Instrumentation

1. Blowfish Introduction Introduction The Blowfish neutron detector array was built to study the photodisintegration of light nuclei, such as Helium-4. Photodisintegration is a reaction where a gamma-ray photon interacts with an atomic nucleus and causes it to lose one or more of its composite particles. Blowfish is designed to detect neutral particles, such as neutrons emitted during photodisintegration, and is not sensitive to protons, because these particles are absorbed by the target walls. Since we detect only neutrons, we measure the photoneutron cross section, which is related to the probability that a gamma-ray photon breaks apart a nucleus and releases a neutron. This photoneutron cross section has a large resonance, called a giant dipole resonance, at energies ~20MeV. This resonance is due to the dipole absorption of a photon by a nucleus. Photonuclear physics is very concerned with experimental measurements and theoretical predictions of this resonance. Blowfish is able to measure the total-photoneutron cross section as well as angular dependence. A cross section with angular dependence is called a differential cross section. The instrumentation of the Blowfish neutron detector array allows us to study the physics described on the left side of this poster. In the past, much attention has been paid to the total cross section of photodisintegration reactions. Less time has been devoted to the differential cross section due to the absence of appropriate experimental facilities. Polarised targets have existed for many years, but it is only recently that polarised gamma-ray sources, such as the Duke Free Electron Laser Laboratory, have become available. Now it is possible to make measurements using a polarised target with a polarised beam. This will lead to a new generation of photodisintegration measurements that take into account the angular dependence and asymmetries of these reactions. The spherical nature of Blowfish allows us to study these angular dependencies and asymmetries in photodisintegration. The 88 Blowfish detectors use the classic scintillator and photomultiplier tube design and the data acquisition system uses a combination of traditional electronics with a new computer interface. The Blowfish Neutron Detector Array The Blowfish neutron detector array consists of 88 organic liquid scintillator neutron detectors. These detectors are arranged in 8 groups of 11. Each group of 11 is mounted on an aluminum support arm as can be seen in the above picture. The Blowfish detectors cover about 1/4 of the 4π steradian solid angle of a sphere. Giant Dipole Resonances Electronics Detectors Giant dipole resonances (GDR) have been studied for some time. They connect theory and experiment. Experimentalists are constantly striving to improve their measurements and theorists are working on mathematical techniques to make the difficult calculations tangible. The ultimate goal is to model the nuclear potential as accurately as possible. GDRs have been measured for many nuclei1. Experiments with Blowfish will concentrate on light nuclei with atomic mass number  4. Helium-4 is an inexpensive material with a rich nuclear structure, making it an ideal candidate for photodisintegration experiments. Previous studies, shown in the graph on the left, have generated contradictory results. New experiments are needed to understand the form of the Helium-4 GDR. Blowfish can go beyond measuring the total cross section. With its spherical coverage, Blowfish can look at the angular dependence and asymmetries of neutrons emitted from photodisintegration. This allows us to further study the structure of the giant dipole resonances. The Blowfish electronics digitise the information from the 88 detectors. When a signal is received from a detector, it triggers a discriminator which outputs a digital-logic pulse. The signal is also sent to an integrating analogue-to-digital converter that can be read by a computer. The logic pulse is sent to the electronics to inform that a particle has been detected. It is used with a signal from the accelerator to measure the time-of-flight for the particle. Along with a technique called pulse shape discrimination, the time of flight can tell us whether it was a neutron or scattered photon that caused the signal. Saskatoon People The detectors used in Blowfish are designed to detect neutrons. The liquid organic scintillator in the cell is rich in hydrogen. When a neutron enters the cell, it may collide with a hydrogen nucleus and transfer energy to the proton. The proton then moves through the scintillator, depositing energy. The scintillator emits photons that travel through the light guide and silicon cookie and strike a photocathode which emits electrons. These electrons are accelerated by the potential difference in the photomultiplier tube and strike dynodes in the tube to release more electrons. This amplified signal is then sent to the electronic modules for analysis. See W.R. Leo Techniques for Nuclear and Particle Physics Experiments, 2nd Ed. Springer-Verlag, Berlin (1994). Diagram courtesy of Jen Robb. Giant Dipole Resonance for Helium-4: Experiment and Theory2 The Experimental Subatomic Group - Summer 2004 From left to right: Daron Chabot, Jennifer Robb, Mike Barnett, Rob Pywell, Ward Wurtz, Tavi Mavrichi, Ru Igarashi (missing is Brian Bewer) 4He(,n)3He cross sections up to 120MeV (a) and 35MeV (b): full result (solid curve), Born approximation (dotted curve), experimental results (data points). 1. B.L. Berman and S.C. Fultz, Rev. Mod. Phys 47, 713 (1975). 2. S. Quaglioni, et al. Phys. Rev. C 69, 044002 (2004). New VME Electronics The Blowfish electronics are currently undergoing an upgrade. FASTBUS and CAMAC electronics modules are being replaced by newer and faster VME modules. The VME modules are connected to a Pentium-4 PC though an optical fibre cable. The PC runs RTEMS, a real time operating system that takes data from the VME modules, converts it into a useful format, and sends it to the user’s computer for online analysis and saving. Conclusions The Blowfish neutron detector array will be an important tool in the next generation of photodisintegration measurements. With its unique spherical design and the advance of polarised gamma-ray beams, studies of the angular dependence and asymmetries of photodisintegration reactions will shed new light onto old problems. Combined with powerful theoretical techniques, Blowfish will help us better understand the structure of light nuclei. Blowfish Neutron Detector ArrayW. Wurtz1,3, R. Pywell1, N. Kolb1, R. Igarashi1, B. Bewer1, D. Chabot1, J. Ives1, B. Norum2, B. Sawatzky2 Instrumentation Nuclear Physics Acknowledgements Homepage: nucleus.usask.ca Funding Agencies: NSERC of Canada University of Saskatchewan University of Virginia Facilities: Duke Free Electron Laser Laboratory Triangle Universities Nuclear Laboratory Author Legend: 1. University of Saskatchewan 2. University of Virginia 3. Poster Author