Measurements of the neutron-proton analyzing power in the energy range from 17 to 50 MeV

Measurements of the neutron-proton analyzing power in the energy range from 17 to 50 MeV

Nuclear 0 Physics A425 (1984) 458468 North-Holland Publishing Company MEASUREMENTS OF THE NEUTRON-PROTON ANALYZING IN THE ENERGY RANGE FROM 17 T...

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Nuclear 0


A425 (1984) 458468







Karlsruhe, Institut ftir Kernphysik 1, D-7500 Karlsruhe, West Germany Received

30 November


Abstract: Measurements of the analyzing power A, for neutron-proton scattering in the energy range from 17 to 50 MeV are reported. These data improve considerably the precision of the np data base in this energy range. Preliminary phase-shift analyses indicate reduced uncertainties in the np ‘P(T = 1) phases and in the aD(T = 0) phase shifts.



REACTIONS H(polarized n,n), E = 17-50 MeV; scintillator target, polyethylene target.




1. Introduction In recent years the NN interaction has received considerable attention, particularly the np system which bears on the isospin singlet (T = 0) part of the 50 MeV has received particular interaction. The energy range around attention ’ -4) b ecause ambiguous and/or anomalous values of the 3S, -3D, mixing parameter, ei, and the iPi phase-shift were obtained in earlier phasedisagreed with values derived shift analyses 5, 6). Th ese T = 0 phase parameters from models and with interpolations derived from adjacent energies 6). It was noted in another analysis ‘), which obtained a negative value for &I at 50 MeV (-0.92O), that a rather complex x2 hypersurface exists which could invalidate conventional error analysis. The more recent measurements and analyses ie4) improved the situation so that uncertainties were reduced, particularly in &I and the ‘P1 phase shift. However, the coupling in the determination of these parameters *) remains of some concern. In addition, another recent analysis9) indicated that the 3S, phase near 50 MeV is

* Permanent ** Permanent

address: address:

Dept. of Physics, Dept. of Physics,

U. C. Davis, CA 95616, USA. Texas A & M Univ., College Station, 458

TX 77843, USA.

J. W~~~z~nskiet al. 1 np anatysitzg power


out of line with a predicted smooth energy variation. It is clear that in general more data of higher accuracy are needed to reduce phase-parameter uncertainties and resolve present ambiguities. A program at the Karlsruhe cyclotron using the POLKA lo) polarized neutron facility is underway to accomplish this. A, and A,, are being measured and ~(0) measurements are in the planning stage. The precision which is being achieved in these measurements will eventually produce an accurate set of 7’ = 0 and, instead of pp (T = 1) phase shifts as used until now, of np (T = 1) phase parameters at these energies. The latter, for example, will allow the investigation of anomalous (i.e. due to strong interactions) isospin-violating effects in the NN interaction. Recent calculations 11) have used quark mass differences to derive an anomalous isospin-violating potential, and it appears the effects on the 3P phases should be observable

9. Wiiczynski ct at. / np a~lys~~~ power TABLE t

Poiari~a~~on of the neutron beam from POLKA Energy bin (MeV)

Average energy (MeV)

17.0$: 1.0 19.0+ 1.0 22.ozt: 1.5 25.02 I.5 27.5 & 1.0 3o.oc 1.5 33.01 I.5 36.0 & 1.5 40.0 + 2.0 50.0 s 2.0

13.0 19.1 22.1 25.0 27.4 29.9 32.9 35.8 39.7 50.0

17 8

Fig. 1. Distribution

w ‘

22 t

25 30 ‘,I,‘*

23.7 t0.5 30.0 kO.9 35.0+_1.1 42.0 +_I.2 43.4k1.3 44.2f 1.3 44.411.3 44.1 f 1.3 44.7 & 1.3 44.0*1.5


50 I


of neutron flux and polarization in the continuous energy neutron beam from POLKA. The width of the energy bins used in the off-tine data analysis is indicated at the bottom.

J. Wilczynski et al. / np analysing power


With this total neutron spectrum, data can be taken simultaneously for neutron energies from 2 15 MeV to N 52 MeV and then divided into energy bins according to the requirements of the experiment. The neutron beam is collimated by annular tungsten, copper (and some polyethylene) inserts placed in a stainless steel tube. The openings in these annular inserts taper down to provide a collimating throat, and then taper up in diameter to provide an antiscattering section at the exit end of the collimator. In the present case a throat of 14 mm diameter at a distance of 1.75 m was used to produce a beam spot of about 60 mm diameter on the scintillating target located 6 m from the source. For measurements with the CH, target, where proton detection was used to take data at large c.m. angles, a smaller collimator throat produced a beam of 21 mm diameter on the CH, target located 3.4 m from the source. The first measurements of A, [ref. ‘“)I were carried out by having the collimated neutron beam impinge on a target consisting of a vertical cylinder (7.6 cm high x 7.6 diameter) of NE 213 liquid scintillator at 6 m from the fi source. Scattered neutrons were detected at 2 m by 7 pairs of NE 213 liquid scintillators (vertical cylinders 20 cm high x 14 cm diameter) at 7 angles covering the range from 16Oto 71” lab. Each event ( a target-detector coincidence) was characterized by the source-target (or incident) time of flight (TOF), the target-detector TOF, the pulse heights in the target and detector scintillators, the pulse shape in the detector scintillator, the detector number, and the n spin direction. The incident TOF, which was measured relative to the cyclotron RF, determined the incident neutron energy which was divided into 10 energy bins in the off-line data analysis (see fig. 1). For two backward tip c.m. angles, A, was measured by detecting and identifying recoil protons from a 3 mm thick CH, polyethylene target in AE-E telescopes at f 14” and + 20” lab. In this case the incident TOF, the AE and E pulse heights, telescope number, and fi spin direction were measured. In each experiment all parameters were written on magnetic tape, via an ND 4420 multichannel analyzer system (Nuclear Data). Spectra from the deuteron polarimeter were stored separately on disc via CAMAC by an LSI 11/23 microcomputer system (Digital Equipment). On-line monitoring of single and twoparameter spectra was carried out. In the off-line analysis the data were first corrected for time shifts due to phase changes in the cyclotron and then separated according to energy bin (by cuts in the incident TOF). If necessary, corrections were made for gain changes in the recoil pulse-height spectra in the target scintillator, &or shifts in the TOF of the scattered neutrons (as indicated by time shifts in the y-ray peak), and in the pulseshape spectra. Then the appropriate restrictions were applied. For example, in the pulse-shape versus pulse-height matrix, neutrons were separated from y-rays, and in the AE versus E matrix, protons were separated from deuterons. In the case of neutron detection, backgrounds, due mainly to double scattering, were

! np anaiysing power Experiment

27.5 MeV lOS.PofCM)



,,,.. ..:.:&,&&,.. -




Fig. 2. Monte Carlo simulation of np single scattering and of multiple neutron scattering on carbon and hydrogen compared to experimental data at E, = 27.5 MeV and Be,,= 52.7”. The broad distribution at small E, is due to 12C(n, n’) processes; the ridge below the np true event peak stems from recoil protons produced close to the walls of the scintillating sample. Both effects were taken into account in the Monte Carlo calculations but have not been plotted.

estimated from the target pulse-height versus scattered TOF matrix as can be seen in fig. 2. Backgrounds under the true-event peak in these matrices ranged from 1 to about 10 % and were typically a few percent. The data were corrected for finite geometry and multiple scattering effects by an extensive Monte Carlo simulation of the experiment 14). It predicted the true events and the multiple scattering background to about 10 % accuracy and provided a good verification of the method of data analysis. In the case of proton detection (at + 14O and &ZOO) the backgrounds (S 1%) due to carbon and air were measured and corrected for. The effective neutron scattering angle was calculated from the finite geometry. Finally from events for each energy bin and each pair of left (L) and right (R) detectors and for neutron spin up 7 and down 1, the usual asymmetry E = (,/m,,/m/(,/m + ,/m) and hence A, = + e/P, was calculated. The +/sign applies when a neutron/proton was detected. This yielded measurements of A, in 10 energy bins whose center energies were 17, 19, 22, 25, 27.5, 30, 33, 36, 40, and 50 MeV and for the angular range of E 33” to 151* c.m. The resulting values of A, are given in table 2. The total absolute errors given are mainly statistical in origin but contributions from uncertainties in the geometry are also included. A scale error for each energy can be obtained from table 1 where the absolute uncertainties in P, are given. The data at 151” and 1410 are

J. Wiiczynski


et al. / tzp analysing power

TABLE 2 Analyzing power for np scattering and (absolute) uncertainties both in “I, 0 (cm.)



0 (c.m.)



33.1 50.9 69.1 87.1 105.4 122.9

17.0+ 1 MeV 1.90 2.43 2.95 4.52 2.99 1.34

1.15 0.95 0.97 1.07 0.98 1.20

33.1 50.9 69.1 87.1 105.4 122.9

19.0& 1 MeV 2.56 4.57 3.26 2.96 2.66 2.41

0.76 0.66 0.66 0.75 0.60 0.75

33.1 50.9 69.1 87.1 105.4 122.9 141.0 151.4

22.02 1.5 MeV 4.14 5.44 5.50 5.32 3.20 2.61 1.89 1.28

0.47 0.43 0.39 0.46 0.43 0.48 0.23 0.38

33.1 50.9 69.1 87.1 105.4 122.9 141.0 151.4

25.0+ 1.5 MeV 4.75 6.41 7.14 6.95 4.71 2.68 1.56 0.90

0.42 0.39 0.35 0.41 0.39 0.42 0.17 0.36

33.1 50.9 69.1 87.1 105.4 122.9 141.0 151.4

27.5 i: 1 MeV 5.60 8.11 7.74 7.68 5.42 3.60 2.37 2.25

0.52 0.49 0.44 0.51 0.50 0.53 0.21 0.32

33.1 50.9 69.1 87.t 105.4 123.0 141.0 151.4

30.0 + 1.5 MeV 5.94 8.57 9.27 8.49 6.44 4.11 2.33 0.90

0.53 0.48 0.44 0.52 0.55 0.54 0.19 0.31

33.1 50.9 69.1 87.1 105.4 123.0 141.0 151.4

33.0,i.S MeV 8.02 10.58 11.86 10.62 8.11 4.61 2.65 1.36

0.65 0.59 0.54 0.66 0.63 0.66 0.23 0.27

33.1 50.9 69.1 87.1 105.4 123.0 141.0 151.4

36.0 & 1.5 MeV 9.64 12.23 12.85 13.08 8.59 5.86 3.36 2.22

0.79 0.70 0.66 0.83 0.78 0.91 0.30 0.46

33.1 50.9 69.1 87.1 105.4 123.0 141.0 151.4

40.0 k 2 MeV 7.97 14.58 15.65 14.29 10.88 6.01 3.57 3.15

0.90 0.83 0.78 0.99 0.95 0.99 0.36 0.59

33.1 50.9 69.1 87.1 105.4 123.0 141.0 151.4

50.0+2 MeV 12.15 21.35 23.21 21.06 14.53 7.89 4.94 1.86

1.24 1.15 1.12 1.42 1.45 1.67 0.51 0.85


J. Wilczynski

et al. 1 np analysing power

from recoil proton measurements. The recoil data at the lowest partly cut off by the energy losses in the target and telescopes included.

energies were and are not

3. Discussion Data at 17 MeV from the present experiment are in good agreement with the precise data from Duke i5). For neutron energies above 20 MeV our present A, data improve considerably the existing np data base for this observable. At 50 MeV our results agree well with the data from Davis 3, i6). There is also very good agreement at 25 MeV with recent results from the Madison group 17). Our very accurate data at large c.m. angles (from recoil proton measurements) appear to rule out the possibility of a zero crossing which the early Harwell data I*) may have indicated and produced in some phase-shift predictions. Values of A, (as %) for incident neutron energies of 50 and 30 MeV are shown in figs. 3 and 4. For comparison other data from earlier work 3, i6, i8-“) are also shown. In general the POLKA results have much smaller uncertainties particularly at 30 MeV where the fi beam intensity is large. The data at 22, 25, 27.5, and 33 MeV have uncertainties AA, 5 0.5 “/,. The largest AA, 5 1.5 “/ are at 50 MeV. The uncertainties in the 141° and l5l0 data from p-detection will be further

8 c.m. Fig. 3. fip analyzing power at 50 MeV. The new data of this work (full circles) are shown together with the results of ref. 3, (crosses) and ref. 16) (triangles). The curve represents the results of a preliminary np phase-shift analysis ‘I).


klcm Fig. 4. iip analyzing power at 30 MeV. The new Karlsruhe data (full circles) are compared to the results of ref. 20) (crosses: neutrons detected, triangles: protons detected), ref. 19) (diamonds) and ref. Is) (squares). Curve: preliminary np phase-shift analysis *‘).

reduced in future experiments, where the proton telescopes are used as polarized neutron flux monitors. This method gives high statistical accuracy and is free of the large multiple scattering corrections required for neutron detection data at similar c.m. angles. In figs. 3 and 4 the curves through the data are from our preliminary phase-shift analyses21). These results and those from more recent analyses 22), including the new Madison data ’ ‘> at 25 MeV, imply ‘an improved determination of the np 3P phases (independent of pp data) and of the 3D phases, particularly the 3D3. In fig. 5 our new A, data at 25 MeV are compared to distributions calculated from several phase-shift sets by our group 22), Signell 23), Paris 24), OBE (Bonn) 25) and Arndt 26, 27). All show good qualitative agreement with the data. However, the quantitative differences in the shapes of the various predictions show that precise data can discriminate among these predictions. The shape of the A, distributions depends strongly on the sD phase shifts. These parameters will be constrained by new phase-shift analyses which are carried out at present. We conclude that these new measurements of A, considerably improve the np data base in the energy range 20-50 MeV. Preliminary phase-shift analyses indicate much reduced uncertainties in the 3P np and 3D (11 = 0) phase shifts. It is hoped that inclusion of new A,, data, presently being measured, will result in a richer data base for phase-shift analyses which should provide phase parameters for precise model test or for model construction. For example, the 3P waves for

J. Wilczynski e’t al. / np anaiysing power






.....I....... ,$rn&



KA 83 . . . . .._ &ndt

np (221

l( 9 c.m.


Fig. 5. hp analyzing power at 25 MeV. Our experimental results are shown in part (a) together with predictions from several potential models, i.e. the Bonn OBE potential 25) (solid line) and the Paris potential 24) (dashed line) as well as from phase-shift analyses by Arndt =) (dotted line) and by Signell 23) (dashdotted line). In part (b) the data are compared to the results of new Katlsruhe phase-shift analyses 22). Solid fine: our full np analysis in the energy range from 20 to 30 MeV; dotted line: Amdt’s”) single energy solution at 25 MeV, with t&resuIts of ref. I”) and of this work added to the data base.

2. ~~~c~ynskj et al. / np analysing power


np, can be compared with those from pp data ‘to check the predictions “) of anomalous isospin violations due to quark mass differences. The authors would like to thank Drs. V. Bechtold and L. Friedrich for providing the polarized deuteron beam. The help of A. Bischoff, K. Hofmann, H. Krupp, Ch. Maier, W. Nitz, and M. Oexner during the data-taking is gratefully acknowledged. F. P. Brady and J. C. Hiebert would like to thank Prof. B. Zeitnitz and the Institut fiir Kernphysik, Kernforschungszentrum for their hospitality and support. F. P. Brady would also like to acknowledge the hospitality and support of the University of Karlsruhe and to thank the American and German Fulbright Commissions for their support during his year in Germany.

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