Physica 13 17f'~ (1992) 78 92 North-Holland PHYSICA On neutron interferometer partial absorption experiments P.N. Kaloyerou and H.R. Brown Utm'ersily ~t' O.tjbrd, Suh-Facul(r ~f Philo,~ol~hy, I~ Merton 3,1., ().t#,tU (LVI 4,1.1, ~,K Rcccivcd 11 March 1991 Rcviscd nlanuscripl icceivcd I Jul\ 1991 In intriguing ueulrt)n intcrfcronlelry cxpcriincnts ~,uullnhanlinci. t~,aueh and luppillgci l~*tilitt ih~il the iutcrlcrci/cc amplitude depends Oll the method used to alicnuale line of thc ll~_.tltlOl] I~C{IlIIk, uxplainil]g the dill:ere'nee in results as duc Io lhc diffcrcnl path information aftorded by the dilfercnl atlcllualors, lnslcad, w¢ sugg¢~,l Ihal Ihc diffeicnccs should hc underslood ill terms of the differcnl mtcrfercncc duc to diffcl-cnl \,~,a~c prolilcs. I. Introduction It is well known that path determination in ;.i typical two-slit cxperiment results in the loss of interference. Woottcrs and Zurck [1] had the interesting idea of investigating "intermediate" cxperimental conligurations where the path is only partially determined, arriving at thc surpris- ing result that a significant interference pattern is retamed even when the path is determined with 9c)c~7 certainly. The results of such experiments were explained by relating the amplitude of in- terfcrence to the degree of path information obtained from a given experimental configura- tion. Actual photon experiments of this inter- mediate type followed I2]. The application to neutrons was suggcsted by Greenberger [3], and Zeilinger [4] following an idea by Wheeler [5] of using partial absorbers :is a means of path de- termination. In this article we shall be concerned with an mtriguing series of neutron interferometry ex- perimcnts carried out by Rauch and his group [6-8], in which wtrious types of absorbcr were used by attenuate one of the neutron beams, allowing partial path determination. In the first experiments [6], Rauch and Suture- hammer (RS) used :111 absorbing block and a chopper (a rotating toothed wheel) as at- tenuators. They found, surprisingly, thai for ~l ~ivcn allenualiol] (i.e. when thc same number ot neutrons is removed from the attenuated he;All]), the absorber experiment gives rise to a grealcr amplitude of intcrfcrcncc than does ihc chopper experiment. Spccilically, RS found that the ab- sorbing block is charactcriscd by a square root rcl~.Hion between the bc~.llll attenuali(m and the interference aniplitudc, whilst the chopper is cl]aracieriscd by a direct i~roportionality ielation bolas,con attenuation and interfercncc amplitude. li is an interesting question how the diffcrencc in results is it> bc cxplaincd. RS suggested :i possible explanation in terms of the path mfot +- mat:on afforded by the diffcrcnt absorbcrs. In :t later article, Summhammcr, Rauch and l'ul+- pinger (SRT) [7] introduced the notions of sto- chastic and deterministic absorption, relating the degree of path information with the type ~+1 absorber. In this vicw the absorbing block is regardcd :is a slochaslic absorber, whilst the chopper is regarded as a deterministic absorber RS [{~1 has already raised thc interesting pos- sibility lhat increasing the chopper fl-cquencv might lead to a loss of path information, thcrcbv giving rise io ;~111 increasing iillerfercncc :lm- pliludc with increasing chopper lOtalion fi-c'- quency. More generally I71 SRr asked \vhcthcr it would be possible to obtain relations between 0921-452~'i/92/$(15.0{I ~) 1992 Hsevicr Science Publishers B.\,. All rights rcscrxcd