Efficiency optimized low-cost TDPAC spectrometer using a versatile Routing/Coincidence Unit

M. RENTERÍA; A. G. BIBILONI; G. N. DARRIBA; L. A. ERRICO; E. L. MUÑOZ; D. RICHARD; J. RUNCO

Foz do iguazú, Brasil

Conferencia; XIV International Conference on Hyperfine Interactions & XVIII International Symposium on Nuclear Quadrupole Interactions, Foz do Iguazú; 2007

A highly accurate, stable and low-cost  g−g  TDPAC spectrometer, PACAr, using a versatile EPROM-based routing/coincidence (RC) unit, optimized for 181Hf-implanted low-activity samples is presented. It is based, in addition, on 4 BaF2 detectors, 8 home-made fast constant-fraction differential discriminators (CFDD) [1], a TAC and an 8K MCA-card (Nucleus PCAII-8000) hosted in a 486 Intel-processor PC, connected in a fast-fast coincidence logic [2]. The BaF2 crystals are truncated conical-caped cylinders of 30 mm height with 90º full cone angle, being their shape chosen to optimize “the signal (anisotropy) to relative accuracy (statistical error) ratio” assuming the use of a sample holder/oven of f=20mm and relevant  energies of 181Hf, but also of 111In and 99Mo TDPAC probes [3]. A serious limitation of a great number of commercial MCA cards is their impossibility to address events to different memory groups during acquisition, i.e., one can only select a memory group when the acquisition is stopped and it remains fixed during acquisition. Although the routing technique is very well-known, for face to these cases we developed a RC unit where the internal signals of the MCA that normally determine a particular memory group are replaced by the specific combination of logic signals produced in the coincidence circuit of the RC unit [4]. This procedure enables the time events storage in different memory groups, during continuous data acquisition, depending on the specific coincidence validated in the RC unit. A compact instrumentation of this idea was presently developed and implemented, replacing the mentioned circuits by an EPROM (erasable programmable read-only) memory programmed to know which combination of input logic signals the MCA needs to route the event to a certain memory group. If the replacement of the MCA card by a different one is needed, the RC unit can be quickly adapted with no cost reprogramming the EPROM. The excellent energy resolution, high efficiency and “information gain”, and very good timing resolution of PACAr are analyzed and compare with advanced and already tested fast-fast [2] and slow-fast [3] PAC spectrometers. We gratefully acknowledge T. Butz, R. Vianden, and A.F. Pasquevich for fruitful suggestions and advice during the design of the detectors. [1] Constructed upon a modification of a fast CFD (Dissertation Th. Schaefer, Univ. of Bonn, 1987, based on a circuit suggestion by J. Pouthas and M. Engrand, NIM 161 (1979) 331) adding a 3rd comparator to perform pulse energy selection, N. Mart´ýnez, G. Renzi and A.F. Pasquevich (1988), private communication. [2] A. Bartos etal., Nucl. Instr. and Meth. A330, 132 (1993). [3] T. Butz etal., Nucl. Instr. and Meth. A284, 417 (1989). [4] Our first RC unit is a modification of those described in many PhD Thesis done before 1987 at the H-ISKP (Univ. of Bonn).