r^^us5;NV= or SLO* NEITTRCNS WITH CYLINDRICAL ZONE PLATES
A.G. Klein, F.D. Kearney, and C.I. 0:nt
School OJ. Physics, University of Melbourne,
Victoria 3052, Australia
and
R. Cahler
Institut Laue-Langcvin, 3S042 Grenoble, France
A3STRACT
he report on the successful focussing of a slov neutron beam by neans of a cylindrical Fresnel tone plate. IT. 2n earlier paper [l] we reported cr. th.- successful fccussing and imr ging of slow neutron- usir.g z phase-shifting Fresncl zone plate t2j. In applications where the source is in the fom of a slit, much higher bca= intensities may be obtained by using a cylindrical focussing structure. Ke now report on the construction and tejtir.- of a cylindrical zone plate with a 2.5 m focal length for neutrer.s of 20 A wavelength.
Our ;one plates were produced by a photc-lithographic technique of the type used in the manufacture of r.icrc—electrr r.ic components. A computer drawn zone plate of the tyre shew- :r.
Fig. 1 was reduced photographically by a factor of 20 tc product an intermediate patter*. This was then nsurted ir z 1C to 1 reduction "step-repeat" camera and carefully sligr.od so that contiguous exposures resulted in one lor..; hu-iir structure. A thin layer (*\. U P»>) of copper was evtr.vrated onto a silicon substrate, previously flashed with chrosiin to enhance adhesion, to provide an electrically conductive rath.
The substrate was then coated with a positive phrtcTesist
(Shipley Kl 1350 J) and exposed to UV light threes'- the photographic cask. After dissolving away the erased photopolymer, a layer of copper was electrolyticaliy deposited onto the exposed surface to the thickness recuirsc tc £ive a
180° phase shift to neutrons traversing it. For a material with N scattering centres per unit volunc, each with a scattering length b, the thickness required to jive a pha D(2jAJ = W(n-l) where the refractive index n is given by A2Nb n = 1 - . For example, for copper at /. = 20 A wc have D(V.j = 2.4 urn . The final zone plates produced in this way had the foilcwir.r, characteristics: width d = 2 ax, length 1-35 am, nurcber of zones !, = 200, focal length f(n) = 50/A(S) = 2.5 m at ). - 20 A, width oi outermost zone e - 2.5 ura, F-number =1,25 The experimental tests of our zone plates were done or, the lor.j? wavelength neutron beare, HIS, at the Hijjh Flux Research Reactor of tiie Institut Lauc-I.an^cvin in Grenoble. '!!'.•• main feature;, of the apparatus l3j ar<- S'-;JW:I i:> i • j;. 2. The ,?onoc!'.i'c~,ato: , consisting of a 1 2o quartz prism with entrat"..c and exit slits, was used to select a 1 A wide wavelength sprr.'id centred on 20 i\. The Bonoclironiator cxi' slit, sot tc SO -.n. vv-.dr;;, served as the object to be jisaged by the zone o. plate at 1:1 magnification, that is, the object ar.c! inacr distances were both 5 m. The iraage was scanned in 1C u' steps by a 30 pas vide slit, placed in front of a shielded EP^ proportional counter. The measured intensity distribution in the inage plane is showi in Fig. 3. The graph is plotted so that the horizontal axis covers the region of the geonetric shadow oi the lens. The resolution licit of the zone plate for monochromatic neutrons is given by fA/d = 2.5 un . The effects of chromatic aberration result in a smearing of the image so that, for a 1 A wide wavelength spread, the radius of the circle of least confusion is approximately 25 jsn. Thi* would then give a FWIM = 28 urn. Combining this with the resolution limits set by the finite slit sizes we get FWHM * /c28)2+(50)2*(30)2 * 65 urn i-ompared with the oeasured value of 68 jin. Measurements of the efficiency of focussing were not carried out, but the results of previous tests on circular zone plates showed it to be close to the theoretical value. .\it.".C'U.<;h the effective aperture c:" s-eh s lens is nther S!r;.li, as seer, from its rtiati\sly iar»;e F-ni^nber, it nay, nevertheless, find application ir. neutron optical experiments. An exasple of such ar. application is the Billet split-lens neutron interferometer *h:ch will be reported separately iAj • Other possible arplications a.:y be envisaged, r.ariely as a condenser-mennchroTuatcr combination in which a cylindrical zone plate is used in cor:;ur.ction with a defining slit ir. the focal piano, correspond: r.c tc the wavelength to be SLitctcu. I-'urtrierrore, by :;aKinr, the phsse-shifrinc, material ffrrc".i^;ict.' •:, different focal length: r.?.y be obtained for different states of polarization if tr.c material is unifonr.lv r.^r.ctiscc. TIius, a pol ari;er-cc:iti«ns}ng ITS combination cay be achieved. As siiown elsewhere [5], the effective aperture cf a :c:vc plate lens is determined by the width of the outcrnost :or.e, d, according to the relation F-nurrjer = 6/A. In our present device, J is limited to 2.5 \im by th? :-.•..:•.-iitho/.r:;;':••*.•: proce;..".. 0:1K; rv:\ i-.:va:i;"t :! rrecesses cjr'-.-ntl;. *".-ir:.'. developed for the r.i :r: -electronics inJ:istry -i.. a]]-/* t!.'. reduction of t.;u.- b> a.- r..c'v a.« r.u ;;rdi r cf ::,aAr. ti.d'j . Such advances wvjj i J-jac t;< the er.har.ccj aj plicabiii ty f :i:;:i- ' IMC. to neutron; cf i'irr'.c: •::.".'. e n--11 h . J. Wt wish to thank the directors u°..: staff ~f ::._• Ir.stitut Laue-Lanjjevm for their cooperation •".: j-asrita'. :t_- re are grateful to the Australian Telecom Research La'rcrit^r: and the Royal Melbourne Institute of Technology for allc-.--. to use their photo-lithographiC facilities. This vor» »•.- supported by the Australian Research Grant* CeT_-ittee. [1] Kfsrncy, P.D., Klein, A.G.. Qp-st, G.I., and Gahler, P.. Nature 287, 313 JlSSOj. [2] Kir;, J., J.O.S.A. fc£, 3CI (1974). LJJ Gahler, R., Kalus, J. and Macpe, K. J. Phys. E33, 546 (1950; . [4] Klein, A.G., Kearney. P.O., Opat, G.I., Cinsnino, A. and C.-Jiier, R. (to be published; . L'-'j ••';ein, A.G. and Op;tt, Z.l. ir. "Neutron Interferon try", L. Honse and K. fcauch (F.ds) O.U.!'. (1379), p. 97. HSL'RE CAPTIONS Fig. I Cylindrical zone plate pattern. Fi,.;. 2 Schesatic layout of neutron focussing experixent. Fit;. 3 Neutron intensity in the image plane. The full width of the horizontal scale represents the projected boundary of the lens. R , v, l,:>^U'::1;;:ii;.i;iil!iiii!II!l >!iiiiii|;il!!if w&$s»fOr&~-s,x ; i!!j¥«i"iii • • ..-'. " ! (•I..-- ,:J Hi. 'Hi .,a.j c. *-... 2 o (LI «-» Of o CO CD £ o in flU ai rvi a. « E o a E .9 o c_ 1 JZ -i u * O c o 5: 4000 3000- c O <-68 pm 2000 1000- r -r——, ,,..—., ,——,—•••»"•••' T-—r~-• -r T 1 ( T p r "~i r—i 0 2 3 SI i I" Position (mm)