PHYSICAL REVIE% C VOLUME 16, N UMBER 4 OCTO BE R 1977

Nuclear gamma rays from V levels populated by 200 MeV 7r+ inelastic scattering'

H. O. Funsten and %. J. KosslerDepartment of Physics, College of William and Mary, Wi)liamsburg, Virginia 23185

B. J. LiebDepartment of Physics, George Mason University, Fairfax, Virginia 22030

H. S. PlendlDepartment of Physics, Florida State University, Tallahassee, Florida 32306

C. E. StronachDepartment of Physics, Virginia State College, Petersburg, Virginia 23803

V. G. LindDepartment of Physics, Utah State University, Logan, Utah 84322

(Received 21 March 1977)

Nuclear y rays from 200-MeV m'+ inelastic scattering on "V were measured. Observed were "V levels at0.32, 0.93, 1.61, 1.81, and 2.70 MeV. y ray cross sections were similar for both 7r+. If the interactionwere direct with no distortion on a (1f»,)' proton configuration, the m cross sections would be 1/9 the m+

cross sections.

NUCLEAR HEACTIONS ~iV(&', 7(~'y); F„*r-200 MeV; detected p s; Ge(Li);measured 90' o for 320, 930, 1610, and 2700 keV "V levels.

I. INTRODUCTION

Because of the isospin character of the n'-N,

n(1232), resonance, 200-MeV pions may be a use-ful probe to study nuclear configurations. In par-ticular, inelastic nuclear scattering of pions atthis energy may be sensitive to the relative protonto neutron components of the nuclear level wavefunctions. In an extreme picture in which the pioninelastic scattering occurs in lowest order, e.g. ,single direct m-X scattering in the nucleus with nodistortion of the pion wave function, r' inelasticcross sections mould reflect the behavior of thefree &-N scattering, yieMing proton and neutronconfiguration information. For example, in anucleus whose excited levels are obtained from theground state by a recoupling of only a certaingroup of protons, inelastic n'/v cross sectionratios to these levels will simply be equal to theratio of free m'-proton cross sections —', , sincepion charge exchange scattering would not occur.Effects such as the apparently strong surface ab-sorption of 200-MeV pions on medium and heavynuclei, yielding diffraction-like angular distribu-tions, multiple scattering, etc. , appreciably modi-fy the above idealized picture.

A measurement of 200-MeV n' inelastic scatter-ing on "V was obtained at the SREL synchrocyclo-tron by examining nuclear deexcitation y raysfrom the target. "V mas chosen because of its

apparent simple structure, three valence protonsoutside a closed core. This nucleus has been stu-died extensively both theoretically and experimen-tally. -~ i~.&7 Low lying levels arise mainly fromrecoupling among the three valence protons. Theground state (& =—,

' ) and negative parity states atE,(MeV), &=0.32, 2; 0.93, —,'; 1.61, —"; 1.81, —,';and 2.70, —", are characterized as 90-95% (lf», )'proton configuration outside a closed "Ca core,the small remaining parts of these wave functionsbeing higher 1f, 2P proton configurations. An addi-tional ~ state at 2.41 MeV is considered as beinga (1f~, )'(1P@,) or a (1f~,)'(1f~) configuration. Mea-sured values of &(E2) obtained from y transitionstudies of the first group of levels can be explainedin terms of the above configuration [or a 100%%uo

(1f,~,)' proton configuration] and an effective protoncharge e~/& =1.8. All these states have the groundstate isospin T =-,'.

If pion excitation of the T =4 "Ca core is mini-mal, direct inelastic m scattering, to lowestorder, should therefore be 9 times stronger thanthe corresponding m scattering to these states.

II. EXPERIMENTAL METHOD

Due to the inherently poor energy resolution ofpion beams, inelastic pion scattering on "V wasobtained, as mentioned above, by observing & raysfrom decay of the "V excited levels. This method

16

1522 FUNSTEN, KOSSLER, LIES, PLENDL, STRONACH, AND LIND 16

ClC.

O

~ I.O

C

0

IO

I

0

I

l.55

0

OO

|i 60 I.65 l.70

E nergy ( MeV)

OI

I

l.75 l.80

FIG. 1. Portion of a beam-coincident p ray spectrumfor a & run. Transition assignments are labeled abovepeaks.

III. NEUTRON CONTAMINATION

Neutrons with energies of several MeV have arelatively large cross section, of the order ofseveral hundred millibarns, for inelastic scatter-

has been used previously' for examining pion reac-tions on nuclei. In brief, y rays were detected bya 105-cm Ge(Li} in coincidence with a pion inci-dent upon the target. The incident pion beam wasdefined by a tmo counter telescope, consisting of anupstream 20- &25- &0.3-cm scintillator and adownstream 13- &13- &0.2-cm scintillator adja-cent to the target. The y detector was placed atan angle of 90' with respect to the incident pionbeam and surrounded by an anticoincidence scin-tiiiator to veto charged particles in the Ge(Li}.The distance between the target center and thecenter of the Ge(Li} sensitive volume was 10 cm.The m' energies were 180 and 215 MeV, respec-tively. Tmo "V targets were used in order toeliminate effects of neutron contamination, as ex-plained in Sec. III. A y spectrum noncoincidentwith the v beam was also taken to eliminate con-tributions from target P decay, room background,etc. A y ray with random timing with respect to anincident pion, the 2223.3-keV, &+P- d capture yray, produced a ratio of noncoincident-coincidentspectra peak area of -6/1. Peaks in the beam-coincident y spectrum were then identified indi-cating levels in a variety of residual nuclei pre-viously observed with a larger "V target. ' Figure1 shows a ~ spectrum section, 1.5 MeV-E~ «1.9MeV.

Using published y decay schemes and branchingratios for the residual nuclei, cross sections forpion excitation directly to these levels were deter-mined, assuming isotropic y ray distribution. t or-rections were made for y ray absorption in thetarget. No y transitions from other residual nu-clei mere observed to interfere with the inelasti-cally produced deexcitation y rays in "V.

ing on "V. This is an order of magnitude greaterthan that for 200-MeV pion inelastic scattering asdetermined by this experiment. Neutrons can thuscontribute a substantial contamination componentto the observed "V y rays. Those originating out-side of and inelastically scattering in the "V targetand telescope will be randomly timed with respectto an incident pion in the beam telescope and willyield contaminating y ray peaks predominantly inthe non-beam- coincident spectra. Neutrons origi-nating within the target, secondary target neu-trons, will yield inelastic peak components thatare beam eoineident, as are the components aris-ing from pion inelastic scattering. (The predomi-nant coincident neutron contaminating componentarises from secondary target neutrons rather thanneutrons originating within the downstream beamtelescope which had &5 the thickness, in g/cm' ofthe "V target. The upstream seintillator was -1m from the target. } However, the cross sectionfor the peak component due to secondary targetneutrons will vary with target size. In the case ofa thin target (thickness negligible compared withwidth or height) the secondary neutron componentwill increase linearly with target thickness. (Sucha shape yields maximum variation of secondaryneutron cross section component with thickness. )The residual y ray cross section, due only to inci-dent pions, is then obtained by linearly extrapola-ting to zero target thickness the measured coinci-dent y ray cross sections versus target thickness.Two 10-cm diam disk "V targets were used, thesmaller having 1.024-g/cm' thickness (1.6 mm},the larger having a thickness five times thisamount, 5.120 g/cm'.

As a further check, cross sections for six prom-inent spectra peaks arising from single and multi-ple nucleon emission were evaluated; see Table I.The negative Q values for the reactions producingthese peaks varied from 20 to &70 MeV. These aresufficiently large to limit contributions induced byneutrons which are expected to have an evaporationspectrum with mean energy of only several MeV.It was found that the cross sections for these peakshad little variation with target thickness (Table 1}and the peaks were prominent only in the beam-coincident spectra.

In view of the above qualitative results, crosssections for pion excitation of "V levels were eval-uated as follows. '

For both m' and m, beam-coincident and nonco-incident spectrum areas were evaluated for five"V transitions that yielded discernible peaks, ' seeTable I. Level and transition energies and branch-ing ratios were obtained from Ref. 7. Areas inthese spectra were also obtained for the sixnon-"V peaks, arising predominately from the

NUCLEAR GAMMA RAYS FROM 5'V LEVELS POPULATED BY.. . 1523

TABLE I. Cross sections for p transitions from V level for a ~' and & run sequence.Values have been rounded off to the nearest integer in mb. Cross section errors are + 30%.R and R' are the ratios of five disk f&&(5)) to one disk [0&(1)] transition cross sections.

R,R'&y (0)(mb)

0&(1)(mb) R,R'

&p (0)(mb)

"V 0 3200.9281.6091.8131.090

48Ti 0 9381.312

Ti 0.88938Ar 2.1684~Ca 1.1572Ca 1.524

147

1053

472626

82012

521531178

553029

82213

3.72.23.13.12.81.21.11.11.01.11.1

95

1044

382127

61312

291324119

452535

72215

3.12 ' 52.42.72.31.21.21.31.21.71.3

first excited state to ground state transitions inresidual even-even nuclei. Ratios for coincidentto noncoincident areas for the five "V transitionswere -—", and equaled, to within +15%, correspond-ing ratios for the six non-"V transitions. Peaksassociated with non-beam-coincident y rays, suchas the 2223.3-keV y+t) - d+y capture y ray,511-keV annihilation radiation, and the 2613-keVy ray from the calli. matar o Pb 3 to ground statetransition, had ratios less than 0.5.

Beam-coincident areas mere then used to obtaincross sections for the 1.024- and 5.120-g/cm'targets farmed, respectively, by one and five10-cm diam& 1.6-mm vanadium disks, positionedat 45' with respect to the incident pion beam.After correcting the cross sections for target yabsorption, ratios R and R', for five disk /onedisk cross sections were obtained for the six non-"V and five "V levels, respectively. The former,v:ith relatively good spectrum statistics, did notvary by more than +10@in most cases, and theiraverage A,„was formed. This, when divided intothe R' ratios, yielded ratios 8" normalized forvariation of target —Ge(Li) detector distance with

target size. Finally the five "V level cross sec-tions were linearly extrapol, ated to zero targetthickness by multiplying the one disk "V crosssection o(1) by the factor e(5-R").

The ratio of the number of secondary targetneutrons n„ to the number of incident pions n„-for one target disk can be obtained from the aboveand 18

ng 2o (1) I (Rll ] )n, - a~„. 'where the factor 2 accounts for secondary neutranscattering in —,

' the target, and o„~. is the -3-MeV

inelastic neutron scattering cross section to a "Vlevel. o (1) and R" values obtained for v to thefirst excited state of "V and o„„.to this state fromRef. 5 yielded n„/n;= 0.02 per disk.

A less reliable estimate of n„/n, - may be madeby utilizing the area a of a 600-keV triangularshaped spectrum peak caused by inelastic neutronscattering to the first excited state af "Ge withinthe Ge(Li) detector. ' (The internally converting700-keV "Ge 1 —0 triangular peak could not be usedsince its lifetime, 0.3 p, s, caused it to appearmostly in the non-beam-coincident spectra. ) Astraightforward evaluation of n„/n„- per disk usinga yields

n„8ma AG,eff xs . (n, ') 6 x N P . ol .) '

--0.01 per disk

where R is the target-detector distance, eff is theGe(Li) relative photopeak efficiency, oo, (n, n') isthe -2-MeV neutron inelastic cross section to Ge Istate (Ref. 9), and all other symbols are conven-tional. n„/n, + yield approximately half thesevalues.

IV. PROTON CONTAMINATION

The contribution of secondary target protons to"V y peaks is expecte. d to be small since, at aproton energy E~= 6 MeV yielding the maximumin the inelastic proton cross section, the protonrange in the target is only =0.1 g/cm', approxi-mately ~0 the minimum target thickness. Further-more, if the production of secondary target nucle-ons with energies &20 MeV is mainly by evapora-tion, the proton flux would be +~» the neutron flux.'

An estimation of secondary target proton eontam-

FUNSTEN, KOSSLER, LIEB, PLENDL, STB, ONACH, AND LIA D l6

ination was made in a manner similar to that de-scribed in Ref. 11. It used a recent measurement"of proton emission from 235-MeV m' on Ni, dis-playing exponentially decreasing yield with E~. An

excitation curve for inelastic proton cross sectionsto the first excited (2') states for "Fe and "Fe for3 MeV «E~ ~50 MeV" was formed and also ap-proximated exponentially. A double integration ofthe product of this fit and the Ni n' induced protonemission cross section over proton range and pro-ton emission energy yielded an inelastic crosssection to the first excited Fe states &0.1 mb. Thepath length of target secondary protons was setequal to the proton range and was not limited bytarget geometry. Proton contamination in the in-cident pion beam had an energy of -40 MeV and

was less than 5Q of the pion flux, as determinedby time of flight.

V. RESULTS

Table II lists values for m' cross sections ob-tained for the y rays from levels of "V extrapo-lated to zero target thickness and corrected forbranching and y feeding from observed higherlevels. Also shown for comparison are cross sec-tions for 17.5-MeV protons and 42-MeV aparti-cles." A search of the spectra for y decay of allother "V levels of E, ~ 3.5 MeV to the ground stateor excited states up through 1.61 MeV showed no

peaks within the limits of experimental uncertaintyof =0.6 mb extrapolated to zero target thickness.

Relative to each other, the cross sections forthese five levels follow the same general patternfor both m' and also for proton and e excitation.The 320-keV -', level has the largest cross section,with the 1610- and 930-keV le:els somewhat less,'each of the two other levels observed has a consid-erably smaller cross section. Since, as notedabove, shell model calculations of the inelasticcross sections for the five strongly excited levelsreasonably reproduce the proton and a data, thepion data indicates a reaction apparently proceed-ing similarly. However, the ratios of the m' to m

cross sections listed in Table II are approximatelyequal. Since these ratios should be -', on an extreme

TABLE II. Cross sections o' for the production of ")t'levels by 200-MeV m' inelastic scattering, obtained byaveraging o'& {0) over two run sequences. They have beencorrected for branching and p feeding from higher levelsand have errors + 30'i~. Also shown are inelastic protonand «cross sections for comparison.

E o+ o.

(MeV) J {mb) {mb) o+/odo'{p,P') '

dQdo{A, «') '

dQ

0.32

1,81

2.70

52

3

ir2

8

2i5

13 0.7

6 1.05 0.6

1.3

0.4

1.0

0.5

0.2

2.8

1.6

' Differential cross section in mb/sr at 0, ==-35 forprotons, 0,,~.=20' for «particles. Ep=--17.5 MeV, E~=42 MeV. Protons had similar angular distributionshapes v itii the maximum at 0, ,„,--35'. «particles alsohad similar shapes with the second maximunl at 0;. .=-20 . Data are from Ref. 14.

no-distortion direct-interaction basis as notedabove, the measured ratios are, on this basis, un-expectedly small hy a factor of -9.

Several effects may appreciably modify the abovesimple interaction basis, in particular multiplescat ter ing of the incident pion and appre c iable nu-

clear distortion of the pion wave function. Thishas been shown to produce a strong nuclear surfacereaction yielding diffraction-like inelastic angu.'ardistributions. " In connection with the latter, adistorted wave impulse approximation calculation"showed great sensitivity of the magnitude of thepion inelastic scattering cross section to smallcharges in the outer slope of the nuclear form fac-tor. An increase of -50/~ in the latter produced atenfold increase in the former. The presence ofthe closed (1f,i2) neutron shell could then be ini-portant. A tt'/m cross section ratio calculated by

simply assuming that all nucleons outside the "Caincoherently participate in the reaction yields

o (w')/o'(n' ) = (1 x 8 + 9 x 3)/(9 x 8 + 1 x 3) = 1/2. 2 .

*Work supported by the National Science Foundation,NASA, and HEW.

J. D. McCull~n, B. F. Bayman, and L. Zamick, Phys.Hev. 134, B515 (1964).

~H. N. Horoshko, D. Cline, and P. M. S. Lesser, Nucl.Phys. A149, 562 (1970),

3B. A. Brown e& 4., Phys. Bev. C 9, 1033 (1974).4A. S. Goodman and D. J. Donahue, Phys. Rev. C 5, 875

(1972).

'B. J. Lich e~ ~., Phys. Rev. C 14, 1515 (1976).A. W. Barrows e~ a/. , Nucl. Phys. A107, 164 (1968).

'C. Rossi-Alvarez and G. B. Vingiani, Nuovo Cimento17A, 731 (1973); J. N. Mo e~ a/. , Nucl. Phys. A147,129 (1970); A. S. Goodman and D. J. Donahue, Phys.Rev. C 5, 875 (1972); R. N. Horoskho e& 4., Nucl.Phys. A149, 562 (1970); G. L. Horchert e& A. , Z.Naturforsch. 30a, 274 (1975) ~

"C. Chasman et 4 ., Nucl. Instrum. Methods 37, 1 (1965).

N

UNCLEAR

GAMMA RA "I S FROM ''V LEVELS POPU LATED 8 Y. . . l525

~K. C. Chung eI'4. , Phys, Bev. C 2, 139 (1970).E. Vogt, Advan. Nucl. Phys. I, 261 (1968).

"H. Funsten eI' 4, (unpublished).P. G. Bizzeti ef' 4., Nuovo Cimento 26A, 25 (1975).

~F. D. Sev ard, Phys. Bev. 114. 514 (1959); K. Amorand B. Smith, Nucl. Phys. A226, 519 (1974); P. G.Greaves eI' 4., ibzd. A179, 1 (1972).

' B. J. Peterson, Ann. Phys. (N. Y.) 53, 40 (1969); Phys.Bev. 172, 1098 {1968}.

~~H. J. Weber {private communication).~67. H. Lee and F. Tabakin, Nucl. Phys. A226, 253

(1974).~~B. M. Preedom e& 4., Phys. Bev. C 2, 166 (1970).