A PROPOSAL FOR HOSTING THE AGATA DEMONSTRATOR AT GANIL

Transcription

1 A PROPOSAL FOR HOSTING THE AGATA DEMONSTRATOR AT GANIL 1. Introduction This document outlines a proposal for hosting the AGATA demonstrator at GANIL starting from The scientific program associated with this request has been discussed within the EXOGAM collaboration and among a larger international community during the various workshops on prospects for high-resolution γ-ray spectroscopy at GANIL. The GANIL facility offers a large variety of radioactive and stable beams, ranging from 6,8 He to 238 U, over a wide range of energies from a few MeV/u up to 100 MeV/u. This wide spectrum of beams combined with the availability of state-of-theart spectrometers, γ-ray and particle detectors were seen to make GANIL the ideal choice for hosting the AGATA demonstrator and eventually a larger AGATA (sub-)array. Hosting the AGATA demonstrator will give a strong boost to the field of γ-ray spectroscopy at GANIL. While the coupling of the AGATA demonstrator to the existing instrumentation has a high potential for cutting-edge nuclear structure research, the perspectives for γ-ray spectroscopy at GANIL should also be seen in the light of SPIRAL2, the fully-funded next-generation Radioactive Ion Beam (RIB) facility. The present proposal will deliberately focus on physics programs that can be performed only at GANIL due to the unique character of the facility. A brief overview of the available beams and instrumentation will be given in the next chapter. While there are several options to couple the AGATA demonstrator to the available instrumentation and operate it at various locations within GANIL, this proposal concentrates on its coupling with the VAMOS spectrometer and the EXOGAM γ-ray array. The technical feasibility of this coupling has been studied in detail and will be presented in the next chapter. VAMOS is a highly flexible spectrometer, allowing a wide range of beams and techniques to be used. Selected physics cases will be discussed in detail, illustrating the rich opportunities offered by the operation of the AGATA demonstrator at GANIL. More detailed GEANT simulations have been performed for several cases and are presented in an annex to this proposal. This proposal is divided into the following sections: A brief overview of the GANIL facility in terms of available beams and their quality, experimental areas and existing detectors (section 2) Selected physics cases for the AGATA Demonstrator at GANIL supported in some cases by the results of Geant-4 calculations (section 3) A technical proposal considering all technical aspects related to the hosting of the AGATA demonstrator at GANIL (section 4) 1

2 2. Overview of the Facility A. Beams The present GANIL facility comprises three cyclotrons: CSS1, CSS2, and CIME. The former two accelerate intense stable beams of ions ranging from C to U at energies up to 100 MeV/u. Neutrondeficient and neutron-rich radioactive beams are produced by fragmentation and are either separated in flight or reaccelerated up to 15 MeV/u with the CIME cyclotron, which can also be used to accelerate stable beams. After acceleration, the beams are momentum analyzed in the alpha or Z spectrometer, providing a precise energy measurement with ~ 1 per mille resolution. Many of the stable beam intensities are among the highest available in the world. Beam species and intensities 1 are being continuously developed and improved. Fast fragmentation beams can be produced on two different targets, SISSI or LISE, and have typically energies of MeV/u. Since 2001, SPIRAL 1 is a unique facility for producing reaccelerated radioactive beams at energies around the Coulomb barrier with a very high purity. Species already produced 1 are summarized in Figure 1. Further developments of new beam species like Li, C and others are planned and the increase of beam intensity is an ongoing continuous effort. The intensity and high quality of the ISOL beams were demonstrated in recent experiments and make them ideally suitable for γ-ray studies. Figure 1: Spiral I beams The SPIRAL 2 project comprises a new superconducting linear accelerator for very intense deuterons and stable heavy ions, and an UC x ISOL target to produce fission fragments, which will be reaccelerated with the CIME cyclotron. SPIRAL 2 will become operational in 2011 and make GANIL an ideal place to operate AGATA at a later stage. B. Experimental areas Both stable and radioactive ion beams are available in all experimental areas of GANIL. The outline of the facility and the location of the major equipments are shown in Figure 2. Three spectrometers are available at GANIL: VAMOS, SPEG and LISE, each of which is unique in its domain. The spectrometers are coupled with other particle or γ-ray detectors for specific studies, e.g. VAMOS + EXOGAM, SPEG + Château de Cristal, and LISE + EXOGAM. Additional charged-particle detectors like TIARA, DIAMANT, MUST(1/2), and INDRA or neutron arrays like DEMON are also used in 1 See 2

3 conjunction with these spectrometers. The Euroball Neutron Wall was recently commissioned at GANIL and is used in conjunction with EXOGAM and the DIAMANT detectors. The G1 area where the VAMOS spectrometer is installed is the preferred location for the AGATA demonstrator along with a partial EXOGAM array. VAMOS comprises two quadrupoles, a Wien filter, and a dipole magnet. It has a large angular acceptance of ~100 msr and a momentum acceptance of ±5 %. It can be rotated up to ~90 degrees with respect to the beam axis and be operated both in a dispersive or non-dispersive mode. Several detection systems exist to detect and identify the reaction products at the focal plane of VAMOS, for example secondary electron detectors (SeD), ionisation chambers, and silicon and plastic scintillator walls. A large highly pixelized silicon wall for the study of heavy nuclei with the Recoil-Decay Tagging (RDT) method is currently under construction (MUSETT). The 16 Clover detectors of EXOGAM consist of 4 large germanium crystals each, which are segmented into four sectors. It is expected that EXOGAM will be equipped with digital electronics by The large-acceptance multi-mode operation of VAMOS and the planned improvements for EXOGAM make this coupling a very natural and obvious choice. The possibility to operate the demonstrator in other GANIL areas is not excluded and still under evaluation, but will also depend on the interests of the AGATA community. A drawing of the planned implantation of the AGATA demonstrator coupled to VAMOS and part of EXOGAM is shown in Figure 3. A direct beam line from CIME to the experimental areas G1 and G2 (VAMOS and EXOGAM) is foreseen as part of the SPIRAL 2 project and expected to be operational in With this new beam line, parallel beam delivery from CIME to G1 or G2 and CSS or fast fragmentation beams to other experimental areas will be possible, increasing the amount of available beam time for the AGATA demonstrator significantly. Figure 2: Ganil experimental areas. 3

4 C. Proposed configuration of the AGATA demonstrator Geant-4 simulations have been performed in order to determine the optimum distance of the AGATA demonstrator from the target and to decide whether it should be coupled with EXOGAM. The results are summarized in Annex A of this document. The main conclusions are that the demonstrator should be placed very close to the target in order to optimise the total efficiency. This can be achieved without a severe loss in spectrum quality. Most of the physics examples call for a coupling to the VAMOS spectrometer. Given the mechanical constraints in the VAMOS area, a distance of 10 cm can be reached if the AGATA detectors are mounted around 90 degrees and above the beam line (see Figure 3). This mounting will allow for a rotation of VAMOS up to 45 degrees and using up to 8 AGATA triple clusters. Alternatively, up to 6 EXOGAM detectors could be added to the 5 AGATA triple clusters as long as no rotation of VAMOS is required. The 5-module configuration of the AGATA demonstrator will have a photo-peak efficiency of 5-8% depending on the specific reaction (see Annex A). Even though the efficiency is smaller than that of the (full) EXOGAM array, the much better energy resolution and peak-to-total ratio result in a superior overall spectrum quality, which can be quantified as the resolving power of the instrument. Experiments will also benefit from the higher rate capability of the AGATA demonstrator. Figure 3: Drawing of the AGATA demonstrator coupled to the large-acceptance spectrometer VAMOS. 5 AGATA triple clusters are shown in a compact geometry mounted above the beam line together with 5 Clover detectors from the EXOGAM array. Without EXOGAM clovers up to 8 triple clusters could be accommodated in this compact geometry. The coupling of VAMOS and EXOGAM has been very successfully used in many experiments covering a wide physics program, for example the study of neutron-rich Ca nuclei in deep-inelastic reactions and inverse kinematics, the study of neutron-deficient nuclei around A~130 using fusionevaporation reactions induced by radioactive beams, (d,p) reactions in inverse kinematics to study the evolution of single-particle states in the neutron-rich Ne isotopes, and a successful test of the Recoil Tagging method for the spectroscopy of heavy nuclei. The experience gained from these programs both in physics and experimental techniques will be beneficial for the coupling of the AGATA demonstrator to VAMOS. The significant improvement that can be expected from the AGATA demonstrator compared to EXOGAM will be discussed for a few selected examples in the next section. 4

5 3. Selected physics examples In this section we briefly describe some of the possible physics cases that can be pursued combining the AGATA demonstrator with the existing detection systems at GANIL. The possible programs best utilizing the various facets of GANIL range from the spectroscopy of transuranium elements using fusion evaporation, the characterization of exotic nuclei from deep-inelastic collisions using the high intensity stable beams, gamma spectroscopy with reactions at intermediate energies, exotic shapes at high angular momentum and the studies of high-spin structure of neutron-deficient nuclei produced using radioactive beams at the Coulomb barrier. A. Gamma-ray spectroscopy with reactions at intermediate energies Gamma-ray spectroscopy at intermediate energy is an important tool to explore the nuclear structure at extreme isospin. Investigations using Coulomb excitation led to the discovery of new regions of deformation and the disappearance of shell closures e.g. at N=28 and N=20. In-beam γ-ray spectroscopy after fragmentation reactions was extensively used to explore the most exotic regions of the nuclear chart. In nuclei with extreme isospin, one can get information on the shift of shell closures and other related effects. For instance, some breakthrough observations are: The discovery of the N=28 shell closure breakdown at Z=14, The observation of the weakening of the N=20 shell gap and the appearance of the N=14/16 spherical shell effects gap with decreasing proton number, The observation of extremely large mirror energy difference in quasi-bound states in 36 Ca. The other important aspect of using intermediate energy reactions is that inelastic proton scattering combined with Coulomb excitation allows the isospin decomposition of the nuclear excitations. Detection of γ-rays instead of the scattered protons made it possible to go much farther from the valley of stability and the discovery of valence neutrons decoupled from the core in nuclei like 16 C or 17 B. All experiments using this large spectrum of reaction types rely on the combination of a selective magnetic spectrometer for the identification of the reaction products and a high-resolution, highefficiency γ-ray array for the measurement of excited states in the produced nuclei. In this case the advantage of using γ-ray tracking capabilities of the AGATA demonstrator is probably most obvious, since the γ-rays are emitted by recoils that are moving with a large velocity and very high counting rates can be encountered in the detectors. At GANIL, intermediate energy reactions include inelastic scattering of radioactive ion beams, fragmentation reactions of both stable and radioactive ion beams and one or two nucleon removal reactions from radioactive ion beams. In addition, GANIL offers the possibility to produce nuclei using deep-inelastic collisions at Fermi energies, a production mechanism whose potential has just started to be explored. The high-intensity stable ion beams available at GANIL, the availability of high-resolution spectrometers such as SPEG and VAMOS and the foreseen high-resolution γ-ray array comprising the AGATA demonstrator and several EXOGAM clover detectors, will allow in the future extending the study of very exotic nuclei, both at the neutron-rich and neutron-deficient side of the valley of stability. The major improvement that will be achieved with the AGATA modules is an increase of the resolving power allowing, e.g., studies of odd-mass exotic nuclei, which have been often neglected in the past. In these nuclei, the excitation energy spectra reflect, in a more straightforward way, their intrinsic structure and complement very well the information brought by the spectroscopy of neighbouring even-even nuclei. In addition, more spectroscopic information, such as lifetime of excited states will become accessible using gamma-ray line shapes due to velocity changes through a stack of targets with a well-defined thickness. We have performed a Geant-4 simulation comparing the experimental results obtained with the Château de Cristal BaF 2 array with the expected performance of the AGATA demonstrator (see annex B) both at the standard distance of 23.5 cm and a much closer distance of 11.5 cm. The main result is that the close distance is the preferred solution and that the much better energy resolution of the demonstrator (<20 kev vs. 100 kev at 1 MeV) overcompensates 5

6 for the ~3.5 times smaller efficiency. This is true in particular for odd-mass nuclei where it is almost impossible to obtain nuclear structure information from the low-resolution BaF 2 spectra. The most promising area opened by such an upgrade from the point of view of the intermediate-energy γ-ray spectroscopy would be the possibility to study even-even and odd-mass nuclei around N=50 and as close as possible to 78 Ni. Measuring excited states in unknown heavier N=Z nuclei is the other spectroscopic study that will highly benefit from the availability of the AGATA demonstrator at GANIL. B. Gamma-ray spectroscopy of very neutron-rich nuclei populated in deepinelastic reactions and inverse kinematics Extending the knowledge of nuclear properties towards very neutron-rich nuclei is of fundamental interest, in particular to refine for nuclear models, which have been derived mainly based on the properties observed close to stability. New experimental data will deepen our understanding of the evolution of nuclear structure towards larger asymmetries of protons and neutrons. It is experimentally very well established that deep-inelastic reactions performed at energies of about 10-20% above the Coulomb barrier are especially suited to produce neutron-rich nuclei and to populate high-spin states. The production cross-section peaks for beam-like and target-like nuclei and generally follows the N/Z equilibration line. Numerous experiments have shown that states in quasi-target and quasi-projectile residues are populated up to about 20 ħ and 8 ħ, respectively, and follow the yrast line. The small production cross-sections of the most exotic nuclei and the large yield from a large number of other channels (including fission) limit the selectivity obtained by γ-γ coincidences. A very significant increase in sensitivity can be obtained by using thin targets in conjunction with a large-acceptance magnetic spectrometer (VAMOS) and a high-efficiency γ-ray spectrometer (AGATA or EXOGAM). The spectrometer is rotated at the grazing angle in order to detect the recoiling fragments, which are uniquely identified in Z and A. The measurement of the recoil velocity can be used to further improve Doppler correction. Such a powerful setup enables in a single experiment, i) the firm assignment of γ- ray transitions of unknown nuclei via γ-recoil (or γ-γ-recoil) coincidences and ii) to determine the structure of neutron-rich nuclei and to measure their properties as well as their modification along isotopic or isotonic chains. GANIL is a unique place where nuclear structure studies for exotic nuclei can be performed with deepinelastic reactions in inverse kinematics. It has been shown that these reactions are powerful for such studies using Pb or U beams at moderate energies (~5.5 MeV/u). Some of the advantages of inverse kinematics for these studies are: The almost complete absence of beam like particles in the focal plane The phi-angle acceptance increased by a factor ~2 as compared to direct kinematics The larger recoil velocity of the light fragment enables a better identification in VAMOS and reduces the detection threshold. Recently, a thin-target experiment using deep-inelastic scattering was performed at GANIL, aiming at understanding the N=32, 34 sub-shell closures by studying the low-lying excited states in the Ca isotopes. This experiment used the EXOGAM γ-ray array composed of 11 clover detectors (εω ph ~10 %) and the VAMOS magnetic spectrometer (large geometrical and momentum acceptance of ~100 msr and ±5%, respectively). The most neutron-rich Ca isotope previously studied from deep-inelastic scattering was 50 Ca from a thick target GAMMASPHERE experiment. This experiment was performed in inverse kinematics using a 5.5 MeV/u U beam. New transitions were identified in 50 Ca. Gamma rays in 52 Ca were also observed and the observational limit is estimated to be as low as 1µbarn for the 2 + state in this latter isotope. The levels obtained in 52 Ca are in agreement with recent measurements from 2 proton knock-out reactions at MSU and β-delayed neutron work at ISOLDE. The upgrade of this setup with the AGATA Demonstrator will further improve the sensitivity. It is anticipated that the EXOGAM electronics will be upgraded with digital electronics on the segment 6

7 channels. A simple pulse-shape analysis will allow determining the first interaction point in x and y with a spatial resolution of ~5 mm, about four times smaller than the size of the segments. The energy resolution in the EXOGAM array will be improved from ~20 kev to ~12 kev at 1 MeV and for recoil velocities of v/c~15%. The AGATA Demonstrator will be composed of five triple-clusters located at 10 cm from the target. The total photo-peak efficiency is somewhat smaller as with EXOGAM, but the peak-to-total ratio (>50%) and the energy resolution (~ 4 kev at 1.3 MeV and recoil velocities of v/c~15%) will be much better than today. Defining the figure-of-merit, F by: F = (efficiency) x (P/T) / (FWHM), the new setup will be around three times better than the present setup with 12 EXOGAM clover detectors. C. Exotic nuclear shapes Studying nuclei at the limits of angular momentum and extremes of isospin enhances the understanding of the nuclear many-body problem. The combination of intense radioactive beams from SPIRAL, and later SPIRAL2, and new instrumentation such as AGATA will help our understanding of exotic shapes, collective phenomena, and new symmetries in several ways. While many of these studies will be fully addressed with neutron-rich beams from SPIRAL2 we are planning to exploit fusion reactions with the AGATA demonstrator already with SPIRAL1 for beams that have, and those that will have, the required intensity of ~10 6 pps. Fusion-evaporation reactions with intense neutron-rich beams make extreme values of spin accessible, since the fission barrier increases with N/Z ratios in the compound system. Up to ten additional units of angular momentum can be transferred to nuclei over a wide mass range using beams like 94 Kr, 132 Sn, and 142 Xe as compared to similar entrance channels using stable beams. This spin regime, which is not accessible today but possible with SPIRAL2, has strong potential for new physics. It is at the extreme values of spin, well above 60ħ that exotic shapes such as hyperdeformation are expected to occur. The observation of hyperdeformation at high spin would be a major achievement in the field of nuclear physics. A key experiment would be the investigation of extreme shapes around 114 Cd using 94 Kr+ 26 Mg at 500 MeV. For these nuclei both proton and neutron (N+3) hyperintruder orbitals are expected to be occupied, and which can hold more angular momentum than 108 Cd, one of the most deformed nuclei observed so far. Favorable macroscopic liquid-drop properties, manifested in the Jacobi shape transition, are a prerequisite for the population of hyperdeformed shapes. The A~120 region is also from this point of view the most promising, as all recent calculations show a minimum in the potential energy surfaces at β~0.9. Experimental signatures for the Jacobi transition are the energy splitting of the GDR and a giant backbend of the moment of inertia when plotted as a function of rotational frequency. New types of symmetries like tetrahedral and octahedral shapes are predicted by mean-field models for islands of nuclei far from stability, e.g. for both very neutron-rich and very neutron-deficient Zr isotopes. Experimental signatures for such shapes, which are expected to occur at low spin (I~10ħ), include shape isomers and static octupole moments and no dipole moment. However, most experiments to study exotic nuclear shapes involve large γ-ray multiplicities. Consequently, not only high efficiency, but also a high granularity is needed for a γ-ray spectrometer for these investigations. For the studies at high angular momentum, a larger number of triple clusters (up to 10) is envisaged. A possible plan could be to organize a specific campaign with the AGATA demonstrator for these types of experiments, so as to operate a larger array for high-multiplicity experiments towards the end of the hosting period of the demonstrator at GANIL. This bigger array would comprise as many AGATA modules as possible in addition to the EXOGAM Clover detectors to obtain a maximum efficiency. For nuclei at and beyond the proton dripline, studies of excited states by means of γ-ray spectroscopy may reveal the predicted strongly modified structures arising as a consequence of the interaction between bound states and proton continuum states. Studies locating the major regions of large deformation away from closed shells and near the proton drip line is an important aspect of nuclear structure physics and could address questions concerning the interplay of weak binding and 7

8 deformation (this can be considered as the analogue of 42 Si case which is at the other extreme of isospin). The use of fusion-evaporation reactions induced by stable or radioactive beams makes highspin states of very neutron-deficient nuclei accessible and allows the probing of nuclei for predicted regions of large deformations. Another question concerning neutron deficient nuclei is that of T=0 proton-neutron pairing with many consequences beyond nuclear structure. The signature for such a new pairing phase could come from np transfer reactions on heavy N=Z beams, but effects are also expected to be observed at high angular momenta for which T=0 pairing is predicted to be enhanced. Some of these questions may already be addressed with beams available from SPIRAL1. Experiments performed at GANIL using radioactive ion beams near the Coulomb barrier with the EXOGAM γ-array have shown that it is possible to observe the low-lying states of these neutrondeficient nuclei near the proton drip-line, where no excited states are known. For these experiments, one has to deal with a large background due to the possible use of radioactive beams and the large number of channels open in the reactions. Therefore the use of ancillary detectors as a light chargedparticle detector (DIAMANT) and a spectrometer to detect the evaporation residues (VAMOS) is necessary to extract the relevant information. To follow the evolution of the deformation in the proton drip-line nuclei at relatively high spin, the coupling of the AGATA Demonstrator to EXOGAM represents a solution to provide sufficient granularity and efficiency to observe the rotational bands up to high frequency. High-spin states in neutron-deficient rare-earth isotopes up to the proton drip-line can be explored by means of fusion-evaporation induced by 74,76 Kr radioactive beams, as soon as these latter will be available with sufficient intensity (~ pps). Reactions like 58 Ni + 74,76 Kr at a bombarding energy of 4-5 MeV/A with the set-up composed of the AGATA Demonstrator with a maximum number of triple-clusters coupled to the light charged-particle detector DIAMANT and the VAMOS spectrometer will be used for these studies. Geant simulations show (see annex C) that the higher efficiency of the (complete) EXOGAM array is clearly overcompensated by the much better spectrum quality (resolution, peak-to-total) obtained with the AGATA demonstrator. This result is not unexpected since EXOGAM was optimised for high efficiency and low-spin reactions. Both a smaller peak-to-total ratio as consequence of rather thin BGO shields and a somewhat worse energy resolution were accepted in order to maximise the efficiency. D. Spectroscopy of heavy elements towards super heavy elements at GANIL Nuclei beyond fermium (Z=100) are unique in that they are stabilised entirely by shell effects and predictions of their shapes, structure and lifetimes are highly dependent on the accuracy of the models. For example the position of the spherical gap for super heavy elements (SHE) remains uncertain as the Nilsson-Strutinsky, Hartree-Fock and Relativistic Mean Field theories give different results due to different treatments of the Coulomb and of the spin-orbit interactions. Macroscopic-microscopic calculations tend to predict an enhanced stability at Z = 114, relativistic mean-field models yield Z = 120, whereas Hartree-Fock calculations with Skyrme forces predict Z = 126. The developments of 4π Ge arrays in the last two decades coupled with advanced detection setups for evaporation residues have made a tremendous impact in this area of research. Complementary to the search for super-heavy elements, spectroscopy of prompt γ-rays or conversion electrons of these very heavy elements provides an additional access to the understanding of nuclear deformation and fission barriers. The sufficiently high production cross-sections of the transfermium nuclides make them ideal laboratories to probe important questions such as: Which orbitals are involved in the configuration of these nuclei and what is their role? What is the influence of the collectivity on nuclei located around the small islands of deformation (centred around 254 No and 270 Hs)? What is the role of K-isomerism in this mass region and in particular on their stability? What is the angular dependence of the fission barriers? Experimentally such studies involve both an extremely low cross-section (few nb) and a large background from competing reaction channels (mainly fission, with cross-sections from a few mb up to several barns) requiring the detection of the evaporation residues. At GANIL, the coupling of 8

9 VAMOS (also using the Wien filter) and the EXOGAM spectrometers has recently been proven to be highly competitive for very asymmetric reactions, like O or Ne beams on Pb or U targets. For these cases a transmission larger than 50% can be obtained. Improvements in the detection part of VAMOS, like the highly segmented Si wall MUSETT located in the focal plane, are currently in progress and will be used to study 255 No using Recoil Tagging and Recoil Decay Tagging techniques in 2006/07. The use of light beams on uranium targets will offer a unique opportunity to populate and study less neutron-deficient nuclei closer to the predicted island of stability. Developments of a rotating target to adapt to high-beam intensities and the use of exotic targets like Pu and Cm are also planned. It is envisaged that the demonstrator (and EXOGAM) will be triggered by the VAMOS spectrometer. One can estimate a triggered crystal rate of a few 100 Hz, while the untriggered rate may reach 50 khz. At the same time, the upgrade of EXOGAM to digital electronics (SPACE cards) will provide larger rate handling capabilities. One may therefore expect an improvement of a factor of ~5 in rate as compared to the present EXOGAM array. We estimate that the coupling of the AGATA demonstrator with EXOGAM and VAMOS will allow us to push the observation limit down to 10 nb or less, for a one week experiment. This clearly illustrates the capabilities of using the AGATA demonstrator coupled to EXOGAM and VAMOS and offers unique opportunities for γ-ray spectroscopy towards super heavy nuclei. The experiments that are proposed lead to the production of heavy nuclei with tiny cross-sections, populated at very low excitation energy and medium angular momentum, where the total efficiency is a key parameter. Obviously, a larger number of AGATA modules (up to 8 triple cluster modules can be accommodated by the proposed geometry) will provide a gain in statistics and in particular in coincidence efficiency, which is essential for the study of odd-mass nuclei. Typical cases which can be studied are 238 U( 22 Ne,xn) 255,256 No and 238 U( 26 Mg,xn) 259,260 Rf. 4. Technical proposal This section describes the infrastructure necessary to install and maintain the AGATA Demonstrator (AD) at GANIL. The presentation follows the items of the general document Basic infrastructure for the AD called BIAD in the following. 1. Experimental area The AD will be installed in the G1 area which comprises all the systems associated with VAMOS and EXOGAM and in particular the target chamber, the pumps and the Germanium cooling system Space constraints for the AD The AD is composed of 5 TC (Triple Clusters) placed as close as possible to the target in order to have the maximum efficiency. All the BIAD requirements (available space including digitisers, crane, Ge cooling ) are fulfilled. The installation details are presented in the following part ( 2) Other items The G1 area has a LN2 terminal and a Ge-detector autofill system which fulfills a basic requirement for the implementation of the AD. The pre-processing ATCA racks will be installed in the room very near the experimental G1 area. In this condition, the maximum distance between the experimental and the acquisition areas will be less than 50 meters. 9

10 2. Mechanics and beam line The AD must be placed close to the target and around 90. Detailed integration studies have led to the configuration presented in Figure 4. Note that the assembly of the 5 TC is not around the pentagonal hole but taken out from another part of AGATA in order to have compact geometry which can be placed at the nearest distance of 100 mm. This configuration allows the rotation of the VAMOS spectrometer and the introduction of (at least) one more TC. The AGATA collaboration will provide the mechanical support for each TC. In collaboration with the infrastructure team, GANIL will design and construct the support structure for the proposed configuration and its adaptation to the VAMOS platform. Depending on the final design, it could be necessary to move back by a few cm the VAMOS spectrometer. GANIL will provide the beam pipe, the target chamber, the beam dump and associated pumping system and valves. A crane is available and can be used to install and dismount the AD. Figure 4. Implementation of the AD (5 or 6 TC) in a compact geometry on the VAMOS platform (view from above). The spectrometer has been rotated at 45 The coupling of the AD to (parts of) the EXOGAM array is under study and will necessitate modifications of the existing infrastructure. In Figure 3 a possible assembly of the AD with 5 TC and 5 EXOGAM clovers was already presented. Note that the positions of the clovers (close to the floor) are not the EXOGAM standard ones and thus must be studied particularly on the mechanical aspects and associated costs. 3. AD cabling, HV and low-bias power supplies Each crystal is 36-fold segmented leading to 37 preamplifier output signals including the core signal. Seven 3M-MDR-26 Camera Link Style cables (SCSI like) transfer data from the detector to the corresponding digitiser box. These SCSI cables (10 m long) as well as the HV and low-bias power systems will be provided by the Collaboration. An environmental slow-control system will be used to manage the Ge HV system, the preamplifier power system, the Ge and cryostat temperatures control and the Ge autofill system. It is named Detector Support System (DSS) and will be also provided by the Collaboration and adapted at the specificities of the G1 area. 10

11 4. Electronics The front-end electronics is composed of the Digitiser and Pre-processing systems which will be provided by the Collaboration. Each crystal is associated with its own digitiser box (60cm width, 35cm depth and 11cm height). The 15 boxes needed for the AD will be implemented in 2 racks placed closed to the detectors (connection length not exceeding 10 m). Note that at least 2 spares will be necessary for maintenance purposes. Each digitiser may develop up to a maximum of 600 W per crystal and a total of 9 kw must be considered for cooling and UPS (Uninterruptible Power Supply). Each digitiser is linked to the corresponding pre-processing cards through multi-fibres optical cables which will be provided by members of the Collaboration. The crate standard for the preprocessing cards is ATCA (PCI Express). A 16-slot crate is well suited for two full TC and three such crates are needed for the AD. The pre-processing power dissipation for one TC is 1500 W and a total of 7.5 kw must be considered. The ATCA crates and their racks will be provided by the Collaboration. They will be implemented in the room 018 located outside the controlled experimental area (see Figure 5) but within a distance compatible with the 50 meters length of the optical cables. Figure 5. Possible implementation of different measurement areas 5. Cooling The water-cooling for the digitiser boxes and the pre-processing crates circulate in a secondary water circuit. Its temperature is maintained constant through a secondary/primary water/water heat exchanger. GANIL will provide cooling water at 25 for the digitiser boxes and 20 for the preprocessing crates. 11

12 6. Acquisition and Control area GANIL will provide the adequate hardware for general experiment control, online analysis and data storage. The dedicated processing power beyond the standard one at GANIL which is required to handle the AD data will be provided by the Collaboration. During the stay of the AD at GANIL, a control area (rooms in Figure 5) will be available. 7. Networking The AGATA Collaboration required a network architecture security and access such as the RFC 1918 which is the system used at GANIL. GANIL will be responsible for maintaining and securing the routing / VPN connections to the system. 8. Uninterruptible Power Supply The UPS available at GANIL is sufficient to deliver power to the AD and its associated equipments. The time autonomy for the front-end part (HV power, digitiser boxes and cooling) will be about 15 minutes for 40 kva. The equipments installed in the detector laboratory will also benefit from the GANIL UPS. However, the security rules at GANIL do not actually allow a private system devoted to specific equipment. Discussions have started with the security staff in order to meet the requirements of the AGATA Collaboration. 9. Germanium Cooling With the present 4.5 litre TC-dewar design, two or three fillings per day are necessary and similar AD LN2 consumption as EXOGAM has to be considered. GANIL will provide water-free LN2 from the existing 5000 litres LN2 tank and the distribution to the G1 area. As for EXOGAM, a buffer tank of 200 liters can be used. Using existing EXOGAM manifolds the AD autofill will be driven separately from EXOGAM with a dedicated 200 liters buffer tank. 10. Detector laboratory A detector laboratory is necessary for the maintenance of the detectors. It can be subdivided in three parts: an annealing area, a measurement area and a stocking area. At GANIL, the three areas (in Figure 5) will be located in three different rooms dedicated to Germanium detectors. Presently, the annealing area (room 006, 12 m 2 ) is equipped with a cryostat pumping station and a capsule annealing system. It will be completed by a table for Ge detector mounting and dismounting. A room for the measurement area is available (room 016, 12 m 2 ). It is equipped with standard electronics and data acquisition (4 channels) and routinely used for EXOGAM. The room 014 is sufficient for the measurements on a TC but needs all the specific AGATA equipment (1 Digitiser mechanic and cooling, 1 DSS simplified) which should be provided by the Collaboration during the stay of AGATA at GANIL. The stocking area is under discussion at GANIL but one (at least 12 m 2 ) will be available when necessary. 11. Spatial occupancy All the items are presented and discussed in the previous sections. 12

13 12. Maintenance and assistance to experiment For all the experiments, GANIL provides a technical support during the installation and the experiment itself. This support is provided by the STP Support Technique pour la Physique Department of 45 persons. Today, there are two persons (1 engineer, 1 technician) working full time for the EXOGAM detector assisted by the electronics and the data acquisition groups. Those teams will be adapted and trained to the specificities of the AD at GANIL. The requirements underlined in the BIAD (Support to the mechanical installation, Detector autofill, data processing ) will be fulfilled. Concerning the detector laboratory, not only the standard support such as the annealing will be assumed but a training investment in the specificities of AGATA (digitiser) is foreseen soon (by the end of 2006) in order to be ready when the AD will arrive at GANIL. 13

14 Annex A: Generic simulations for experiments with the AGATA demonstrator at GANIL This annex summarizes the generic Monte Carlo simulations that were performed for AGATA Demonstrator (AD) at GANIL. For the simulation the AGATA code (developed by E. Farnea) was used in order to generate the interactions points in the AD and the forward tracking code (developed by A Lopez-Martens) was used in order to perform tracking. The physics which has been taken into account is the following: Shell structure far from stability using deep-inelastic and fragmentation reactions (low multiplicity and high v/c) Nuclear structure towards superheavy nuclei using fusion-evaporation reactions (medium multiplicity and low v/c) Exotic shapes at high spin using fusion-evaporation reactions (high multiplicity and low v/c) Several simulations with different values of v/c, multiplicities and gamma ray energies have been performed: 8 different distances between the target and the demonstrator were chosen and 3 angular positions of the AD coupled to VAMOS have been used (90, 135 and 180 o with respect to the beam direction). The best results were found for the configuration at the closest distance (8.5 cm), but the mechanical constraints require a distance of 10 cm. The position at 90 o is better for high velocities (due to the Lorenz boost), whereas the position at 135 o has advantages for for small velocities (in order to take into account the angular distributions of photons). In all cases gammas have been simulated. As a reference the results for E γ =1 MeV, M γ =1 and 4 values of v/c (2, 10, 20 and 30 %) were adopted. The results are shown in A.1. The absolute efficiency, P/T and FWHM as a function of v/c and targetdetector distances are displayed in 3D. The upper right panel corresponds to the resolution (at v/c=10%) with a point source and a diffused one (1cm 2 size) together with a recoil cone opening angle of 10 o. Simulations were performed for specific cases involving various recoil velocities and γ-ray multiplicities. The results are listed below and compared to the EXOGAM performances. Comparison of the AGATA demonstrator and Exogam For a Coulomb excitation experiment, where a 46 Ar beam at 160 MeV is used on a 208 Pb target, corresponding to a v/c of 7.5% one would expect the following performances for a 1.3 MeV γ-ray and a target-detector distance of 10 cm for the AGATA demonstrator. Ph-efficiency (%) P/T (%) FWHM (kev) EXOGAM AD For a typical fusion-evaporation reaction in a inverse kinematics (the inverse kinematics is used in order to optimize the acceptance of the recoil spectrometer in conjunction with EXOGAM for such experiments), with a 72 Kr beam at 280 MeV on a 40 Ca target, corresponding to a v/c value of 5.8% one expects the following performances for 1.3 MeV γ-ray and medium multiplicity (15) Ph-efficiency (%) P/T (%) FWHM (kev) EXOGAM AD For typical fusion-evaporation reaction at low v/c (2%) and medium multiplicity (15) for the very heavy nuclei spectroscopy one obtains the following performances: 14

15 Energy (kev) Ph-efficiency (%) P/T (%) FWHM (kev) Exogam AD Exogam AD Taking into account that with AD, one can increase the beam intensity by a factor of 5, one would expect much better performances in terms of counting rate. Figure A.1: Simulated performances of the AD. The Photo-peak efficiency (Ph-Efficiency), P/T and FWHM are shown as a function of v/c (X axis) for different distances (Y axis). The 8 values of Y correspond to target-detector distances of 23.5, 22.5, 20.5, 18.5, 13.5, 11.5 and 8.5 cm, respectively, and the 4 values of X axis correspond to v/c values of 2, 10, 20 and 30%, respectively. The upper right panel compares the resolution (at v/c=10%) of a point source to a diffused one (1cm 2 size) together with a recoil cone opening angle of 10 o. 15

16 Annex B Simulation of the γ-ray detection with the AGATA demonstrator after fragmentation reactions with fast beams Abstract Simulations have been carried out to compare the performance of the Château de Cristal detector array of 74 BaF 2 crystals with EXOGAM and the AGATA demonstrator in experiments using fast fragmentation beams. A recent experiment to study extremely protonrich nuclei around 36 Ca after fragmentation of a 40 Ca beam followed by a secondary nucleonremoval reaction was used as a basis for GEANT simulations. In a first step the reaction mechanisms and sources of background were modeled to reproduce the experimental spectra of the Château de Cristal. In the second step the BaF 2 detectors were replaced by the EXOGAM spectrometer or the five triple clusters of the AGATA demonstrator placed at different distances to the target. The results show the excellent Doppler correction capabilities of the AGATA demonstrator, resulting in a significantly improved energy resolution and overall quality of the spectra. Experimental data A primary 40 Ca beam of 95 A MeV was fragmented on the SISSI target and the resulting secondary beam was purified in the α spectrometer optimised for the selection of 37 Ca. The fragments are identified event by event through a time-of-flight measurement. Further nucleons are removed on a secondary 9 Be target at energies around 61 A MeV. The secondary fragments are selected and identified in the SPEG spectrometer by time-of-flight and energyloss measurements. The γ rays from the secondary fragments are measured with the Château de Cristal array of 74 BaF 2 detectors arranged around the 9 Be target. The fragments have an average velocity of v/c 0.3, with a relatively broad velocity distribution due to the stopping in the Be target of 1 mm thickness. If known, the lifetimes of the short-lived excited states were taken into account when calculating the stopping in the target. A large number of different nuclides was produced in the experiment. The left part of Figure B.1 shows the experimental spectrum for 33 Cl which was produced abundantly and for which the level scheme is well known. The five strongest γ rays visible in the spectrum have energies of 810, 1541, 1986, 2838, and 2975 kev, the latter two of which are not resolved. Figure B.1: Experimental spectrum from the Château de Cristal spectrometer gated on the fragments identified as 33 Cl (left) and GEANT simulation (right). The binning of the spectra is 20 kev per channel. 16

17 Simulations Château de Cristal The 37 Ca fragments were assumed to arrive at the Be target with an energy of 60 MeV/u, and the interaction point is assumed to be equally distributed over the target thickness. The secondary fragmentation reaction populates the nucleus of interest in an excited state, where the relative population of the well-known states is modeled to best reproduce the experimental spectra. A realistic modeling of the various sources of background such as bremsstrahlung turned out to be very difficult. Instead an exponential background was assumed. The parameters of the background were fitted to the experimental data. The simulated spectrum is shown and compared to the experimental spectrum in Figure B.1. The background is well described in the interesting energy range and only at low energies below 500 kev it is overestimated. The Doppler correction and add-back between the crystals was performed in the same way as for the experimental data. The agreement between the simulation and the experimental data is excellent, showing the validity of the approach. The energy resolution in the simulated spectra is very similar to the experimental data. The numbers are compared in the table below together with the results for EXOGAM and the AGATA demonstrator. EXOGAM The same assumptions about the reaction mechanism, the population of the excited states, and the background were used in the GEANT simulation for EXOGAM in its close configuration with a distance of 11.4 cm between the segmented clover detectors and the target. The same number of events was simulated as for the Château de Cristal, so that the peaks integrals reflect the efficiencies of the detectors. To perform the Doppler correction, the crystal segment with the highest energy deposit was assumed to be the first hit. The γ-ray energy was assumed to be the sum of all four crystals of one clover detector. Events in which more than 50 kev was deposited in the Compton suppression shields were discarded. The resulting spectrum is shown in Figure B.2, and the energy resolution and peak integrals are summarized in the table. The energy resolution is significantly better than for the Château de Cristal, but still not sufficient to resolve the two transitions at ~3 MeV. Figure B.2: Simulated spectrum of 33 Cl for EXOGAM. The binning is 5 kev per channel. 17

18 AGATA demonstrator The same simulation was performed for the AGATA demonstrator comprising five triple modules at their nominal distance of 23.5 cm from the target position at 90º with respect to the beam axis. A second simulation was performed for closer distance of 11.5 cm, again at 90º. The interaction points of the γ rays in the germanium detectors were determined in a GEANT simulation and then packed and smeared following the prescription of the simulation package by E. Farnea. The γ rays were reconstructed using the tracking algorithm mgt by D. Bazzacco. The resulting spectra for both distances are shown in Figure B.3. Even though the efficiency of the AGATA demonstrator is smaller than that of EXOGAM, and much smaller than for the Château de Cristal, the overall quality of the spectra has improved drastically due to the high position resolution of the AGATA detectors. The two close-lying high-energy transitions are resolved for both distances. Approaching the AGATA demonstrator 12 cm closer to the target from its nominal distance enhances its efficiency by approximately a factor of 3, while the energy resolution deteriorates by less than 50%. It should be noted that the transition at 1541 kev becomes only visible at the closer distance. The operation of the AGATA demonstrator at a close distance to the target seems therefore the best compromise between the best possible energy resolution and the highest efficiency for experiments using fast fragmentation beams. Figure B.3: Simulated 33 Cl spectra for the AGATA demonstrator at its nominal distance of 23.5 cm (left) and at the distance of 11.5 cm (right). The binning is 4 kev per channel. Exp. Château EXOGAM AD 23.5 cm AD 11.5 cm E γ (kev) E (kev) E (kev) counts E (kev) counts E (kev) counts E (kev) counts Table 1: Comparison of the energy resolution (FWHM) and the statistics for two transitions in 33 Cl for the different detectors. The same number of events has been simulated for all cases. 18

19 Annex C Simulation of fusion-evaporation experiments with radioactive beams Geant4 simulations have been performed in order to compare the AGATA Demonstrator to Exogam for fusion-evaporation experiments using the example 76 Kr + 58 Ni at 4.3 MeV/A. This experiment (labelled E404as) has been realized in 2004 at GANIL with the radioactive 76 Kr beam provided by the SPIRAL facility. The five triple modules composing the AGATA Demonstrator were placed at their nominal distance (23.5 cm) at 0 degree with respect to the beam axis. The AGATA program (provided by E. Farnea) has been used in order to generate the interaction points in the AGATA Demonstrator and the forward tracking code (developed by A. Lopez-Martens) in order to perform tracking. Exogam was composed of 16 segmented clovers in configuration A (at 11.7 cm from the target position) with back catchers (CsI) and rear side shields (BGO) allowing Compton rejection. Photopeak efficiency and peak-to-total ratio have been compared to previous simulations (Geant III) and real measurements in different cases probing the reliability of this Geant4 simulation. As an input for the detector simulations, fusion-evaporation events have been generated by using the physics generator available in the GammaWare package ( With this Monte-Carlo code discrete γ ray cascades are generated randomly by using the level scheme associated to a given nucleus. The cross sections for the various fusion-evaporation channels were calculated using the code PACE. The five fusion-evaporation residues with a production cross section greater than 10 mb were taken into account: 130 Nd, 127 Pr, 131 Pm, 128 Nd and 129 Pr. Their level schemes come from the ENSDF database. A simple feeding (flat continuum of E1 and E2 transitions) has been introduced to reach the entry point (50 MeV, 50 ħ) which was the same for all the residues. The produced data set, consisting in events, have been shouted in both detectors as emitted from a moving source with velocity 5.3 % of the speed of light. The mean multiplicity of the gamma-ray cascades was then about 15. At such multiplicities previous Geant3-based EXOGAM simulations have shown that addback in clovers does not improve the peak-to-total ratio (add-back factor close to 1). Thus, in our analysis, each crystal is considered as a detection unit, the Doppler correction being applied using the mean crystal angle. While in EXOGAM, each crystal is electrically segmented in four regions, this information has not yet been included in the detector simulation. It could reduce the full width at half maximum (FWHM) of the peaks by about 20%. However, it does not change drastically the comparison between AGATA demonstrator and EXOGAM simulations. In Figure C.1 the gamma-ray spectra obtained for EXOGAM (upper part) and AGATA (lower part) are presented. While the large EXOGAM efficiency allows collecting more statistics, the much better peak-to-total ratio and the very much improved FWHM for the AGATA demonstrator lead to a spectrum of very high quality. Peaks that are hardly resolved with EXOGAM at low energy are well separated by AGATA. In addition, the AGATA demonstrator does not suffer from the Doppler broadening even at energies around 1 MeV, while finer peak structures already disappear at 700 kev for EXOGAM. The resolving power of both detectors has been compared by building γ-γ matrices and by setting gates on known γ-ray transitions of the most strongly populated nucleus ( 130 Nd). The result is depicted in the Figure C.2, where red boxes indicate the applied gates and blue lines the expected γ-rays. 19

20 Figure C.1: Total gamma spectra for EXOGAM (top) and the AGATA demonstrator (bottom). Both spectra are without background subtraction. Figure C.2: Single-gated gamma-ray spectra for EXOGAM (top) and the AGATA demonstrator (bottom). Both spectra are without background subtraction. 20