112 Nucl ear I nst rument s and Methods mPhysi cs Resear ch A252 ( 1986) 112- 118 Nor t h- Hol l and, Amst er dam A WATER CHERENKOV COUNTERSENSI TI VE TONONWAVESHI FTED ULTRAVI OLET CHERENKOV PHOTONS C .R . BOWER, R .M . HEI NZ and J . REYNOLDSON Physi cs Depart ment , I ndi ana Uni ver si t y, Bl oom i ngt on, IN 47405, USA S. MUFSON and J . PETRAKI S Ast r onomy Depart ment , I ndi ana Uni ver si t y, Bl oom i ngt on, IN 47405, USA S.P . AHLEN, J .J . BEATTY and A . TOMASCH Physi cs Depart ment , Bost on Uni ver si t y, Bost on, MA 02215, USA S. BARW I CK, G . GERBI ER, D . LOWDER, H .S. PARK, P.B. PRI CE and M .H . SALAMON Physi cs Depart ment , Uni ver si t y of Cal i f orni a, Ber kel ey, CA 94720, USA J . MUSSER and G . TARLÉ Physi cs Depart ment , Uni ver si t y of M i chi gan, Ann Ar bor , MI 48109, USA Recei ved 19 June 1986 We descr i be a pr ot ot ype wat er Cher enkov count er whi ch has been bui l t and t est ed with r el at i vi st i c cosm ic r ay muons . An anal ysi s of t he expect ed phot oel ect r on yi el d is descr i bed . The pr edi ct ed r esul t of 315±31 phot oel ect r ons is compar ed with t he experi ment al r esul t of 272±30 phot oel ect r ons . We f i nd t hat over 70% of t he Cher enkov phot ons det ect ed have wavel engt hs l ess t han 400 rim 1 . I nt r oduct i on The pr ot ot ype Cher enkov det ect or descr i bed her e is a smal l er versi on of a wat er Cher enkov count er whi ch is bei ng desi gned and bui l t f or a bal l oon- bor ne exper i - ment, PBAR . The pur pose of PBAR is to measur e t he f l ux of l ow ener gy (0.1 to 1 .5 GeV ki net i c ener gy) ant i pr ot ons in t he pri mary cosm ic r ays in or der to conf i r m an ear l i er ant i pr ot on measur ement [ 1] whi ch coul d be i nt er pr et ed as evi dence of supersymmet ri c par t i cl e anni hi l at i ons [ 2, 3] . The i nt egr at ed PBAR det ect or is a magnet i c spec- t romet er consi st i ng of t hr ee separ at e det ect or s : a sub- nanosecond t i me-of -f l i ght ( TOF) sci nt i l l at or syst em; a densel y packed, mul t i l ayer dr i f t t ube hodoscope l ocat ed in t he bor e of a 15 k G, t wo r i ng super conduct i ng magnet ; and a 15 cm t hi ck wat er Cher enkov count er in t he shape of a t r uncat ed pyr am id with an oct agonal base 1 macr oss . The hodoscope will gi ve t he r i gi di t y of a t r aver si ng par t i cl e and t he si gn of t he par t i cl e' s char ge . The TOF sci nt i l l at or s will gi ve t he magni t ude of t he par t i cl e' s char ge and t he par t i cl e' s speed, and will r ej ect upwar d goi ng ( al bedo) par t i cl es . The Cher enkov coun- 0168- 9002/ 86/ $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) t er will gi ve t he speed f or a par t i cl e movi ng f ast er t han 0 . 67c and t hr eshol d i nf ormat i on ( no si gnal ) f or par t i cl es movi ng sl ower t han t hi s val ue . The r edundancy of t he TOF and Cher enkov si gnal s will r educe t he er r or in t he vel oci t y measur ement when t he vel oci t y is gr eat er t han 0. 67c in addi t i on to addi ng convi nci ngness to t he r esul t s f or event s bel ow t hi s Cher enkov t hr eshol d . Water has been chosen f or t he Cher enkov r adi at or f or sever al r easons . The i ndex of r ef r act i on (1 . 34 at 400 ri m) pr ovi des a good Cher enkov t hr eshol d f or par t i cl es whose vel oci t y can be accur at el y det er m i ned by t he TOF det ect or , and t he Cher enkov si gnal f or par t i cl es movi ng above t hi s t hr eshol d ai ds in t he vel oci t y de- t er m i nat i on in t he r egi on wher e t he TOF vel oci t y r eso- l ut i on begi ns to degr ade . In addi t i on, wat er has a l ong l i ght at t enuat i on l engt h, does not sci nt i l l at e, is c hem- i cal l y i ner t , nonvol at i l e, and r eadi l y avai l abl e . Al so, it is unaf f ect ed by t emper at ur e changes under t ypi cal det ec- t or oper at i ng condi t i ons . Last l y, t her e exi st s a vast l i t er at ur e quant i f yi ng t he physi cal pr oper t i es of wat er . For t he PBAR experi ment we r equi r e a Cher enkov count er with a hi gh degr ee of spat i al uni f or m ity of r esponse ; t her ef or e, we consi der ed pl aci ng t he wat er