392 Nucl ear I nst rument s and Methods in Physi cs Resear ch A263 ( 1988) 392- 396 Nor t h- Hol l and, Amst er dam PERFORMANCE OF A BaF 2 CALORI METER W I TH PHOTODI ODE READOUT AT ENERGI ES BETWEEN 60 MeV AND7 GeV J. GI EHL, D . HAUFF and J .M . HENNEBERG Max- Pl anck- I nst i t ut f ur Chem i e, Abt . Ker nphysi k, Mai nz, FRG E . LORENZ and G . MAGERAS Max- Pl anck- I nst i t ut f ur Physi k and Ast r ophysi k, Muni ch, FRG H . VOGEL Car negi e Mel l on Uni ver si t y, Pi t t sbur gh, PA, USA Recei ved 26 May 1987 We r epor t t est r esul t s of a smal l BaF 2 cal ori met er usi ng i nci dent beam ener gi es bet ween 60 MeV and 7 GeV. A r eadout s c heme consi st i ng of a combi nat i on of wavel engt h shi f t er pl at e and si l i con phot odi ode is empl oyed . Measurement s of t he l i near i t y, ener gy r esol ut i on f or el ect romagnet i c shower s, el ect r on/ hadr on separ at i on and time r esol ut i on ar e pr esent ed. 1 . I nt r oduct i on The use of bar i um f l uor i de ( BaF 2 ) cr yst al s in sci nt i l - l at i on det ect or s has at t r act ed consi der abl e i nt er est in r ecent year s [ 1] . At t ent i on has f ocussed on t he pr esence of a f ast , sub- nanosecond decay component at 220 nm [ 2] , and on i ndi cat i ons [ 3] t hat t he r adi at i on r esi st ance of BaF 2 compar es f avor abl y with t hat of ot her wi del y used i nor gani c sci nt i l l at or s . In a r ecent publ i cat i on [ 4] we r epor t ed on t he per f or - manc e of a smal l BaF 2 cal ori met er in t he ener gy r ange 2 to 40 GeV. A novel r eadout s c heme was used, consi st i ng of a f l uor escent f l ux concent r at or ( FFC) coupl ed to a si l i con phot odi ode ( PD) . The advant ages of PDs over phot omul t i pl i ers ar e t hat t hey wor k in magnet i c f i el ds, ar e ver y compact and exhi bi t no dr i f t . Thei r mai n lim i t at i ons ar e t he smal l ar ea and r el at i vel y hi gh noi se . FFCs [ 5] , or wavel engt h shi f t er pl at es, i mprove t he si gnal - t o- noi se r at i o by concent r at i ng t he l i ght f l ux ont o a smal l vi ewi ng ar ea . A f ur t her benef i t is t hat t he l i ght is shi f t ed to l onger wavel engt hs, wher e t he PDs ar e mor e ef f i ci ent . In t hi s r epor t we ext end t he i nvest i gat i on of our BaF 2 cal ori met er to l ower i nci dent beam ener gi es . 2 . Experi ment al set up The t est s wer e per f or med in t he T7 beam of t he CERN pr ot on synchr ot r on ( PS) , and t he t agged phot on beam of t he Mai nz M i cr ot r on (MAMI ), t he l at t er oper - at ed j oi nt l y by Johannes- Gut enber g- Uni ver si t at and Max - Pl anck- I nst i t ut f ur Chem ie . 0168- 9002/ 88/ $03 . 50 © El sevi er Sci ence Publ i sher s B.V. ( Nor t h- Hol l and Physi cs Publ i shi ng Di vi si on) The cal ori met er set up f or t he CERN PS was t he s ame as descr i bed in our pr evi ous r epor t [ 4] , whi ch we br i ef l y out l i ne her e . The cal ori met er consi st ed of f i ve cyl i nder s of BaF 2 cr yst al s [ 6] , t he first 10 cmin di amet er and 10 cml ong, t he remai ni ng cyl i nder s each 13 .5 cmin di amet er and 7 .5 cm l ong, f or a t ot al of 19 .5 r adi at i on l engt hs. Each cyl i nder was r ead out by a combi nat i on of FFC and PD, f ol l owed by char ge sensi t i ve preampl i f i er, shapi ng ampl i f i er (wi t h 1 As shapi ng time const ant ) , and peak- sensi ng CAMAC anal og- t o- di gi t al conver t er ( ADC) . The di gi t i zed si gnal s wer e r ecor ded on magnet i c t ape . The f i ve si gnal s f r om t he preampl i f i ers wer e al so passed t hr ough a s umm i ng ampl i f i er and shapi ng ampl i f i er, t hen i nt o a LeCr oy qVt mul t i channel anal yzer ( MCA) . Dur i ng s ome r uns t he time devel opment of t he s ummed si gnal was r ecor ded by means of a LeCr oy 2261 pul se shape r ecor der ( PSR) , f or t he pur pose of time r esol ut i on and beam pi l eup st udi es. Two sci nt i l l a- t i on count er s in f r ont of t he cal ori met er gener at ed a gat e si gnal , whi l e a vet o count er with a2 cm di amet er hol e def i ned t he beam ar ea. Two Cher enkov count er s wer e used f or sel ect i ng el ect r ons or pi ons . The t ypi cal beam f l ux was 2500 par t i cl es per 500 ms bur st , with an r ms moment um spr ead t , p/ p of l ess t han 1%. El ect r on cont ent of t he beam var i ed f r om 90% at 1 GeV to 10% at 7 GeV. For t he MAMI beam t est t he cal ori met er was r e- duced to f our BaF 2 cyl i nder s, each 13 .5 cmin di amet er by 7 .5 cm l ong, and t he shapi ng time const ant was i ncr eased to 2 As . Separ at e t est s wer e made in whi ch one and t hr ee phot odi odes wer e at t ached to each FFC, in or der to compar e t he noi se l evel s . The phot on beam