Project Description
During the past decade it has been demonstrated that reactions with exotic secondary beams are an important tool for exploring the properties of nuclei far from stability, and allow detailed spectroscopic information to be extracted. The physics motivation for studying reactions with exotic nuclei is described extensively in various reports in the context of next-generation facilities, see, e.g., the 'Conceptual Design Report' (CDR) (http://www.gsi.de/GSI-Future/cdr) for the future FAIR project or those for SPES (http://www.lnl.infn.it/~spesweb/index.php/what-is-spes/tdr-2008) and SPIRAL2 (http://pro.ganil-spiral2.eu/spiral2/what-is-spiral2/physics-case/view ). In addition, fusion-evaporation reactions induced by high intensity neutron-rich beams from SPIRAL2 will make it possible to populate exotic compound nuclei at a much higher initial angular momentum than currently achievable with stable beams. This will be of strong benefit in the study of vibrational and rotational collective phenomena at high spins and finite temperature, such as the Giant Dipole Resonance or exotic shape changes (e.g. Jacobi and Poincare transitions) induced by fast rotation. Heavy-ion radiative capture and reaction dynamics studies will also benefit considerably from the availability of high-intensity neutron-rich beams.
As gamma-ray detection constitutes an important experimental probe common to all these physics topics, the powerful future accelerator facilities require a new generation of gamma detector arrays capable of exploiting the full potential of these highly exotic or high intensity beams. Very recently, new technologies, materials and techniques have been developed and large international collaborations have been formed for the development and construction of large detector arrays like CALIFA (R3B) at FAIR or PARIS at SPIRAL2. The R3B set-up at FAIR will concentrate on experimental reaction studies with exotic nuclei far from stability, with particular emphasis on nuclear structure and dynamics and reactions of astrophysical interest. The R3B programme will focus on the most exotic short-lived nuclei, which cannot be stored and cooled efficiently, and on reactions with large-momentum transfer allowing the use of thick targets. The proposed experimental setup is adapted to the highest beam energies delivered by the Super-FRS, thus making full use of the highest possible transmission efficiency of secondary beams. A crucial part of the R3B set-up is the gamma-ray spectrometer CALIFA that surrounds the reaction target. This spectrometer is designed to stop gamma rays up to 30 MeV and protons up to 300 MeV, with high angular resolution to permit a full kinematic reconstruction. Due to the relativistic energies involved, the gamma rays are strongly Doppler-broadened which is why a very high segmentation is needed.
The main aim of the PARIS collaboration is to develop and construct a dedicated gamma-calorimeter with dynamical range from 100 keV to 50 MeV. PARIS is designed to be used primarily at SPIRAL2 but could be portable and be used at other facilities such as FAIR or HIE-ISOLDE. At SPIRAL2, PARIS will be used in conjunction with other detectors such as AGATA, NEDA, and GASPARD. PARIS therefore needs to be a highly modular and versatile device. The full technical design and specification of a device such as PARIS can only arise from an intensive R&D program, with GEANT4 simulations and prototype testing being a key component of this work.
Objectives of the project and expected results
In-beam and decay studies of the rarest isotopes produced at next generation radioactive beam facilities will shed light on the structure of nuclei approaching the drip-lines. This is central to a deeper understanding of the isospin dependence of nuclear forces, and our understanding of different nucleosynthesis processes in our Universe. Gamma-ray detection constitutes an important experimental probe common to all these physics topics. High-resolution gamma-ray spectroscopy is still one of the most important tools in nuclear physics. The unique opportunities of the next generation RIB facilities bring with them strong experimental challenges and the obligation to make best use of the high investment in delivering RIBs. The optimum gamma spectrometer will therefore combine a maximum of solid angle with good rate capability and energy resolution. New scintillator materials and photon detector technologies in combination with high granularity will push forward the experimental limits of Doppler shift at relativistic energies at a quite reasonable amount of investment. Detector setups need to be versatile to satisfy the demands of a wide range of different experiments ranging from the detection of low energy gamma rays from single particle excitations, high energy gamma rays associated with different collective modes, up to the detection of charged particles emitted from the reaction zones. In the case of quasi-free scattering reactions, low-energy gamma rays have to be detected with high resolution in coincidence with protons of up to 300 MeV. On the other hand, in Coulomb-excitation experiments, it is necessary to fully absorb high energy gamma rays from collective modes that are Doppler shifted by up to a factor of three. When using relativistic energy beams, it is also very helpful to have a good neutron-gamma separation to suppress background as well as a E/E measurement to identify charged particles without additional assumptions on the reaction. Decay experiments dealing with extremely low production yields of the nuclei of interest, and suffering from enormous radiation from strong competing channels as well as from the environment, requires excellent background suppression capability.
A crucial part of the R3B set-up at FAIR is the gamma-ray spectrometer CALIFA that surrounds the reaction target. This spectrometer is designed to stop gamma rays up to 30 MeV and protons up to 300 MeV, with high angular resolution to permit a full kinematic reconstruction. Due to the relativistic energies involved, the gamma rays are strongly Doppler-broadened which is why a very high segmentation, especially in the forward direction is needed. CALIFA is divided into two parts: a BARREL region covering 40-130 degrees (polar angle) and a forward END-CAP covering 6-40 degrees (polar angle). The collaborating partners have almost completed the design and are close to beginning the construction of the BARREL part of CALIFA. The present project in the scope of the NuPNET initiative will deal with the extra R&D efforts needed to fully develop the END-CAP part of the detector. Here the Doppler shift is largest, the magnetic field is up to several hundred mT, the mechanical space is limited by the magnet housing and most of the particles and gamma rays are kinematically boosted into this region. This is a challenge not to be solved with standard technologies.
The full technical design and specification of a device such as PARIS can only arise from an intensive R&D program, with GEANT4 simulations and prototype testing being a key component of this work. Two scenarios are presently under consideration: a) A double-shell calorimeter, with inner (hemi-)sphere, highly granular, made of novel short crystals (such as LaBr3(Ce), LaCl3, or other materials that are under development). The readout might be performed with APDs or very compact PMT coupled with digital electronics that would offer the possibility of pulse shape analysis. The outer (hemi-)sphere, with lower granularity but with high volume scintillators will measure high-energy photons. The inner-sphere will be used as a multiplicity filter, sum-energy detector and will also serve as an absorber for the large detectors behind. b) A single-shell calorimeter, consisting of Phoswich-type detectors, i.e. with two (or more) layers of scintillators with different emission wavelengths read by common photosensors. These layers will act as the two shells from the scenario a), but the whole design could be more compact and more cost-effective.
Fig.1 Possible geometry of the PARIS array (left), consisting of 220 LaBr3(blue)+NaI(red) phoswiches) and the CALIFA calorimeter array (right) divided into a BARREL part made up from 3.000 CsI crystals with APD readout and a Forward END-CAP of 800 Phoswich made e.g. from LaBr+LaCl phoswiches. Both arrays need a very high segmentation for Doppler correction of gammas emitted in flight at relativistic energies. Further, they also need to reconstruct the energies of charged particles at even higher energies. It is clear from the design studies performed so far of the CALIFA and PARIS detectors that an intense R&D in order to optimize the efficiency and readout performance of these detectors is needed. The objective of the GANAS project is to study new scintillator materials and readout systems in order to obtain and supply the needed information. Modern techniques of digital data acquisition have been found to be a major step forward in the next generation of gamma-ray tracking arrays based on high-purity germanium detectors. These concepts based on digital pulse shape analysis (PSA) have triggered a major development effort in related fields. The position information obtainable by PSA allows i) determination of the first interaction point of a gamma ray for improved Doppler correction; ii) tracking of gamma rays to disentangle multiple hits, distinguish high-energy photons from a shower of low-energy transitions in fusion-evaporation reactions, and to distinguish gamma rays from charged particles; and iii) through tracking, imaging of gamma ray sources can be used to strongly suppress background. Certain advantages like flexible trigger concepts, high rate capability and pile-up reduction, larger dynamic range and highly integrated concepts of data reduction already in the frontends maximise the information stemming from the detector response. Nevertheless, analogue signal conditioning is still an issue when it comes to fast timing signals. The project working packages are organized in an increasing level of complexity so that each successive WP is dependent on the previous package, but the work can naturally proceed in parallel as the necessary materials and experience already exist within the collaboration. Strong communication within the collaboration will ensure the frequent exchange of information. The GANAS project has several clear objectives, related to different work packages. WP1 will focus on identifying and studying new scintillator materials. This will be done in collaboration with existing research that is going on worldwide. Under WP2, we will combine these materials with different photo-sensors in order to obtain an optimum readout and thus the best possible performance of the scintillator. In WP3, we will study how pulse shape analysis can be used to distinguish between different type of radiation (gammas and particles) and how to distinguish interactions in the different halves of a phoswich detector. Further, we will investigate if direct pulse shape analysis can improve upon the dynamic range, in order to simultaneously detect gamma radiation with energies starting at 100 keV and particles of several 100 MeV. In WP4, we concentrate on position sensitivity in large volume novel crystals, in particular, the possibility to identify the point of interaction within the crystal volume. WP5 is looking at the sum of the other work packages, where we will gather the outputs of the different WPs to design and construct a segmented scintillator array prototype. asdfasfasfasd asdfasfasdf asdfasfsdf