER: A novel particle-detection system for transfer-reaction experiments at ROSPHERE T. Beck a ,∗, C. Costache b , R. Lică b , N. Mărginean b , C. Mihai b , R.E. Mihai b , O. Papst a , S. Pascu b , N. Pietralla a , C. Sotty b , L. Stan b , A.E. Turturică b , V. Werner a , J. Wiederhold a , W. Witt a a b
Institut für Kernphysik, TU Darmstadt, Schlossgartenstr. 9, D-64289 Darmstadt, Germany Horia Hulubei National Institute of Physics and Nuclear Engineering - IFIN-HH, R-077125 Bucharest, Romania
ARTICLE
INFO
Keywords: Gamma-ray spectroscopy Nuclear structure of exotic isotopes Doppler-shift lifetime methods
ABSTRACT A customizable and cost-efficient particle-detection system for transfer experiments at ROSPHERE has been designed and tested. Results from the commissioning experiment employing the 2𝑛-transfer reaction 94 Zr(18 O,16 O)96 Zr are presented and discussed. The device allows for an efficient suppression of contributions from fusion-evaporation reactions and an improvement of the peak-to-background ratio by about one order of magnitude. The construction, characteristics, and performance are presented and further developments are outlined.
1. Introduction The precise knowledge of nuclear level lifetimes provides an essential insight into the structure of nuclei. It sheds light on the underlying processes of nuclear dynamics and serves as a test case for predictions of theoretical models. For low-abundant or radioactive nuclei, knowledge on electromagnetic transition rates can be especially scarce, but is of utmost importance in order to access nuclear shape information. The lifetimes of low-energy nuclear states are typically in the range of femto- to a few picoseconds. In this domain, the Doppler-Shift Attenuation Method [1] (DSAM) and the Recoil-Distance Doppler Shift [2] (RDDS) method are applicable. Both are based on the measurement of photons emitted in the deexcitation of excited nuclei in flight. In many cases, for the population of off-yrast nuclear states of lowabundant or unstable nuclei close to the valley of stability (𝑝, 𝑛, 2𝑝, 2𝑛, 𝛼, etc.)-transfer reactions have proven to be suitable. In order to exploit such a transfer reaction with reasonable cross-section a (light) donor nucleus is accelerated to a kinetic energy closely below or above the Coulomb barrier of the projectile-target system. Naturally, even below the Coulomb barrier transfer reactions are in competition with subbarrier fusion-evaporation reactions. The latter’s cross-section exceeds the former’s by about one order of magnitude [3,4]. In order to discriminate between these two reaction mechanisms particle-𝛾 coincidences can be employed. The variety of experimental techniques, reaction processes, and kinematical properties fuels the need for a cost-efficient and customizable particle-detection setup. For this purpose, the SORCERER array
(SOlaR CElls for Reaction Experiments at ROSPHERE), based on commercial photodiodes, has been constructed, implemented and commissioned at the ROmanian array for SPectroscopy in HEavy ion REactions [5] (ROSPHERE). The following Section outlines the technical layout of the SORCERER array. Its performance in a first experimental application to investigate nuclear level lifetimes of 96 Zr is described in Section 3. Naturally, SORCERER’s range of application is not limited to DSAM and RDDS measurements. In fact, all transfer and scattering experiments where the necessity for using more sophisticated particle detectors is absent (cf. Section 4) might profit from the array introduced in the following. 2. Technical layout ROSPHERE is a versatile detector array installed at the 9 MV Tandem accelerator [6] at the ‘‘Horia Hulubei’’ National Institute of Physics and Nuclear Engineering (IFIN-HH) in Bucharest, Romania. It allows for the installation of up to 25 Compton-suppressed high-purity Germanium (HPGe) detectors or fast LaBr3 :Ce scintillators and can be equipped with a conventional reaction chamber for DSAM experiments and the Köln-Bucharest plunger device [5] for RDDS measurements. At energies around the Coulomb barrier the recoiling particles after the transfer process are scattered backwards with respect to the direction of the incoming ions if the mass of the projectile is small compared to the target. For the detection of the ejectiles a dedicated array of silicon photodiodes has been constructed. The mount was designed to best take
∗ Corresponding author. E-mail address: [email protected] (T. Beck).
https://doi.org/10.1016/j.nima.2019.163090 Received 21 August 2019; Received in revised form 31 October 2019; Accepted 5 November 2019 Available online 11 November 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.
T. Beck, C. Costache, R. Lică et al.
Nuclear Inst. and Methods in Physics Research, A 951 (2020) 163090
Fig. 1. Schematic drawings of the mounting structures for DSAM (left ) and RDDS (right ) experiments. Apart from the mounting lugs both versions of SORCERER are identical. The inner cone, housing the six photodiodes, has a maximum diameter of 19 mm and a height of 11.5 mm. The characteristic kinematic of the 2𝑛-transfer reaction is depicted schematically in the middle part. The initial 18 O ion beam propagates from below. Recoiling 16 O ions are detected by the photodiodes at backward angles with respect to the beam axis. The fusion-evaporation (FE) residuals conserve the momentum of the projectile and travel in the opposite direction with respect to the particle detectors.
advantage of the characteristic reaction kinematics (cf. middle part of Fig. 1). The holding structure itself is manufactured by the selective fusing of polyamide PA 2200 in a granular bed of the same material. This procedure is often addressed as selective laser sintering. Components produced by this technique have a significant advantage over fused deposition modeling as the occurrence of sealed holes is prevented. Thus, the mount can be exposed to pressures down to 10−6 mbar, which is crucial in the environment of accelerated ion beams. Additionally, PA 2200 does not fumigate in vacuum. The CAD (Computer-Aided Design) drawing of the individual versions of SORCERER for DSAM and RDDS experiments are shown in Fig. 1. The constructed mounts are tailored to the existing scattering chamber and the Köln-Bucharest plunger device. They house six Advanced Photonics PDB-C613-2-ND [7] diodes, which are equally spaced and oriented around a central beam hole with a diameter of 5 mm. Each photodiode has an active area of 8.8 × 9.8 mm2 which is about 89% of its surface area. Due to the specific design of the holding structure, the cells are pairwise electrically isolated and kept in place without the usage of adhesive. Hence, an easy exchange of degraded photodiodes is ensured. In operation, each diode is read out individually. The signals are delivered to Canberra 2003BT preamplifiers and mesytec MSCF-16-LN modules for amplification and discrimination. Afterwards, the signals are fed into mesytec MADC-32 and MTDC-32 digitizers for energy and time information, respectively. For each event in the photodiodes it is validated whether a trigger signal from the HPGe detectors is received. In order to cope with the different response times of the HPGe detectors of ROSPHERE and SORCERER’s photodiodes (cf. Table 1), the latter’s signals are stored by the digitizers until the trigger is received [8]. Subsequently, events in the photodiodes are only recorded if they lie within a 256 ns wide acceptance window. Due to the geometry of the device, the individual readout offers a position sensitivity of ±30◦ in the polar angle of the recoiling ion. With respect to the impinging ion beam polar angles between 121.7◦ and 163.5◦ for DSAM and 130.4◦ and 165.5◦ for RDDS experiments are covered by SORCERER. In total, a solid angle coverage of about 15.4% for DSAM and 11.0% for RDDS experiments is achieved. A compilation of characteristic parameters is given in Table 1.
Fig. 2. Gamma-ray energy spectra of ROSPHERE without (upper panel) and with (lower panel) time and energy conditions on ejected 16 O ions in the photodiodes of SORCERER. Without the application of particle conditions the spectrum is dominated by transitions from the fusion-evaporation products 108 Cd (▾) and 109 Cd (▴). In the properly gated spectrum, however, these reactions are effectively suppressed. Instead, transitions stemming from 96 Zr (★) and 98 Mo (⧫) produced in 2𝑛 and 𝛼 transfer reactions, respectively, are clearly visible besides inelastic excitations of the target (■). The inlays focusing on the 2+1 → 0+1 transition of 96 Zr highlight the effect of the energy and time conditions on the recoiling 16 O. Clearly, the peak-to-background ratio is improved (cf. Fig. 6) and peaks which are not originating from nuclei produced after transfer reactions are removed.
Fig. 3. (color online) Summed spectrum of SORCERER’s six photodiodes. The ejectiles after the transfer reactions, mainly 14 C and 16 O, are found in the plateau of the spectrum. The similar mass of 14 C and 16 O ions along with the large angular coverage of each individual photodiode results in a wide energy distribution of the detected ejectiles. Hence, different transfer processes are not distinguished but jointly selected by application of an energy condition (cf. Fig. 5). The energy range selected in the lower part of Fig. 2 is indicated in green.
excited states of 96 Zr were populated. The beam energy was chosen slightly below the projectile-target system’s Coulomb barrier of about 51 MeV. In this energy regime the fusion-evaporation channel, which is highly favored above the Coulomb barrier, is severely hindered. Nevertheless, the cross-section is still about one order of magnitude larger than for the desired transfer reaction [4]. In order to remove events stemming from electromagnetic decays of nuclei produced by fusion evaporation, particle-𝛾 coincidences are employed. The 94 Zr target thickness was 7.22 mg/cm2 . This ensures that target-like particles are stopped completely within the target, while 16 O ejectiles can leave the target and are detected with SORCERER for DSAM experiments (cf. Fig. 1 left).
3. Performance In the first DSAM experiment using SORCERER as described in Section 2, the intruder band of the nucleus 96 Zr was investigated. The structure of 96 Zr has recently attracted a great deal of attention [9–11]. At the IFIN Horia Hulubei 18 O ions were accelerated to 49 MeV by the Bucharest 9 MV Tandem and delivered to the 94 Zr target at the center of the reaction chamber. By the 2𝑛-transfer reaction 94 Zr(18 O, 16 O)96 Zr 2
T. Beck, C. Costache, R. Lică et al.
Nuclear Inst. and Methods in Physics Research, A 951 (2020) 163090
Table 1 Characteristic parameters of SORCERER and ROSPHERE. Specifications of the photodiodes are taken from the data sheets [7]. Details on ROSPHERE are found in Ref. [5]. Quantity
Unit
Value SORCERER for
Mean distance to target Active area Polar-angle coverage wrt beam axis Solid-angle coveragea Bias voltage Junction capacitance Response time
mm mm2
15.7
◦
msr V pF ns
121.7 − 163.5 1933+191 −173
Quantity
Unit
DSAM
Value ROSPHERE for
◦ ◦
Solid-angle coverageb a Assuming b Assuming
msr
130.4 − 165.5 1366+126 −117 −5 350 50
◦
1, 3 & 5 2 & 4
18.9 518
HPGe Number of detectors Polar angles of rings Azimuthal angles of rings
RDDS
2143+1 −101
BGO 25 in 5 rings 37, 70, 90, 110, 143 0, 72, 144, 216, 288 36, 108, 180, 252, 324 6836+25 −138
a displacement of ±1 mm parallel to the beam axis for uncertainty estimation. a displacement of +5 mm for HPGe detectors and +2 mm for BGO scintillators for uncertainty estimation.
Fig. 4. (color online) Time difference spectrum of HPGe and particle detectors with arbitrarily chosen zero. The readout of the detectors is triggered by the detection of a photon in one of the 25 HPGe detectors. A condition on the time difference in order to select events from 2𝑛-transfer reaction is indicated in green. Random coincident events from the region highlighted in red are subtracted.
Photons emitted in the decay of excited nuclear states were recorded by the 25 Compton-suppressed HPGe detectors of ROSPHERE. The 𝛾ray spectrum produced in the reaction of a 18 O beam on a 94 Zr target at 49 MeV recorded in singles mode is shown in the upper panel of Fig. 2. The most prominent peaks in the spectrum at 633.0, 875.5 and 203.4, 259.4, 522.4, 835.9 keV stem from 108 Cd and 109 Cd, respectively. Based on a calculation using the PACE4 [12,13] computer code, these nuclei are the most frequent products of fusion evaporation of 18 O with 94 Zr. For each photon detected in a single germanium detector, all events of the HPGe and particle detectors are collected. Typical singles particle and time difference spectra are shown in Figs. 3 and 4, respectively. The 𝛾-ray spectrum gated on charged-particle ejectiles is shown at the bottom of Fig. 2. Clearly, the 𝛾-rays of 96 Zr from the 94 Zr(18 O, 16 O) two-neutron transfer reaction are enhanced with respect to fusion-evaporation products. Based on this data, particle-𝛾 matrices are sorted applying a coincidence time-window. A matrix built from the 96 Zr data is shown in Fig. 5 along with the individual projections on each of the two axes. Applying coincidence conditions on energy and timing of the particle detectors results in an effective suppression of fusion-evaporation events (cf. lower panel of Fig. 2), as it places a constraint on the two-body scattering kinematics. In the present example, gates on the timing of the photodiodes were placed as shown in Fig. 4 and contributions from random coincidences (red band) were subtracted from true coincidences (green band). The energy condition was set to the interval between 750 and 2250 keV as determined from the relation of particle and 𝛾 energies (cf. upper panel of Fig. 5). In the 𝛾-ray spectrum with particle conditions peaks arising from the 3− → 2+ and 2+ → 0+ transitions of 96 Zr are identified at 146.7 and 1 1 1 1
Fig. 5. (color online) The particle-𝛾 coincidence matrix for all photodiodes of SORCERER and all HPGe detectors of ROSPHERE. The projections on the particle and HPGe detectors are shown in blue in the upper and right panel, respectively. In addition, the energy spectrum of the particle detectors in coincidence to 𝛾-rays from the 2+1 → 0+1 transition of 96 Zr and the 15∕2− → 11∕2− transition of 109 Cd are shown in the upper panel in green and red color, respectively.
1750.5 keV, respectively. In addition, peaks assigned to the nucleus 98 Mo prove the occurrence of 𝛼-transfer reactions with cross-section comparable to 2𝑛-transfer. Almost exclusively due to the removal of non-resonant background and fusion-evaporation events the statistics is reduced by a factor of roughly 50. The peak-to-background ratio for transitions of nuclei produced in transfer reactions is greatly enhanced due to the kinematical focus on backward angles. This improvement of the peak-to-background ratio is more than one order of magnitude, independent of the 𝛾-transition energy as shown in Fig. 6. Naturally, this is surpassed by spectra obtained after application of an energy condition on the 𝛾-ray energy of the 2+ → 0+ transition. Though, parti1 1 cle detection offers decisive advantages as it enables the selection of a specific reaction channel. Thus, the resulting spectra are cleaned from strongly perturbing contributions of competitive reaction mechanisms, mainly fusion evaporation. In the case of plain energy conditions on 𝛾-ray energies, these contributions can still dominate the 𝛾-ray spectra. In addition, the spectra after application of conditions on the particle 3
T. Beck, C. Costache, R. Lică et al.
Nuclear Inst. and Methods in Physics Research, A 951 (2020) 163090
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank the technical staff of the precision mechanical workshop of the Institute for Fluid Mechanics and Aerodynamics, TU Darmstadt for production of the mounting structures and the staff of the Bucharest Tandem accelerator for providing excellent experimental conditions. Additionally, the help of the electrical and mechanical workshop of the Institut für Kernphysik, TU Darmstadt is greatly acknowledged. One of the authors (TB) would like to thank C. Fransen for valuable advice during the commissioning of the photodiodes, P.R. John and M. Schilling for fruitful discussions, and J. Kleemann for providing the picture shown in Fig. 7. This work was supported by the BMBF under grant Nos. 05P18RDEN9 and 05P18RDFN1/9, the Romanian Ministry of Research and Innovation under contract No. PN 19060102, and the European Union within the Horizon 2020 research and innovation programme (ENSAR-2).
Fig. 6. Comparison of peak-to-background ratios from 𝛾-ray energy spectra without (▶) and with (◀) time and energy conditions on16 O ejectiles in the photodiodes (upper panel). The enhancement of the peak-to-background ratios is more than one order of magnitude and independent of the transition energy (lower panel). Uncertainties are within marker size.
References [1] T.K. Alexander, J.S. Forster, Lifetime measurements of excited nuclear levels by doppler-shift methods, in: M. Baranger, E. Vogt (Eds.), Advances in Nuclear Physics: Vol. 10, Springer US, Boston, MA, 1978, p. 197. [2] A. Dewald, S. Harissopulos, P. von Brentano, The differential plunger and the differential decay curve method for the analysis of recoil distance Doppler-shift data, Z. Phys. A 334 (2) (1989) 163–175. [3] F. Pühlhofer, On the interpretation of evaporation residue mass distributions in heavy-ion induced fusion reactions, Nuclear Phys. A 280 (1) (1977) 267–284. [4] W. Witt, Type-II Shell Evolution in 98 Zr (Ph.D. thesis), TU Darmstadt, 2019, https://tuprints.ulb.tu-darmstadt.de/8536/. [5] D. Bucurescu, I. Căta-Danil, G. Ciocan, C. Costache, D. Deleanu, R. Dima, D. Filipescu, N. Florea, D. Ghiţă, T. Glodariu, M. Ivaşcu, R. Lică, N. Mărginean, R. Mărginean, C. Mihai, A. Negret, C. Niţă, A. Olăcel, S. Pascu, T. Sava, L. Stroe, A. Şerban, R. Şuvăilă, S. Toma, N. Zamfir, G. Căta-Danil, I. Gheorghe, I. Mitu, G. Suliman, C. Ur, T. Braunroth, A. Dewald, C. Fransen, A. Bruce, Z. Podolyák, P. Regan, O. Roberts, The ROSPHERE 𝛾-ray spectroscopy array, Nucl. Instrum. Methods Phys. Res. A 837 (2016) 1–10. [6] S. Dobrescu, D.V. Mosu, D. Moisa, S. Papureanu, The bucharest FN tandem accelerator: Modernization and development, AIP Conf. Proc. 1099 (1) (2009) 51–54. [7] Luna Optoelectronics, Solderable Silicon Photodiodes PDB–C613–2, Technical Report, 2019, URL https://lunainc.com/wp-content/uploads/2016/06/PDB_C613_2. pdf. [8] mesytec GmbH & Co. KG, MTDC-32 (Data sheet V2.6_05), Technical Report, 2019, URL https://www.mesytec.com/products/datasheets/MTDC-32.pdf. [9] T. Togashi, Y. Tsunoda, T. Otsuka, N. Shimizu, Quantum phase transition in the shape of Zr isotopes, Phys. Rev. Lett. 117 (2016) 172502. [10] C. Kremer, S. Aslanidou, S. Bassauer, M. Hilcker, A. Krugmann, P. von NeumannCosel, T. Otsuka, N. Pietralla, V.Y. Ponomarev, N. Shimizu, M. Singer, G. Steinhilber, T. Togashi, Y. Tsunoda, V. Werner, M. Zweidinger, First measurement of collectivity of coexisting shapes based on type II shell evolution: The case of 96 Zr, Phys. Rev. Lett. 117 (2016) 172503. [11] W. Witt, N. Pietralla, V. Werner, T. Beck, Data on the structural coexistence in the 96 Zr nucleus, Eur. Phys. J. A 55 (5) (2019) 79. [12] A. Gavron, Statistical model calculations in heavy ion reactions, Phys. Rev. C 21 (1980) 230–236. [13] O. Tarasov, D. Bazin, LISE++: Radioactive beam production with in-flight separators, Nucl. Instrum. Methods Phys. Res. A 266 (19) (2008) 4657–4664, Proceedings of the XVth International Conference on Electromagnetic Isotope Separators and Techniques Related to their Applications. [14] T. Klug, A. Dewald, V. Werner, P. von Brentano, R. Casten, The 𝐵(𝐸2 ∶ 4+2 → 2+2 ) value in 152 Sm and 𝛽-softness in phase coexisting structures, Phys. Lett. B 495 (1) (2000) 55–62. [15] D. Kocheva, G. Rainovski, J. Jolie, N. Pietralla, A. Blazhev, R. Altenkirch, S. Ansari, A. Astier, M. Bast, M. Beckers, T. Braunroth, M. Cappellazzo, A. Dewald, F. Diel, M. Djongolov, C. Fransen, K. Gladnishki, A. Goldkuhle, A. Hennig, V. Karayonchev, J.M. Keatings, E. Kluge, T. Kröll, J. Litzinger, K. Moschner, C. Müller-Gatermann, P. Petkov, M. Scheck, P. Scholz, T. Schmidt, P. Spagnoletti, C. Stahl, R. Stegmann, A. Stolz, A. Vogt, N. Warr, V. Werner, D. Wölk, J.C. Zamora, K.O. Zell, V.Y. Ponomarev, P. Van Isacker, Low collectivity of the 2+1 state of 212 Po, Phys. Rev. C 96 (2017) 044305.
Fig. 7. Photodiodes mounted (cf. right-hand side of Fig. 1) in the plunger device with the target removed. The incident ion beam is directed from the rear right to the front left.
energy contain the complete structural information and not only on the states which are connected to the selected transition, i.e. 𝛾-cascades. 4. Conclusion The presented particle detectors based on silicon photodiodes are a valuable extension of the ROSPHERE detector array for DSAM and RDDS measurements after transfer reactions. Due to the simple production by laser sintering, SORCERER is cost-efficient and customizable to different experimental setups and requirements, e.g. reactions with different kinematical properties. For instance, the implementation into the Köln-Bucharest plunger device [14,15] is shown in Fig. 7. The presented measurement has highlighted the improved sensitivity for 𝛾ray spectroscopy experiments using the device. Another potential area of application for SORCERER in combination with fast scintillators, such as LaBr3 :Ce, are fast-timing experiments [16] induced by transfer reactions. An upgraded version may contain an additional ring of photodiodes [17] simultaneously increasing the solid-angle coverage and the sensitivity to different reaction processes based on their kinematical characteristics. A systematic continuation of this development might result in an upgraded version of SORCERER for reaction studies. In this case, a combination of a collimating system and a larger amount of miniaturized photodiodes with individual readout electronics is desirable. It enables the determination of angular distributions of the ejectiles produced in the studied reaction of projectile and target. In the present configuration SORCERER bridges the gap in available particle detectors if there is no necessity for more sophisticated devices. 4
T. Beck, C. Costache, R. Lică et al.
Nuclear Inst. and Methods in Physics Research, A 951 (2020) 163090
[16] N. Mărginean, D.L. Balabanski, D. Bucurescu, S. Lalkovski, L. Atanasova, G. Căta-Danil, I. Căta-Danil, J.M. Daugas, D. Deleanu, P. Detistov, G. Deyanova, D. Filipescu, G. Georgiev, D. Ghiţă, K.A. Gladnishki, R. Lozeva, T. Glodariu, M. Ivaşcu, S. Kisyov, C. Mihai, R. Mărginean, A. Negret, S. Pascu, D. Radulov, T. Sava, L. Stroe, G. Suliman, N.V. Zamfir, In-beam measurements of subnanosecond nuclear lifetimes with a mixed array of HPGe and LaBr3:Ce detectors, Eur. Phys. J. A 46 (3) (2010) 329–336.
[17] P. Voss, R. Henderson, C. Andreoiu, R. Ashley, R. Austin, G. Ball, P. Bender, A. Bey, A. Cheeseman, A. Chester, D. Cross, T. Drake, A. Garnsworthy, G. Hackman, R. Holland, S. Ketelhut, P. Kowalski, R. Krücken, A. Laffoley, K. Leach, D. Miller, W. Mills, M. Moukaddam, C. Pearson, J. Pore, E. Rand, M. Rajabali, U. Rizwan, J. Shoults, K. Starosta, C. Svensson, E. Tardiff, C. Unsworth, K. Van Wieren, Z.-M. Wang, J. Williams, The TIGRESS integrated plunger ancillary systems for electromagnetic transition rate studies at TRIUMF, Nucl. Instrum. Methods Phys. Res. A 746 (2014) 87–97.
5