DEVELOPING OF A VME COMPLEX NEUTRON MEASUREMENT SYSTEM AT IFIN-HH TANDEM ACCELERATOR USING A NEUTRON ARRAY DETECTOR * HORIA PETRASCU 1, IULIA COMPANIS 1, ALINA ISBASESCU 1, MICHAEL FAMIANO 2 1 Horia Hulubei National Institute for Physics and Nuclear Engineering, P.O. Box MG-6, Bucharest, Romania, E-mail:hpetr@nipne.ro 2 Western Michigan University, Kalamazoo, MI 49008, USA, E-mail: michael.famiano@wmich.edu Received September 14, 2009 Development of a VME (Versa Module Eurocard) complex neutron measurement system using a neutron array detector based on time of flight method (TOF) at IFIN-HH Tandem accelerator for investigation of neutron-neutron correlations by the interferometry techniques of two identical particles is described. This is based on the fact that the n-n correlation function is extremely sensitive against the space-time dimensions of the area from which the neutrons were emitted. The new features include the use of an array detector with 81 scintillators, a compact multipurpose reaction chamber, a multi-entrances high voltage source, a VME coincidence data acquisition and a standard network of a fast/slow coincidence electronics. In particular, the details of design, fabrication and assembly of components engaged in the setup are presented. Selected experiments planned with the setup are mentioned and briefly discussed. The quantitative determination of the detection s threshold for neutrons energy (energetic calibration), test spectra of neutron coincidences, and fission spectra are illustrated to exhibit the satisfactory performance of the developed setup. Key words: neutron array detector, neutron measurements, n-n correlation function, nuclear fission. 1. INTRODUCTION Major nuclear data centers of the world are cooperating in order to intensify efforts for completing the experimental database using better instruments and more refined theoretical models. In view to solve some issues still not elucidate in the nuclear fission process (e.g. the space-time provenience of the fission prompt neutrons), and envisaging an opening (a strong impact) in the methods for determining nuclear database using multimodular neutron detectors, we initiated the construction of a complex system for neutron measurements to be implemented * Paper presented at the 10 th International Balkan Workshop on Applied Physics, July 6 8, 2009, Constanţa, Romania. Rom. Journ. Phys., Vol. 56, Nos. 1 2, P. 86 92, Bucharest, 2011
2 Developing of a VME complex neutron measurement system 87 at the IFIN-HH Tandem accelerator. The start was given by the neutron multimodular detector, array type, built in IFIN-HH [1], in collaboration with RIKEN, Japan, for n-n coincidence measurements in experiments with radioactive beams, that was successfully used in the investigation of the pre-emitted neutrons from 11 Li halo nuclei, in fusion of these nuclei with light (Si) nuclear targets [2 4]. Our complex neutron measurement system includes, as essential components: a neutron multimodular detector with a multi-entrances high voltage source and a VME coincidence data acquisition and analysis with a standard network of a fast/slow coincidence electronics. 2. DESCRIPTION AND DEVELOPMENT OF THE EXPERIMENTAL SET-UP COMPONENTS 2.1. THE NEUTRON ARRAY DETECTOR The neutron array detector consists of 81 BC400 plastic scintillator cells of 4 4 12 cm 3 coupled to Philips XP2972 photomultipliers. The neutron and γ-ray separation is done only by the time of flight technique (TOF). The detection efficiency which depends on the neutron energy, electronic bias and on the size of the cell, for 10 MeV neutrons is close to 34%. The system time resolution obtained by Monte Carlo simulations [5] is ~ 3.3% and ~ 1ns for a scintillator cell. This system has been specifically adapted for working conditions at the IFIN- HH Tandem accelerator: it has been raised to the axe level of the beam, it can be 3D adjusted, it has been optimised by fixing the distance between two adjacent detectors from 0.8 cm initially to 2 cm these being the result of simulations conducted with MENATE [5] and GEANT [6] computer programs. In Fig. 1 the optimised neutron array detector is shown. Fig. 1. The neutron array detector with 81 components.
88 Horia Petrascu et al. 3 One has to mention that until now, in neutron database measurements predominantly single detectors have been used so far, due to cross-talk effect. The cross-talk (c.t.) is a spurious effect that simulates an n-n coincidence, but in fact is the detection of the same neutron by two or more detector cells. The coincidences between adjacent detectors are denoted as first order coincidences. Coincidences between two detectors separated by one detector are denoted as second order coincidences and so on. The method for cross-talk rejection and the selecting of the true coincidences we have developed, based on by (CT) and respectively (TC) windows was obtained by Monte Carlo simulations with MENATE program [2]. The c.t. can be known with sufficient precision and more important, it was observed that the c.t. could improve the time resolution in TOF measurements [2,3]. An improvement of the resolution by a factor of ~ 1.4 for the first order c.t. and for the second order c.t., by a factor of 1.7, was found. Another benefit in using an array detector is the possibility to measure with much higher efficiency the neutron angular distribution. For the 81 scintillators detectors a universal high voltage multichannel power supply system SY 1527 (CAEN), with 90 entrances was installed. 2.2. THE DATA ACQUISITION SYSTEM After a search for a small, versatile, low dead-time and PC-based acquisition system, for the first time in IFIN-HH Tandem accelerator, a new VME data acquisition and analysis system, based on Software and on VME modules with fast encoding and readout ADC capability, has been chose and put into operation. The VME data acquisition is the last version of NSCL [7], beneficiate of an American expert assistance from Western Michigan University supported by a NATO grant. The advantages of the VME acquisition system is the high speed and high performances, single board computer modules, a faster and efficient data transfer, being a more versatile bus than other major data acquisition standards, NIM and CAMAC. The NSCL data acquisition system, DAQ, consists of three major components. Nscldaq contains the library routines for control of the external electronic modules. With these routines Nscldaq can be used for event triggering, readout to read data, control of electronics, discriminator control, control of the external electronic modules. SpecTcl is a C++ program encapsulated in a TclGUI and is used to read data from disc. The user can create spectra, analyze data, make graphic cuts and even fit data. Spectrodaq provides the necessary link between readout and the analysis interface by setting up a server. The use of the VME bus modules allows fast downloading of data from the converters. The hardware includes 2 Dual-Core Intel Xeon processor, six SATA ports at 3.0 Gb/s via ESB2PC, 4 HDD s of 160 GB each, based on SBS technology interface with optical fibre cable, operating in the Linux Debian, a VME controller. I/O modules and other dedicated subsystems are commercially available, CAEN modules in majority.
4 Developing of a VME complex neutron measurement system 89 The simplified functional neutron schematic diagram of our data acquisition system is shown in Fig. 2. The complete system is controlled by a VME bus unit, multitasking real-time operating system and contains 3 channel multievent Timeto-Digital Converter (TDC) model V775AC, with 32 channels each, 3 Charge to Digital Converter (QDC) model V862AC with 32 channels multievent individual gate, 6 Constant Fraction Discriminators (CFD) model V812B, with 16 channels, an Input/ Output module model V977, 1 ADC, model V785 N with 16 channels, a computer for acquisition and data analysis server type and the universal multichannel power supply system. The electrical signals are given by TDC which records the arrival time of the signal through a CFD module. The CFD module is used with the TDC in order to select the threshold for the scintillators detectors from the array system. These timing are collected and then converted to a neutron spectrum by TOF. The charge analysis is made by QDC modules that are used for light deposit in the material for scintillator of each detector, that the neutrons are detected. Fig. 2. The simplified functional neutron schematic diagram of our data acquisition system. The complete schematic diagram for all 81 detector cells is done by properly multiplying the left side of the VME bus.
90 Horia Petrascu et al. 5 3. TESTINGS AND DISCUSSIONS The assembling of the experimental setup and the running of the whole complex neutron multi-detector-acquisition system was tested for a fission reaction (p,f) induced by protons on actinide targets 235,238 U at the IFIN-HH Tandem accelerator, through TOF. The experimental set-up shown in Fig. 3 include a thick reaction chamber ORTEC type of 0.5 m inner diameter brass, in centre with the actinide target, the fission fragments (FF) detector, Ta collimators and outside, the neutron array detector with 81 detector cells. The target used was 235 U (IAEA) of 100 µg/cm 2 on a thick Ni support, at 45º from the incident protons beam direction. A Si depleted detector of 300 µm was used as FF detector, at 10 cm from the chamber centre. Ta collimators were used, one at the chamber entrance and the other of 0.5 mm collar at the chamber exit for reduction of the protons background on the forward direction of the beam. The neutron array was placed perpendicular of the beam direction at 180 cm from the target. The proton energy was between 10 15 MeV. Fig. 3. The experimental set-up. Testing the experimental setup implied the mounting and testing of all VME electronic modules used for the neutron detector array. For the establishment of the impulse triggering we used the TDC modules that functioned in the COMMON Stop. The START was made by any of the scintillater detectors and the STOP was given by the Si detector for FF. Assignment of the minimum delays (60-70 ns) and the CFD thresholds establishment were made by testing the conditions for the TOF measurements of the energy of the prompt fission neutrons. The optimum high voltage necessary for the alignment of the 81 components of the detector array with a γ source 60 Co, and a quantitative determination of the detection s threshold for neutrons (energetic calibration), using the muonic cosmic radiation s peak of
6 Developing of a VME complex neutron measurement system 91 8 MeVee (12 MeV), observed for each scintillator detector were established. The muonic cosmic peak obtained within this test compared with one obtained one year ago is shown in Fig. 4. Fig. 4. The muonic cosmic ray peak. We have obtained coincidences spectrum with poor statistics and fission spectrum in tree different runs shown in Fig. 5. From the data obtained we can say that the VME complex neutron measurement system described, with the neutron array detector based on TOF is suitable for various other applications where the investigation of n-n correlations by the interferometry techniques is required. Fig. 5. Fission spectrum. The availability of this system will create the possibility of obtaining n-n correlation data of major interest in the emission of scission prompt neutrons in the
92 Horia Petrascu et al. 7 nuclear fission process, a problem still internationally unresolved, approaching neutron investigations in the frontier field of radioactive beams, e.g. four neutron correlations, performing necessary calibrations for neutron detector system, preparing the acquisition and analysis of data and also creating the prerequisites for obtaining more efficiently basic neutron nuclear data. Improvements of this complex system could be done by replacement of the solid scintillators with liquid ones, for better n-γ discrimination and using a suitable actinide target on a thin support, in a bigger thin walled Al reaction chamber with shorter residual radioactivity, for different arrangements. The work is in progress. Acknowledgements. This paper is partially supported by Idei Contract 535 financed by CNCSIS. Most of the VME hardware investments were supported by MEdC (Bucharest) contract no. CEEX-05-D10-48. REFERENCES 1. M. Petrascu et al., Array detector for neutron pre-emission investigations, Rom. Journ. Phys., vol 44, Supplement, p. 115, 1999. 2. M. Petrascu et al., Pre-emission of correlated neutrons in the fusion of 11 Li halo nuclei with Si target, Phys. Rev. C 69, 011602(R) 2004. 3. M. Petrascu et al., Probing the 11 Li halo structure by two-neutron interferometry, Nucl. Phys. A 738, 503, 2004. 4. M. Petrascu et al., Experimental state of n-n correlation function for Borromean halo nuclei, Nucl. Phys., A 790, 235c, 2007. 5. P. Desesquelles, the program MENATE (unpublished). 6. http://wwwasd.web.cern.ch/wwwasd/geant/. 7. NSCL Data Acquisition Documentation http://docs.nscl.msu.edu/daq/.