YAP Imager, an Area Detector for High-Energy X-ray Photons High -energy X-ray beams are outstanding scientific resources of third-generation synchrotron radiation facilities. They provide a powerf ul m eans of investigating the internal structures of thick or high-Z samples, local atomic structures of non-crystalline materials, and substance s containing heavy elements. However, the detection of high-energy X-ray photons is not straightforward because they penetrate through and scatter in all detection media. High-density inorganic crystals with constituents of high-Z values are, therefore, widely used. With a single body CsI(Tl) scintillator attached to a diffractometer, the structure factor of a given sample can be measured by a slit scan method, which however, is highly time-consuming. To be more efficient, an advanced area detector is needed, which should be fast enough to resolve the individual high -energy X-ray photons arriving for background rejection. In this respect, cerium-doped y yttrium aluminum p perovskite (YAlO 3 :Ce, often abbreviated YAP(Ce)) is a promising scintillator, because its decay time ( ～ 28 nsec) is ten times faster than that of a NaI(Tl) scintillator, however, its light yield is 40% t hat of a NaI(Tl) scintillator. Its density (5.35 g/cm 3 ) is as high as that of germanium, and the wavelength of maximum emission (360 nm) is sufficiently blue to illuminate wavelength-shifting fibers as explained below [1]. By assembling 16384 elements of YAP(Ce) crystals, a 2D matrix of [128 128] has been fabricated at the SPring-8 facility as a detection media of an advanced high-energy X-ray area detector, called “YAP imager,” (Fig. 1) [2,3]. Each crystal element is 1 mm 1 mm 6 mm. There are 128 wavelength-shifting Fig. 1. Diagram explaining 2D YAP crystal assembly and wavelength-shifting fibers attached. fibe rs attached to the 128 columns (x-coordinate) on the top surface of the 2D array, and another 128 wavelength-shifting fibers attached to the 128 rows (y- coordinate) on the bottom surface. The 128 fibers of both coordinates are bundled into 8 groups, and are guided to their own photomultipliers. The photomultiplier has sixteen segmented anodes that position-sensitize its photocathode into 4 × 4 regions. The sixteen fibers in each group are attached to their own assigned regions. When high-energy X-ray photons are absorbed in a YAP(Ce) crystal element, scintillation photons are induced (see Fig. 2). These scintillation photons emitted at an acceptance angle enter the corresponding wavelength-shifting fibers on both coordinates. By entering the fibers, the scintillation photons are absorbed and reemitted into the wavelength region from 390 nm to 450 nm. The red-shifted scintillation photons in each coordinate will travel through the fiber, and reach one of the segmented anodes. The arrival location of the incident X-ray photon on the 2D YAP crystal matrix can be uniquely identified by detecting the signals induced on segmented anodes on both coordinates. The scintillation pattern induced on the 2D crystal matrix is thus projected both coordinates. Due to the finite acceptance angle and wavelength-shift efficiency, only a few scintillation photons will reach the photomultipliers, so that the YAP imager does not give energy information of the Fig. 2. Photograph of YAP Imager fabricated at the SPring-8 facility. Wavelength-shifting fibe g g r Wavelength-shifting fiber 9 9 9 9 9 9 9 9 9 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 119 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 1 1 1 References [1] S. Baccaro et al. : Nu c l . Inst r um . Meth . A 361 ( 1 995) 209 . [ 2 ] A . G or in et al. : Nu c l . Inst r um . Meth . A 510 (2003) 76 . [ 3 ] K . Hi ro t a , H . T o y o k a w a , M . S uzuki, T . Kud o , M . N o m ac hi, Y . S ug a y a , M . Y o s o i, A . G or in, I . M a nuil o v, A . Ri a z a ntsev, a nd K . Ku ro d a : Nu c l . Inst r um . Meth . A 510 (2003) 83 . [ 4 ] K . Hi ro t a et al. : Nu c l . Inst r um . Meth . A 513 (2003) 1 87 . K a tsuy a Hi ro t a , Hiden or i T o y o k a w a a nd M a s a y o S uzuki S P r ing-8 / JA S RI E-m a il: t o y o k a w a @sp r ing8 .or. jp Fig. 3. A series of Zn diffraction patterns visualized with the YAP imager while Zn was undergoing the phase transition at approximately 438.6 C. in co ming X- ra y ph o t o ns . At the co st o f l o sing the ene r gy inf or m a ti o n, h o weve r , the YAP im a ge r immedi a tely digitize s the arr iv a l l oca ti o ns o f in co ming X- ra y ph o t o ns o n its 2D cr yst a l m a t r ix with a sp a ti a l r es o luti o n o f 1 mm 1 mm . This e ar ly digitiz a ti o n a ll o ws the YAP im a ge r t o be o pe ra ted a t a high co unting ra te with the dedi ca ted r e a d o ut system des cr ibed bel o w . The X- ra y a ttenu a ti o n length o f a YAP cr yst a l be co mes equ a l t o the thi c kness o f the p r esent 2D YAP cr yst a l m a t r ix a t a pp ro xim a tely 1 20 keV, whe r e the m a t r ix a bs or bs 63% o f in co ming X- ra y ph o t o ns . Alth o ugh a bs or pti o n effi c ien c y in cr e a ses a s the X- ra y ph o t o n ene r gy de cr e a ses, the numbe r o f s c intill a ti o n ph o t o ns gene ra ted in the cr yst a l elements de cr e a ses a t the s a me time, m a king the o ve ra ll dete c ti o n effi c ien c y f ro m 60 keV up t o 1 20 keV t o be ne ar ly co nst a nt a t a pp ro xim a tely 60% . The p o siti o n dependen c e o f o ve ra ll dete c ti o n effi c ien c y w a s o bse r ved o ve r the dete c ti o n ar e a , p r esum a bly due t o the st r u c tu ra l i rr egul ar ities r em a ining in the cr yst a l m a t r ix, the w a velength-shifting fibe r s, a nd the multi a n o de ph o t o multiplie r s . Fl oo d field im a ges o bt a ined by dete c ting tungsten X- ra y flu or es c en c e ar e a n a lyzed t o gethe r with expe r iment a l d a t a t o elimin a te this n o n-unif or mity . In the p r esent d a t a ac quisiti o n system, the 1 28 segmented a n o de sign a ls o n b o th coor din a tes ar e degene ra ted int o 1 6 unified sign a ls by co mbining a ll the n -th segmented a n o de sign a ls int o a n n -th single sign a l . Als o eight dyn o de sign a ls o f the ph o t o multiplie r s f or b o th coor din a tes ar e sent t o the d a t a ac quisiti o n system . The dyn o de sign a ls identify whi c h ph o t o multiplie r is fi r ed, a nd the degene ra ted a n o de sign a ls whi c h segmented a n o de is fi r ed, thus p ro viding suffi c ient inf or m a ti o n t o dete r mine the arr iv a l l oca ti o n o f X - ra y ph o t o ns . In this wi r ing s c heme, the t o t a l numbe r o f sign a ls t o be p roc essed is d ra sti ca lly r edu c ed t o f or ty eight, whi c h is less th a n 0 . 3% o f the t o t a l numbe r o f cr yst a l elements a ssembled . T o effi c iently en co de degene ra ted a n o de sign a ls a nd dyn o de sign a ls int o p o siti o n inf or m a ti o n, a high-speed l o gi c p roc essing m o dule, ca lled the “p o siti o n en co de r ,” h a s been devel o ped a s the c ent ra l m o dule in the d a t a ac quisiti o n system f or the YAP im a ge r [ 4 ]. One o f the imp or t a nt a ppli ca ti o ns o f the YAP im a ge r is the in situ o bse r v a ti o n o f tw o -dimensi o n a l diff rac ti o n p a tte r ns f or med by a s a mple unde r g o ing ph a se t ra nsiti o n . A Zn p o wde r s a mple in a gl a ssy car b o n c ell w a s pl ac ed in a n ele c t r i c fu r n ac e, a nd w a s p o siti o ned o n the diff rac t o mete r o f the high-ene r gy X- ra y diff rac ti o n be a mline BL04B1 . B y using the 11 3 . 4 keV m o n oc h ro m a ti c X- ra y be a m, a se r ies o f Zn diff rac ti o n p a tte r ns we r e o bse r ved while Zn w a s c h a nging its ph a se f ro m cr yst a l t o liquid or f ro m liquid t o cr yst a l a t a pp ro xim a tely 438 . 6 C (Fig . 3) . B e ca use o f the ex c ellent dete c ti o n effi c ien c y o f the YAP im a ge r system, o nly 60 se c o f the exp o su r e time w a s suffi c ient f or the p r esent ca se . The YAP im a ge r is n o w r e a dy f or pe r f or ming v ar i o us types o f time- r es o lved im a ging a t a high-ene r gy X- ra y r egi o n bey o nd 1 00 keV . (436.0 C) (436.7 C) (438.6 C) (439.3 C) (439.9 C) (440.5 C) 120
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