Energy spectra of charged particles emitted following the absorption of stopped negative pions in calcium

Nuclear Physics A381(1982) 317-329 © North-Holland Publishing Company

ENERGY SPECTRA OF CHARGED PARTICLES EMITTED FOLLOWING THE ABSORPTION OF STOPPED NEGATIVE PIONS IN CALCIUM H. RANDOLL, H.I . AMOLS*, W . KLUGE, H. MATTHAY, A. MOLINE** and D . MÜNCHMEYER Kernforschungszentrum Karlsruhe, Institut für Kennphysik und Unioersitdt Karlsruhe, Institut für Experimentelle Kernphysik, Postfach 3650, 7500 Karlsruhe, Federal Republic of Germany Received 4 November 1981 Abstract: The energy spectra of charged particles (protons, deuterons, tritons, and 3 '4He nuclei) emitted following the absorption of stopped negative pions in a 0.022 g/cm2 thick target of calcium are measured from the experimental threshold energy at 2 MeV (for 3He nuclei : 8 MeV) up to the kinematical limit . The experiments are carried out at the pion channel vE3 of the Swiss Institute for Nuclear Research . The data are compared with existing measurements.

1. Introduction The investigation of pion absorption at rest in nuclei is done in order to understand better the mechanism of the absorption process and the subsequent interaction of the emitted particles with the nucleus. In addition to review papers 1-3) recent publications ") confirm the persisting interest in pion absorption at rest. In particular the comparison of experimental data, including our former 12C data') and our calcium data prior to their publication 1°) with a theoretical calculation carried out by Chiang and Hüfner 6), represents an interesting contribution to the understanding of the absorption mechanism. In the present paper we show the energy spectra for protons, deuterons, tritons and 3'4 He nuclei between 2 MeV and the maximum possible energies emitted following the absorption of vr- at rest . A calcium target of natural isotope composition (97ßßo 4OCa) with 0.022 g/cm2 thickness is used. Very few data 11-13 ) from the "pre-meson factory era" with rather low statistics and over limited regions of the total energy spectrum exist for calcium so far. 4°Ca, which is considerably heavier than the thoroughly studied 12C [refs. 4''1], is chosen, because possible different contributions of various reaction mechanisms to the ir- absorption process might be revealed by a comparison of the charged particle spectra from carbon and * On leave of absence from LAMPF, Los Alamos NL; present address Rhode Island Hospital, Eddy Street, Providence, RI 02902, USA . ** Also at Schweizerisches Institut filr Nuclearforschung, CH 5234 Villigen. Jone1982

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H. Randoll et al. / Energy spectra

calcium. It is an interesting question whether the high deuteron, triton and a-particle 12C or typical yields observed for 12C are due to the particular nuclear structure of for the absorption process and the subsequent interactions of the primarily emitted nucleons with the target nucleus. A precise investigation of the calcium spectra helps to clarify that problem. 2. Experimental methods 2.1 . EXPEI2IIMNTAL SET-UP

The experimental set-up is shown in fig. 1 . The pions stopped in the target are identified by a counter telescope. The telescope consists of five scintillation counters, where the detectors S1 to S4 define the incoming pion flux, while S5 is used in anticoincidence to reject pions which do not stop in the target . The carbon degrader (C) and S3, S4 are mounted in the vacuum chamber to get them as close to the target as possible . This minimizes -the effect of beam spreading in the degrader and optimizes the time resolution for time-of-flight measurements between S4 and the particle detectors S6 and S6'. However, particles emitted from S4 must be shielded from the particle detectors S6 and S6' by the aperture A. These detectors consist of a silicon surface barrier detector of 50 mm diameter and of 0.19 mm thickness (S6) and a total energy loss NSI(TI) crystal of the same diameter and 50 mm thickness (S6'). There is no inactive material in front of the NaI crystal which would cause energy losses. The silicon detector acts either as a dE/dx detector for high energetic particles or as a total energy loss detector for lower energies . The distance of about 1 m between the target and S6, S6' allows particle identification by time-of-flight measurements .

WINDOW

VACUUM CHAMBER

Fig. 1. Experimental set-up .

The targets used in this experiment are natural calcium foils of 100 :k 1 wm and 1.3 :1: 0.04 mm corresponding to 0.0219 and 0.285 g/cm2 , if the inclination of the

H. Randoll et al. / Energy spectra

31 9

targets relative to the beam direction of 45° is taken into account. The targets are fixed on 2 mm thick epoxy rings of 10 mm width with an inner open diameter of 180 mm. No measurable background is produced by this support. 2.2 . ELECTRONICS

The electronics enables the measurement of (i) the time-of-flight T6 and T6' of secondary particles between the detector S4 and the Si surface barrier detector and the NaI detector, respectively ; (ii) the energy loss within the silicon detector E6; (iii) the total energy E6' in the NaI detector ; (iv) the time of flight between the RF signal of the cyclotron and detector S4 in order to distinguish between incoming pions, muons and electrons. The large dynamic range of the energies to be measured with the NaI detector and the Si detector covering energies from 0.5 MeV up to 30 and more than 100 MeV, respectively, requires the energy signals from both detectors to be split and the use of two amplifiers with different gain for each signal (denoted in the following by E6, E6 x 3, E6', E6' x 10). All energy and time information is digitized by a CAMAC multiparameter data acquisition system interfaced to a PDP 11/50 computer and is recorded on magnetic tape. Only charged particles are accepted because an energy signal of the silicon detector is required. 2.3 . PARTICLE IDENTIFICATION

The evaluation of the multiparameter data is shown in fig. 2 schematically. In a first step pions are separated from muons and electrons. Then secondary particles stopping in the Si detector and those reaching the NaI detector are distinguished. Fo,r particles stopping in S6 only their energy and time of flight is measured. Protons, deuterons, mass-3 particles, and 414e are separated in two-dimensional plots of the parameter E6 x 3 versus T6 or E6 versus T6. Only tritons and 3He (mass-3 particles) below about 8 MeV are indistinguishable by time-of-flight measurements . For particles reaching the NaI detector the muon background is removed in the twodimensional plots E6 x 3 versus E6' X 10 and E6 x 3 versus E6'. The identification of charge one and two particles is done in the plots E6 versus E6' x 10 and E6 versus E6'. Now the total energy is calculated taking into account the different scintillation yields for charge one and two particles in NaI. For the final particle separation the total energy is plotted versus time of flight . For the "measured" single-particle spectra the results from particles stopped in S6 are added. 2.4 . NORMALIZATION

The number of secondary particles per energy interval must be normalized to the number of stopped pions. This is the counting rate 12345, which has to be

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Event (Tincamingparticle, T6,E6x3,E6,T6',E6'xlQE6')

E6x3

Particle goes through 56 T6

background -a E &x10

- i" EB E

E6

d,t,3He,`Fle

Ef Z=1, 2

t

Z=1 . 2

40.

T6

Calibration of all Energy-Signals E(6+6')'.---- (E6-E6x3)+(E6'vE6'x10)

All particles separated Single particle spectra Fig. 2 . Off-line-processing of charged particle spectra .

corrected for computer dead-time, pion decay between S4 and S5, pion absorption in S4, random coincidences within the time-of-flight window of 300 ns which was necessary for the detection of slow particles. The normalization factor is primarily determined for the thick calcium target . Subsequently, the normalization factor for the thin target is obtained by a best fit of the high energy tail of its proton spectrum

H. Randoll et aL / Energy spectra

321

to that of the thick target . That part of the spectrum is almost not influenced by the finite target thickness. Details of the experimental procedure are found in ref. 1~. 3. Experimental results and discussion The energy spectra of protons, deuterons, tritons, 3He and 4 He nuclei obtained for the thin (0.022 g/cm2) target of calcium are shown in fig. 3. The corrections for the finite target thickness, described in refs. 8'10'15) are applied to the "measured" spectra as obtained after the particle identification procedure discussed in subsect. 2 .3. The corrections for finite target thickness are done for both calcium targets of 0.022 g/CM2 (thin target) and of 0.285 g/CMZ (thick target) and yield acceptable agreement of both unfolded spectra'). The correction procedure follows the method outlined by Schlepütz et'aL 15) . The full lines in fig. 3 denote the particle yields as obtained after the application of the target thickness corrections. The error bars shown in fig. 3 represent the statistical errors only. In addition to these the error due to the particle separation is ±1% for single charged particles, t3% for 4 He and t10% for 3He. The error of the absolute normalization for the thick target is t7%, while the additional error for the thin target due to the fit to the proton data of the thick target is t3%, resulting in a total error of about t8% . The corrections for the finite target thickness are of the order of 15% in total for the low-energy part of the hydrogen spectra. Assuming an error of about 10% for this correction a negligible additional error to the yields is added. For the 4He spectrum the corrections are much larger in the low energy part of up to a factor of 2 below 20 MeV. Assuming again an error of about 10% (mainly due to the inconsistencies of energy-range tables) additional errors for the 4He yields of up to 20% are introduced for the differential yields (i.e. yields per energy interval). In fig. 4 the corrected particle spectra are plotted for comparison. The spectra show a monotonic decrease with energy in the preequilibrium domain and evaporation peaks for protons, deuterons and tritons of low energies are observed . The shape of the proton, deuteron and triton spectra is very similar, whereby the yields of deuterons and tritons are down by a factor of about 5 and 10, respectively, relative to the yields of protons. The shape of the 4He spectrum appears remarkably different from that of the hydrogen nuclei . Furthermore the appearance of protons, deuterons, tritons and a-particles of high energies almost up to their corresponding kinematical limit must be noticed. The low 3He yields compared to the triton yields as observed for the first time in the present investigation, waits for a theoretical interpretation . The particle yields and mean energies of the detected particles, corrected for target thickness, are compiled in table 1. The errors of the yields include the statistical error, the errors introduced by the particle identification and the absolute normalization. The errors of the mean energies include the errors of the unfolding procedure, of the particle identification and of the energy calibration. The charged

1010 . . . 120 ENERGY CMEV ]

Fig . 3 . Energy spectra measured with the 0 .022 g/cm2 thick calcium target. The full lines are obtained after correcting for particle and energy losses in the target.

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ó

10a

1

00

1

100 ENERGY (MEVI

1

120

C

1V

10 -6

1

90

1

100 ENERGY [MEVI

1

120

Fig . 3 .-(oontinued)

TABLE 1 Particle yields and mean energies Target 0 .1 mm

Particles detected p d t 3 He 4 H0

Energy interval (MeV)

Particle yield per stopped

ir

Mean energy (MeV)

0 .744 :0 .062 13 .9 :0 .9 1 .5-121 .5 0 .154 :0 .013 19 .6 -t 1 .4 1 .5-105 .5 1 .5-89 .5 0 .082 :0 .007 12 .3 :1 .4 8 .5-54.5 0.012 :0.002 20.6 :4 .8 0.308 :0 .027 .4 :0 .5 15-66 .8 .5

Mean energy er stopped a (MeV) 10 .3 :1 .13 3 .02 :0.33 1 .01 :0.14 0 .25 :0 .07 2 .59 :0 .27

324

H. Randoll et aL / Energy spectra 10 -, 0 z 0

10 -,

w w 10"' cn w H Q

10

0

20

40

60

80

100

ENERGY [MEV7

120

Fig. 4. Compilation of the energy spectra (replotted from fig. 3) for protons, deuterons, tritons, 3 '4 He. The spectra are corrected for finite target thickness.

secondary particles as listed in table 1 carry away a total positive charge of 1.6, a total baryon number of 2.6 and 17 .1 MeV of kinetic energy . Compared with the total baryon number and the total positive charge available in the lr--4oCa reaction a large deficit of baryons and charges is revealed. To the number of baryons must be added the number of emitted neutrons, not detected in our experiment, which should be about 4 neutrons per pion stop. This number is an estimate based on 16)] . It is the value of 4.5 :0.2 neutrons per pion stop, measured for 59CO [ref. concluded that the major part of the calcium nucleus acts mainly as a spectator to the pion absorption process. Fig. 5 shows a comparison of our corrected energy spectra with results from other counter experiments. The data were taken with calcium targets of 0.06 g/cm2 11) detected protons, [ref. 11)] and of 0.16 g/cm2 [ref. 13)] . Castleberry et al. deuterons, tritons withenergy thresholds of 6 MeV, 8 MeV and 9 MeV, respectively . Schlepiitz et aL 13) measured protons, deuterons and a-particles between 2 and 55 MeV. There exists general agreement between the shape of the particle spectra 13) report higher particle yields of the present and earlier work. Schlepiitz et aL than we do, which might be due to the fact that their particle detector registers also secondary particles from a telescope counter (S4 in our nomenclature), which was mounted very close to the target. Any sort of structure or fluctuations in the particle yields, visible in the spectra of Schlepiitz et al., cannot be confirmed and must be of purely statistical orgin. Table 2 presents a quantitative comparison of our energy-integrated yields and mean energies with the existing measurements t1-13). The integration is carried out according to the energy intervals covered by the measurements of the various authors. The yields in percent are related to

ENERGY (IEV 1

100 ENERGY (MEV7

120

10

i

10'

0

20

40

80

80

ENERGY (MEV ]

100 ENERGY CMEV)

120

Fig . 5 . Comparison of our corrected energy spectra (crosses) with results of other authors : Castleberry et al. 11 ) (triangles), and Schlepiitz et al. 13) (open squares).

80

Z O FK F

O F

104

H. RandoH et al. / Energy spectra

326

TABLE- 2

Comparison of particle yields and mean energies between the present work (left side) and earlier work (right side) . Castleberry et aL

This work

11)

particles detected

energy interval (MeV)

yield per _ 7

yield (% )

mean energy (MeV)

energy interval (MeV)

yield per or-

yield (°k)

mean energy (MeV)

p d t

>6.5 >7 .5 >9 .5

0.361 0.097 0.027

74 .4 20 .0 5.6

24 .5 28 .6 29 .1

6.1-102 7.9-102 >9 .2

0.485 0.130 0 .025

75 .8 20.3 3.9

21 .1 30 .4 28 .9

Budyashov et al. p

d t

>24.5 >24.5 >24 .5

68 .7 23 .8 7.5

47 .6 46 .0 41 .5

>24 >24 >24

12)

70.1 24 .1 5.8

47 .6 48 .5 46 .4

Schlepütz et al. 13) p d 41-le

1.5-55.5 1 .5-55.5 1.5-30.5

0.709 0.144 0 .301

61 .4 12 .5 26 .1

10 .9 15 .6 7 .7

1.4-55.5 2.0-55.5 1 .7-30.5

1.018 0 .162 0 .383

65 .1 10.4 24 .5

10.8 17 .6 8.0

The yields in percent are related to the total yield of those particles only, which are indicated in the first column . The integration over energy was chosen according to the energy interval covered by the measurements of the various authors 11-13 ) .

the total yield of those particles only, which are indicated in the first column. It turns out again that our absolute yields per pion stop (columns 3 and 7) are lower 13) . In contrast to this result than those of Castleberry et al. 11) and Schlepiitz et aL the relative yields (columns 4 and 8) are in good agreement as well as the mean energies (columns 5 and 9). Fig. 6 compares our calcium data with the energy spectra of Tr - absorption in 12C. Both proton spectra are practically identical in the preequilibrium domain, while for calcium .a more distinct evaporation peak appears at low energies. Clearer peaks at low energies are also observed in the deuteron and triton spectra of calcium than in the carbon spectra. Except for protons the integrated particle yields are much smaller for calcium than for carbon . Chiang and Hüfner 6 ) calculated the proton spectrum of calcium, assuming a (vr-, NN) absorption process which results in primary protons and protons from subsequent nucleon-nucleon interactions within the recoil nucleus. Their results are shown in fig. 7 [taken from ref. 6)] together with our data and those of Schlepütz 13) . et aL It turns out that the contribution of the primary protons (denoted as "0" in fig. 7) and the protons from one NN interaction (denoted by "1" in fig. 7)

H. Randoll et al. / Energy spectra

lol

10*

t

10''

1

10'

910-

l0' L 0'

327

10

40

!0

80 100 EMMY [1EY1

120

1

10

'L 10

!0 100 Dl41CT (PEY )

100AM OCa-Target (measured) -100Nm 4°Ca-Target (unfolded ) --¢125gm 12 G-Target (unfolded)

Fig . 6 . Comparison of our calcium and carbon data s) .

120

32 8

H. Randoll et aL / Energy spectra

Fig . 7 . Comparison of the experimental proton data with a calculation of Chiang and Hfifner 6). Our data are shown together with the data of Schlepfitz et al. 13) [taken from ref. 6)].

reproduces the experimental data nicely in the intermediate preequilibrium domain while at higher energies significant deviations occur. 4. Summary and condasions In the present paper the energy spectra of charged particles (protons, deuterons, tritons, 3He, 4He) emitted following the absorption of stopped negative pions on calcium is measured over the entire energy range (except for 3He, where the energy threshold was 8 MeV) with good statistical accuracy. For the integrated yield of heavier particles an upper limit of 10-4 ir- stop is determined. Although the shape of our particle spectra agrees generally with earlier measurements, their remains a difference of about 30% in the absolute yields compared with Schlepûtz et aL'3). This could be due to the improved experimental set-up used in the present work. The spectra differ significantly in their shape from the carbon spectra. There are indications 14) that this can be explained by a model, which assumes for both nuclei the primary (v -, NN) absorption process and subsequent particle pick-up by nucleons . The origin of the remarkably different yields of t and 3He is an open question. The continued interest of A. Citron is acknowledged . Furthermore we thank for discussions H.C. Chiang, F. Hachenberg and J. Hüfner of Heidelberg. For very competent technical assistance we thank A. Höhne and K. Kircher, G. Bûche for his help during the measurements and U. Klein for providing software . The excellent cooperation of our SIN colleagues and the hospitality we encountered at SIN are greatly appreciated.

H. Randoll et al. / Energy spectra

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D.S. Koltun, Advances in nuclear physics, Vol. 3 (Plenum, New York) p. 71 T.I . Kopaleishvili, Particles and Nuclei 22 (1973) 87 J. Hfifner, Phys . Reports 21C (1975) 38 H.S. Pruys, R. Engfer, R. Hartmann, E.A . Hermes, H.P. Isaak, F.W. Schlepütz, U. Sennhauser, W. Dey, K. Hess, H.-J. Pfeiffer, H.K . Walter and W. Hesselink, Nucl . Phys . A352 (1981) 388 5) C. Cernigoi, I. Gabrielli, N. Grion, G. Pauli, B. Saitta, R.A . Rieci, P. Boccaccio and G. Viesti, Nucl . Phys . A352 (1981) 343 6) H.C. Chiang and J. Hüfner, Nucl . Phys . A352 (1981) 442 7) G. Mechtersheimer, G. Bäche, U. Klein, W. Kluge, H. Mattháy, A. Moline and D. Mùnchmeyer, Phys . Lett. 73B (1978) 115 8) G. Mechtersheimer, G. Bäche, U. Klein, W. Kluge, H. Matthây and D. Münchmeyer, Nucl . Phys . A324 (1979) 379 9) U. Klein, G. Bäche, W. Kluge, H. Matthiiy, G. Mechtersheimer and A. Moline, Nucl . Phys . A329 (1979) 339 10) H. Randoll, Report Kernforschungszentrum Karlsruhe KfK 2876 B, Nov. 1979 11) P.J . Castleberry, Ph.D. thesis, Physics Department, University of Virginia (1970), unpublished ; P.J. Castleberry, L. Coulson, R.C. Minehart and K.O.H. Ziock, Phys . Lett . 34B (1971) 57 12) Yu .G . Budyashov, V.G . Zinov, A.D . Konin, N.V. Rabin and A.M . Chatrchyan JETP (Sov. Phys.) 35 (1972) 13 13) F.W. Schlepütz, J.C . Comiso, T.C . Meyer and K.O .H . Ziock, Phys. Rev. C19 (1979) 135 14) F. Hachenberg and J. Hüfner, private communication; F. Hachenberg, H.C . Chiang and J. Hüfner, Phys. Lptt . 97B (1980) 183 15) F. Schlepûtz, Ph.D . thesis, Univ . of Virginia (1977) ; J.C. Comiso, F. Schlepütz and K.O.H. Ziock, Nucl . Instr. 133 (1976) 121 ; J.C. Comiso, T. Meyer, F. Schlepütz and K.O.H. Ziock, Phys . Rev. Lett. 35 (1975) 13 16) H.S . Pruys, R. Engfer, R. Hartmann, U. Sennhauser, H.J. Pfeiffer, H.K . Walter, J. Morgenstern, A. Wyttenbach, E. Gadioli and E. Gadioli-Erbe, Nucl . Phys . A316 (1979) 365