The EUROBALL gamma ray detector array

Abstract

T HE EUROBALL collaboration is a common effort of several European countries to provide a forefront experimental facility for nuclear structure research. Using composite germanium (Ge) counters, the EUROBALL detector is the prototype of a new generation of gamma-ray detector arrays that has set new limits to gamma-detector technology and provided a step forward both in basic research and in applications. As an example of application outside the nuclear physics domain we should mention the recent discovery by the NASA probe, Mars Odyssey, oflarge quantities of hydrogen that most probably is bound in water under the Mars surface. This evidence was obtained through neutron and gamma detection, the latter based on the EUROBALL detector technology (encapsulated "(-ray detectors)-see Fig. 1 [1].The Euroball array consists of 239 Ge crystals geometrically arranged in order to cover 45 % of the total solid angle. Technical details are reported in ref. [2]. Installed at two of the main nuclear structure facilities in Europe, at LNL (Legnaro-Italy) and at IReS-Vivitron (Strasbourg-France), see fig. 2, it has allowed the investigation of atomic nuclei at extreme conditions of angular momentum values and of proton/neutron ratios. This paper will give an overview of the research activities by selecting a number of examples. We, the authors, would like to dedicate this article to the memory of our IReS colleague and good friend Dr. lean-Pierre Vivien who has untimely and brutally passed away when this paper was under discussion. --, Why study nuclei at extreme conditions?The atomic nucleus is the paradigm ofa mesoscopic system (finite many-body system) of strongly interacting Fermions where quantal size effects play a central role. Renormalization effects for the nucleon-nucleon interaction are strongly boosted by making the system more polarizable, that is less bound. A detailed investigation of the nuclear structure of nuclei at extreme conditions, both at the limit of angular momentum and ofisotopic spin (neutron/proton ratio) where specific parts of the nuclear forces are strongly amplified, allows definitive tests to be carried out on the effective interaction acting between nucleons in nuclei. These studies provide the basis for a first principle description of the nuclear structure and are necessary to set stringent limits on fundamental symmetry-violation effects and on the lifetimes of rare processes. With an experimental sensitivity up to 10-5 of the production cross section, such as that ofEuroball, it has been possible to provide an unprecedented study of the nuclear properties under extreme conditions by addressing a number of problems concerning high spins and nuclei far from stability. Nuclear Structure at the limits of the angular momentumThrough the response of the nucleus to rotational stress one can investigate a wide varietyofnuclear structure phenomena showing the different facets of a finite fermionic system. In spite ofthe fact that rotating nuclei have been studied since the early 50's, many new pheno...

Resolution, efficiency and stability of HPGe detector operating in a magnetic field at various gamma-ray energies

“…The most powerful detector for gamma-ray measurements up to a few MeV is the large volume High Purity Germanium crystal (HPGe). In the last few years a rapid progress in nuclear structure studies has been made thanks to the high quality data obtained with gamma-ray spectroscopy experiments using large arrays as EUROBALL [1,2] and GAMMASPHERE [3], based on HPGe detectors. In order to reduce the Doppler broadening in measuring gammaray emission in flight, VEGA [4] and EXOGAM [5] used segmented geometry.…”

Gamma-ray spectroscopy of fission fragments with state-of-the-art techniques

“…A new development phase in experimental techniques began in the early 1990's and continued for the subsequent two decades. The advent of very high efficiency Compton suppressed spectrometers such as GAMMASPHERE and EUROBALL [17][18][19][20][21] achieved unprecedented sensitivity and resolving power through the use of many (> 100) high-volume Compton-suppressed Ge detectors (> 300 cm 3 ). The high detection efficiency of up to 10 percent allowed high-statistics experiments of triple γ -ray coincidences to be performed.…”

“…In high-statistics experiments with large arrays (e.g., EUROBALL [18,19] and GAMMASPHERE [4,21]), using in particular 248 Cm fission sources, multi-fold γ coincidences have been employed to identify yrast cascades in a number of nuclei around 132 Sn, often exploiting prompt and delayed cross coincidences with known γ rays from fission partners. Such experimental approaches have contributed to accessing key ingredients for shell model interpretations, such as single-particle energies, mainly at the highest spins, from the analysis of one-valence particle/hole nuclei (see Fig.…”

“…Fig 19. Spectroscopic information for neutron-rich Sr and Zr isotopes, as obtained from γ -spectroscopy studies of fission fragments produce by spontaneous fission of248 Cm (N = 50-64, EUROGAM-2 data)[141] and fission of238 U in inverse kinematics (N = 66-70, EXOGAM+VAMOS and RIKEN data, the latter in red)[36,79,146].…”

Yeni Geliştirilen NUMEXO2 Dijital Elektronik ile EXOGAM2 Dedektörünün K-Parametresine Bağlı Enerji Çözünürlüğü

“…gibi diğer etkenlerde önemli rollere sahiptir [1]. Özellikle nükleer fizik alanında hem yüksek verimliliğe sahip olan hem de yüksek çözünürlük sağlayan dedektör sistemleri (AGATA, MINIBALL, GAMMASPHERE, GRETA, EXOGAM2, NEDA ve diğerleri gibi) için birçok ülkeden araştırmacılar bir araya gelerek yeni dedektör sistemleri üzerine çalışmalar yapmaktadır [2,3,4,5,6,7]. Bu çalışmada deneysel nükleer fizik çalışmalarında uzun süredir kullanılan ve önemli bir konuma sahip olan EXOGAM2 dedektörünün enerji çözünürlüğünün yeni geliştirilen dijital elektronik NUMEXO2 ile nasıl değiştiği bu dijital elektronik ünitedeki K-parametreleri değiştirilerek araştırılmış ve elde edilen sonuçlar bu çalışmada verilmiştir.…”