Measurement of the analyzing power Ay in neutron-proton radiative capture at En = 68 MeV

NUCLEAR PHYSICS A ELSEVIER

Nuclear Physics A580 (1994) 253-262

Measurement of the analyzing power A r in neutron-proton radiative capture at E n = 68 M e V M. Tuccillo, D. Fritschi, J. G6tz, R. Henneck, J. Jourdan, G. Masson, H. Miihry, L.M. Qin, S. Robinson, P. Steiner, I. Sick, P. Trueb, B. Zihlmann Institut fiir Physik, Universitiit Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland Received 29 October 1993; revised 8 July 1994

Abstract The vector analyzing power Ay for the reaction 1H(fi, T)2H has been measured at five angles between 60° and 140° in the lab at an incident neutron energy of 67.7 MeV. The measurement is of a precision never before achieved (AAy ~<0.01, statistical) for this observable. This precision makes possible a quantitative comparison with meson-exchange theories, thus enhancing our understanding of the role of non-nucleonic degrees of freedom.

Keywords: Nuclear reactions 1H(polarized n, y), E = 67.7 MeV; measured analyzing power A ( y ) vs O. Polarized neutrons, BC400 target.

1. Introduction

It has long been recognized that processes governed by the electromagnetic interaction provide a clean and well-understood method with which to study charge and current distributions inside the nuclei. QED is, in principle, an exact theory, and the small interaction strength of electromagnetic probes affects the dynamics of the investigated system far less than when strongly interacting massive particles are scattered. In particular, np radiative capture and its inverse reaction, deuteron photodisintegration, have been shown to be excellent processes with which to investigate both nucleonic and non-nucleonic degrees of freedom in the low-energy regime of the NN-interaction. Until recently, radiative-capture experiments were limited to various cross-section measurements. Advances in the techniques of polarized-beam and -target 0375-9474/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0375-9474(94)00425-0

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production now make it feasible to study the polarization observables of this reaction as well. In contrast to the unpolarized cross section, which is mainly sensitive to the dominant S-S amplitudes of the transition, polarization observables tend to be sensitive to the AI ~ 0 amplitudes and thus represent complementary information. Furthermore, l + 0 components (D in particular) are often intimately related to meson-exchange currents and other non-nucleonic degrees of freedom; thus spin observables can be expected to offer an excellent window on these small components of the interaction. In the specific case of the vector analyzing power, Ay, calculations by Arenh6vel [1] show that explicit inclusion of meson-exchange currents (beyond the use of Siegert operators) and isobar contributions causes a shift of Ay about 0.04 towards more-negative values over a wide angular range. Previous measurements of h y or Py(n) have been performed at lower neutron energies of a few to several MeV [2-11] and at much higher energies near and above pion-production threshold [12,13]. While the results of these experiments agree with the general trend of theoretical predictions, the experimental accuracies often have not been of a quality that permits critical evaluation of the ingredients entering theoretical calculations. The experiment presented in this paper is intended to improve upon this situation by measuring Ay to an unsurpassed absolute uncertainty of less than + 0.01 over an angular range of 60° to 140°.

2. Experiment The experiment was performed at the monoenergetic polarized-neutron-beam facility of the Paul Scherrer Institute (PSI) in Villigen, Switzerland. A detailed description of this neutron source has been published before [14]; we therefore limit ourselves here to a brief outline. Fig. 1 shows a schematic of the facility and the experimental setup.

experiment house

neutron production house

o

Fig. 1. Paul Scherrer Institute polarized-neutron-beam facility and the experimental setup: proton polarimeter (POL), dipole magnets (D1 and D2), liquid deuterium target (DT), Faraday cup (FC), quadrupole doublet (Q), brass collimator (COL), NaI detectors (GD), active target (AT), and removable doors (RD). The proton beam is transported in vacuum (solid line), while the neutron beam is in air (dashed line).

M. Tuccillo et al. ~Nuclear Physics A580 (1994) 253-262

255

The production of quasi-monoenergetic polarized neutrons is based on the transfer of transverse polarization from a proton beam to neutrons, using the --* breakup reaction 2 H(p, ff)2p at On -~. 0 o where the two final-state protons are left in a quasi-bound 1S0-state. To reduce systematic errors, the neutron polarization was flipped every few seconds by switching r.f.-transitions in the polarized-ion source producing the initial proton beam. The polarization of the protons after acceleration in the cyclotron was continuously monitored in a polarimeter employing pC elastic scattering as an analyzer. The resulting neutron polarization was then determined using the measured proton polarization and the known spin-transfer coefficient K~' = 0.4 of the production reaction. In this experiment, typical values of 71% proton polarization were achieved, leading to about 29% neutron polarization. After the liquid-deuterium target, a dipole magnet was used to deflect the remaining protons into a Faraday cup. This served as a monitor of the proton beam current and hence the neutron flux. A brass collimator limited the neutron beam spot to a diameter of 18 cm at the position of the experimental target. With a typical proton beam current of about 2 wA, a flux of about 5 • 107 n e u t r o n s / s on the target was obtained. The recoiling deuterons produced in the capture reaction were detected in an active target consisting of six plastic-scintillator bars (BC400) placed on a supporting aluminium table and grouped to foim a block of 20 × 50 x 12 cm 3 (W >( H × D). This block was located 2.5 m from the collimator exit and was tilted by 35 ° with respect to the vertical in order to compensate for differences in the signal transit time within the bars. The total effective target thickness was 1.3 g ( H ) / c m 2. Individual bars were read out by fast 3" photomultiplier tubes, allowing each bar to be treated separately in the data analysis. Three pairs of 20 cm diameter × 20 cm long NaI(TI) crystals, placed symmetrically left and right of the target at a distance of 52 cm, served to detect the gamma emitted in the capture reaction. Each NaI detector was equipped with a 8 cm thick lead collimator with 16 cm diameter aperture, reducing the effective solid angle to 73 msr. In order to reduce the effects of background produced in the experimental area, the entire experiment was located in a house with 50 cm thick concrete walls. Openings were provided in the front and back walls to allow the neutron beam to enter and exit. In addition, the NaI detectors were wrapped with a 1 cm thick layer of borated rubber in order to stop thermal neutrons. Charged particles emerging from the target were suppressed by a 1 cm thick aluminium plate placed between the NaI detectors and the target.

3. Data acquisition and analysis 3.1. Data A substantial event rate was produced in this experiment. Singles rates in the Nal detectors were typically 1 kI-Iz, while individual target-bar singles rates were as

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high as 1 MHz. Therefore, an on-line hardware condition was imposed to make a coarse filtration of the incoming data. An event was considered valid if it had coincident signals in one of the target bars and one of the NaI detectors within a 20 ns gate, a target signal exceeding 10 MeVee, and a NaI detector signal of at least 20 MeV. This on-line condition reduced the rate of events accepted by the data-acquisition computer to a manageable 100 to 200 Hz, leading to a dead-time for the data-acquisition system as a whole of about 10%. The information recorded for each valid event served a number of purposes: (1) The time of single target signals with respect to the logical OR of all target signals provided a simple way to identify which bar fired in later off-line analysis. (2) The time of the cyclotron radio-frequency signal with respect to the logical OR of all target signals (referred to as r.f.-TOF) was used to determine the energy of the incoming neutron. (3) The time of the NaI detector signals with respect to the logical OR of all target signals (referred to as NaI-TOF) was used to discriminate between gammas and neutrons observed in the NaI detectors. (4) The integrated amplitude of the target signal provided the recoil deuteron energy. (5) The integrated amplitude of the NaI detector signal provided the gamma energy. (6) The integrated rising and falling part of the NaI detector signal provided pulse-shape information that was used to distinguish between gamma- and neutron-induced signals in the NaI detector. In Fig. 2 we show examples of spectra for the four most important pieces of information. The "raw" spectra are histograms of events conditioned to include coincidences between a single target bar and a single NaI detector. The "filtered" spectra result after additional cutting on all other information to maximize the ratio of capture events to background.

3.2. Background subtraction Despite the stringent on-line requirement, less than 2% of all events written to tape represented real capture reactions. Thus, extensive off-line filtering of the data was necessary in order to extract the desired events. After sorting of the data, in which each target-NaI pairing was treated separately, two classes of spectra of primary interest resulted: the deuteron energy spectra and the gamma energy spectra for the 18 detector pairings ( × 2, because of the two spin states). From these, the remaining background was subtracted by a fit to the data. Inspection of the NaI-TOF spectra and NaI pulse-shape spectra showed that, in addition to accidental coincidences, the residual background in the energy spectra was composed of events with coincidence times and NaI pulse shapes consistent with the passage of a gamma from the target to the NaI detector. These background events are likely to be primarily the result of multi-step processes involving inelastic radiative channels of carbon (e.g. elastic scattering of neutrons from

M. Tuccillo et al. / Nuclear Physics .4580 (1994) 253-262 3000

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Fig. 2. Examples of spectra: (a) target pulse height, (b) NaI pulse height, (c) NaI-TOF, and (d) NaI pulse shape. The capture deuteron peak is clearly seen in spectrum (a). The capture gamma peak in spectrum (b) is less well resolved because of the intrinsic NaI energy resolution and the residual background. The raw spectrum in (c) shows a peak corresponding to gamma-induced events plus a flat accidental coincidence spectrum and the shoulder of a large peak (not shown) due to neutron-induced events. Spectrum (d) shows two bands of events with different pulse shape. One band corresponds to gamma-induced signal pulses; the other is related to neutron-induced signal pulses.

hydrogen followed by 12C(n, nt'~)12C, or 12C(n, p)12B* followed by B* --} B + y) 1. While the amount of accidental coincidence background is correlated with count rate and hence will depend on the scattering angle, the real coincidence background events are expected to exhibit an isotropic angular distribution. In fact, after subtraction of the accidental coincidence background, the remaining background was found to be constant over the angular region covered by the NaI detectors. This suggests that a direct comparison of the six spectra belonging to one plastic-scintillator bar (i.e. in coincidence with any of the six NaI detectors) should show a deuteron-peak

s h i f t i n g a s its e n e r g y c h a n g e s o n a b a c k g r o u n d

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1 TO prove this, several of the BC400 plastic-target bars were replaced with vessels containing a BC517H liquid scintillator, which has a 72% higher H: C-ratio. Data analysed from the two scintillator types showed that the peak-to-background ratio was indeed about 50% larger for the liquid scintillator compared to the plastic, in reasonable agreement with simple calculations.

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i0 3

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Fig. 3. Log-scaleoverlayof energy spectra for a single target bar in coincidencewith three different NaI detectors, placed at 60°, 95°, and 140° in the lab. The spectra are described in the text and demonstrate that the background structure and amplitude are independent of the NaI detector angle. universal shape and amplitude. This fact can be exploited to extract the shape of the background component by simultaneously fitting the regions to the left and right of the deuteron peak in all six spectra; only a small region between 19 and 24 MeV is not covered by this method (Fig. 3). A simple exponential function e -~x was found to reproduce the background over much of the target spectrum equally well as more complicated functions. Therefore the results for the analyzing power quoted here are based on the simultaneous fit of the six spectra for each scintillator, where the fitting function consisted of a skewed gaussian (derived from a Monte Carlo simulation of the experiment) for the deuteron peak and the simple exponential function for the background (see Fig. 4a). As a check, we also determined Ay using the gamma energy spectra. Here, EGS [15] Monte Carlo calculations were used to determine the response function of the NaI detectors, and an exponential function was used to model both the accidental and the 'gamma' background (see Fig. 4b). Excellent agreement within statistical error was found between the two methods. This further suggests that the background contaminant is well understood and that no hidden systematic errors are present in the analysis. 3.3. B e a m polarization

The polarization of the neutron beam was determined from the polarization of the proton beam and the spin-transfer coefficient K ( ' of the neutron-producing --. ~)2p at 0n = 0o. reaction; i.e. 2H(p,

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M. Tuccillo et al. / Nuclear Physics A580 (1994) 253-262 111[11

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Fig. 4. Example of fits to energy spectra. The target energy spectrum (a) is fit by a gaussian with low-energy tail (dot-dashed curve) and a function which decreases exponentially with energy (dashed curve). The combined (solid) curve fits the data between the arrows with a g 2 of 1.34. The NaI energy spectrum (b) is fit with an EGS generated peak shape for the 39 MeV capture gammas (dot-dashed curve) and an exponential function (dashed curve). The combined (solid) curve fits the data between the arrows with a X2 of 1.32.

The spin-transfer coefficient Kyy' varies as a function of the neutron energy, therefore the effective transfer coefficient g ~ ' (i.e. averaged over the accepted incident neutron energy interval) was calculated using the measured energy distribution of the neutrons (see Ref. [16] for details). 3. 4. E r r o r estimation

The absolute statistical errors for the five points range from 0.006 to 0.010. A number of possible sources of systematic error have been identified and are discussed below. When added together in quadrature, they amount to a relative uncertainty of 4.0% to 4.2% depending on the experimental point. To estimate the systematic error in the background subtraction, we compared the results of using different analytical functions in the simultaneous fitting procedure as described above, as well as the results of a Monte Carlo simulation of the background. Little variation in the calculated asymmetry was found, hence we conclude that uncertainties in the fitting and background-subtraction procedure cannot contribute more than a 0.002 (3or) absolute systematic error in the analyzing power and are therefore negligible. Note also that the polarization of the background as extracted from the fitting procedure is zero within statistics. The influence of the position of the integration limits on the asymmetry was studied by altering the gates by ___10 channels on each side, corresponding to a _+30% change in the integration area. In all cases, the asymmetry was affected by less than a one-quarter statistical standard deviation; thus for a realistic uncertainty of _+2 channels in the gate positioning, this effect can be neglected.

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Uncertainties in the determination of the proton-beam polarization will contribute a systematic error. The polarimeter's analyzing power is only known to finite precision ( A y = 0 . 9 8 6 4 + 0.0010 [17]). The statistical uncertainty in the proton-polarization measurement was 10 -4. Errors arising from a dependence on the choice of the integration limits and from possible spin-dependent variations of the photomultiplier gains were studied and found to be of order 10 -3. By far the largest contribution to the final systematic error comes from the uncertainty in the effective spin-transfer coefficient g~'. Pickar et al. [16] measured Kyy' to about 4% using the reaction 4He(ff, n)~He at the same neutron source employed in this experiment. This error manifests itself in this experiment as an overall scale uncertainty in value of Ay.

4. Results and conclusions

Arenh6vel's calculations for the photodisintegration of the deuteron employ two modern NN-interactions, the meson-theoretical Bonn potential and the dispersion-theoretical Paris potential [18]. In both cases, one-boson-exchange (OBE) approximations are used. Only the pure two-body breakup is considered, i.e. the final state consists only of the two nucleons. The calculation of the observables is performed using a standard multipole decomposition of the reaction T-matrix into electric and magnetic multipoles up to order L = 4. The principle of detailed balance is then used to determine the corresponding observables in the inverse reaction, np capture. With respect to the electromagnetic interaction, static meson-exchange currents (MEC) consistent with the potential model, isobar currents (IC), and relativistic corrections (rel) are considered. The use of Siegert operators in calculating the nonrelativistic one-body currents automatically includes part of the MECs for the electric multipole transitions. This leads to the so-called normal part (N) in Fig. 5. Explicit inclusion of MEC beyond the Siegert formalism - ~r- and p-contributions only - accounts for the remainder and for the effects in magnetic multipole transitions (referred to as N + MEC). Here, the coupling of the photon and meson to the nucleon in a single vertex (Kroll-Rudermann term) is considered, as well as the coupling of the photon to the meson in flight (pole term). For the low-energy region considered here (Er < 40 MeV), i.e. far from the region of isobar excitation, the isobar configurations in the deuteron can be treated using impulse approximation (N + MEC + IC). MEC contributions involving isobar configurations, such as the coupling of the photon to the 7r-exchange in NN ~ AA, are also taken into account. Finally, relativistic effects have to be added in order to obtain the full theory (N + MEC + IC + rel). These include (1) the boost of the deuteron wave function into a moving frame (this affecting not only the spin degrees of freedom, but also its internal structure through the Lorentz contraction), and (2) terms of relativistic order in the current operator (spin-orbit currents). For the observable of interest to us and the energy range considered here,

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Table 1 Comparison of measured analyzing powers with theoretical predictions by Arenh6vel as explained in the text ((1) N, (2) N + M E C , (3) N + M E C + I C , (4) N + M E C + IC+rel, and (5) N+MEC+rel). As an indication of agreement between experiment and theory the reduced chi-squared is given for the raw as well as for the 4% renormalized data sets. Note that the best agreement (X2 = 1.3) is obtained for theoretical curve 2 and 5 0c..m Aye~ -+ AASta' -F AASryst A0,1, A (2) A?' A0,4, A (s, 68.9 85.3 103.4 118.6 143.7

- 0.062 5:0.007 + 0.002 - 0.091 -+0.009 __0.004 -0.121+0.006+0.005 -0.174+0.010-+0.007 - 0.246 5:0.008 + 0.010

X2

X,,2 L04 X,,,o.96

- 0.030 - 0.070 -0.113 -0.151 - 0.220

- 0.057 - 0.094 -0.135 -0.172 - 0.242

- 0.075 - 0.109 -0.149 -0.186 - 0.256

- 0.069 - 0.103 -0.143 -0.181 - 0.260

- 0.052 - 0.089 -0.131 -0.169 - 0.246

11.0 16.3 7.2

1.6 1.8 2.8

8.1 5.0 12.4

4.9 2.4 8.7

1.3 1.8 2.2

Arenh6vel finds only a negligible dependence on the potential used, whereas considerable changes occur with the inclusion of the different effects described a b o v e . T a b l e 1 a n d Fig. 5 s h o w t h e r e s u l t s o f this e x p e r i m e n t c o m p a r e d to t h e results of Arenh6vel's calculations. The experimental data points do not agree with t h e full t h e o r y ( N + M E C + I C + rel). A m o n g t h e v a r i o u s t e r m s b e y o n d i m p u l s e a p p r o x i m a t i o n ( M E C , IC, rel), t h e I C c o n t r i b u t i o n is p r o b a b l y t h e m o s t u n c e r t a i n o n e . O m i t t i n g it e n t i r e l y w o u l d l e a d to v e r y g o o d a g r e e m e n t b e t w e e n e x p e r i m e n t

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Fig. 5. Comparison of our data with theoretical predictions by ArenhSvel using the Bonn potential. The error bars reflect statistical uncertainties only. In addition there is a 4% normalization uncertainty. See text for an explanation of the theoretical curves.

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and theory. However, every calculation has intrinsic uncertainties which are difficult to assess quantitatively. We would like to thank Professor Dr. H. Arenh6vel for providing the theoretical calculations and for his many helpful comments. This work was supported by the Swiss National Science Foundation.

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