Nuclear Instruments and Methods in Physics Research A 461 (2001) 413–415 Measurement of photoelectron yield from scintillating fibres coupled to a YAP:Ce matrix N. Belcari a , C. Damiani a, *, A. Del Guerra a , G. Di Domenico b , A. Motta b , N. Sabba b , L. Sartori a , A. Vaiano a , G. Zavattini b a Dipartimento di Fisica, Universit ! a di Pisa and INFN, Sezione di Pisa, I-56100 Pisa, Italy b Dipartimento di Fisica, Universit ! a di Ferrara and INFN, Sezione di Ferrara, via Paradiso 12, I-44100 Ferrara, Italy Abstract In applications where high spatial resolutions in the determination of the position of an interaction are necessary crystal matrices are often used. The readout of such matrices can be difficult especially if these have a large number of pixels (>400). One possible way to solve the difficulty is by reading out the row and the column by coupling wavelength shifting fibres to the front and back of the matrix. Measurements have been carried out on the photoelectron yield of one fibre coupled to a 5  5 YAP:Ce matrix so as to prove the principle. The fibre was read out using a single pixel HPD. The results of these measurements will be presented. # 2001 Elsevier Science B.V. All rights reserved. PACS: 42.62.Be; 29.40.Mc; 85.60.Gi Keywords: PET; Position measurement; YAP:CE; Scintillating fibres; Light yield We will present in this work a set of preliminary studies for a PET scanner prototype. In PET applications, scintillator crystal matrices with a large number of crystals and a large area are often the best solution to increase spatial resolution and field-of-view. We investigated the possibility to readout a large number of pixels (> 400) and a large area crystal with a reduced number of channels. The basic idea is illustrated in Fig. 1: a YAP:Ce crystal matrix made of 2  2  30 mm 3 crystals is coupled to wavelength shifting (WLS) 2  2 mm 2 square section fibres in such a way that each matrix row and each matrix column is read by a single fibre. The WLS fibres are then read and recognized by a multi-pixel hybrid photodiode (HPD). We chose to use HPDs because of their single photon counting capability and their fast response [1,2]. We studied the light yield of the system in detail. Each YAP:Ce crystal is optically insulated from adjacient crystals by a 5 mm thick reflective layer so that the light escaping from each end of the crystal is about 7% of the total light emitted if air is the coupling material between YAP and fibre 1 . The light trapped by the index of refraction difference between air and fibre 2 is about 18:5%. YAP produces 17 000 photons/MeV [3], so a first rough approximation of the *Corresponding author. Dipartimento di Fisica, Universit ! a di Pisa, Piazza Torricelli 2, I-56100 Pisa, Italy. E-mail address: damiani@fe.infn.it (C. Damiani). 1 The fraction of escaping light can be simply calculated as the fraction of solid angle defined by the limit angle between YAP (n YAP ¼ 1:94) and air. 2 n core ¼ 1:59; n cladding ¼ 1:49. The calculation is for a round fiber. 0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0168-9002(00)01261-4 SECTION VII.