Phy51c5Letter5 8 302 ( 1993 ) 390-395 N0rth-H011and PHY51C5 LE77ER5 8 Ev1dence f0r 10w-1y1n9pr01ate 6and51n 188p6 and 186p6 J. Hee5e a, K.H. Ma1er a, H. 6rawe a, J. 6re6052 6, H. K1u9e a, W. Mec2yn5k16, M. 5chramm a, R. 5chu6art a, K. 5p0hr a and J. 5tyc2en h a Hahn-Me1tner-1n5t1tut 8er11n 6m6H, 8er11n, F R 6 6 Henryk N1ew0dn1c2an5k1 1n5t1tute 0 f Nuc1ear Phy51c5, Crac0w, P01and Rece1ved 22 Decem6er 1992; rev15ed manu5cr1pt rece1ved 16 Fe6ruary 1993 Yra5t 5tate5 0fthe nuc1e1 188p6 and 186p6 10cated 1n the m1dd1e 0fthe 82 < N < 126 5he11have 6een 5tud1ed f0r the f1r5t t1me 1n the react10n5 156,1546d(36Ar, 4n) at 175 MeV. 6amma ray5 f0110w1n9 the heavy-10n fu510n-evap0rat10n react10n5 were detected 1n c01nc1dence w1th evap0rat10n re51due5. 80th nuc1e1 exh161t a 5tructure very d1fferent fr0m the heav1er even-even 1ead 150t0pe5 and 5h0w 10w-1y1n9, pr06a61y pr01ate r0tat10na1 6and5. 7he 0ccurrence 0f 1ntruder 5tate5 1n c105ed 5he11 nu- c1e1 cau51n9 5hape c0ex15tence a1ready at 10w 5p1n5 15 exper1menta11y we11 e5ta6115hed and pr0v1de5 an 1n- tere5t1n9 te5t1n9 9r0und f0r nuc1ear the0ry [ 1 ]. 1n the neutr0n def1c1ent even-even 1ead 150t0pe5 10w-1y1n9 0~- 5tate5 have 6een 065erved and a5cr16ed t0 5ha1- 10w 061ate m1n1ma 0f the p0tent1a1 ener9y 5urface5 (PE5) 0f the5e nuc1e1 ar151n9 fr0m the exc1tat10n 0f tw0-part1c1e-tw0-h01e pr0t0n 1ntruder 5tate5 [2-7]. 5uch 0~- 1eve15 are a5519ned a5 the f1r5t exc1ted 5tate5 1n 190-194p6 and have 6een 065erved 1n f1 0r ~ de- cay5 0fthe c0rre5p0nd1n9 81 and P0 150t0pe5 and (a, xn) react10n5 [ 3,6 ]. N0th1n9 ha5 6een exper1menta11y kn0wn 0n the 119hter P6 150t0pe5 where ca1cu1at10n5 [8] pred1ct a150 pr01ate PE5 m1n1ma. 1n the pre5ent w0rk, 5pec1a1 eff0rt wa5 made f0r the 5earch f0r c01- 1ect1ve 6and5 1n the5e very neutr0n def1c1ent P6 150- t0pe5. Pr0mpt 7-rad1at10n fr0m yra5t 5tate5 1n 188p6 and 156p6 wa5 5tud1ed 1n the react10n5 156•1546d(36Ar, 4n) at 175 MeV w1th 1.2 m9/cm 2 tar9et5. Acc0rd1n9 t0 ca1cu1at10n5 w1th the M0nte Car10 evap0rat10n c0de PACE [9 ] the part1a1 cr055 5ect10n51ead1n9 t0 188•186ph are 0pt1mum at th15 6eam ener9y w1th 0n1y weak p0pu- 1at10n 0fthe 189•187p6+ 3n and 187•185p6+ 5n channe15. 7he 36Ar 6eam wa5 pr0v1ded 6y the V1CK51 acce1er- at0r at the Hahn-Me1tner 1n5t1tute 1n 8er11n. 6amma ray5 were detected 1n the 051R15 5pectr0meter c0n- 515t1n9 0f 1 1 C0mpt0n 5uppre55ed 9erman1um detec- t0r5 and a 48 e1ement 860 y-mU1t1p11C1tyf11ter [10]. 1n 5UCh heavy-10n 1ndUCed fU510n-evap0rat10n reac- t10n51ead1n9 t0 very heavy nuc1e1 the d0m1nant 50urce 0fa h19h 7-6ack9r0und are unre501ved y-ray5 fr0m f15- 510n pr0duct5. 1n 0rder t0 5uppre55 th15 6ack9r0und, ),- ray5 were mea5ured 1n c01nc1dence w1th evap0rat10n re51due5 detected 1n a rec011 f11ter detect0r (RFD). 7he RFD c0n515t5 0f tw0 r1n95 0f 6 and 12 1nd1v1d- ua1 detect0r e1ement5 m0unted 73 cm fr0m the tar- 9et under f0rward an91e5 6etween 2.7 ° and 12.1 ° t0 the 6eam ax15. At th15 d15tance, evap0rat10n re51due5 pr0duced 1n the a60ve react10n are we11 5eparated 1n t1me 0f f119ht fr0m a11 0ther react10n pr0duct5. 51nce the RFD re4u1re5 0n1y a narr0w f0rward c0ne (200 mrad), 1t can 6e u5ed w1th a11 48 e1ement5 0f the 860 6a11. 1n each RFD e1ement, nuc1e1 h1tt1n9 a 2 /~m th1ck a1um1n15ed My1ar f011 pr0duce n 5ec0ndary e1ectr0n5 wh1ch are acce1erated 1n an e1ectr1c f1e1d up t0 20 keV and f0cu55ed 0nt0 a th1n (5-10 ~m) p1a5- t1c 5c1nt111at0r 1n wh1ch they pr0duce a 519na1 0f n × 20 keV. Parameter5 mea5ured are the t1me-0f-f119ht w1th re5pect t0 the rad10 fre4uency 0f the V1CK51 cy- c10tr0n and the re5p0nd1n9 detect0r e1ement that de- term1ne5 the rec011 an91e. Ve10c1ty and an91e 0f the nuc1e1 are mea5ured w1th accurac1e5 0f Av/v < 1% and ~e1 .... t/4~ = 6 × 10 -4 and a110w prec15e c0rrec- t10n5 0f the 7-ray D0pp1er 5h1ft5. Deta115 0f the new detect0r are de5cr16ed e15ewhere [ 1 1 ]. F19. 1 5h0w5 a t0ta1 pr0ject10n 0f the 77-5pectrum 390 0370-2693/93/$ 06.00 • 1993 E15ev1er5c1ence Pu6115her5 8.V. A11 r19ht5 re5erved.