Murad Sarsour for CE71 Collaboration

1. Measurement of the Absolute Differential Cross Section for np Elastic scattering near 200 MeV Murad Sarsour for CE71 Collaboration APS Division of Nuclear Physics October 30, 2004

2. B.E. Bonner et al., Phys. Rev. Lett. 41, 1200(1978).  T.E.O. Ericson et al., Phys. Rev. Lett. 75, 1046(1995). —V. Stoks et al., Phys Rev. C48, 792(1993). Motivation • Marred by experimental inconsistencies and cross section normalization difficulties • Partial-wave analyses ignore most of the data! Allow absolute normalization to float by typically 5-10% in the fitting process • NN coupling constant sensitive to the d/d normalization  Los Alamos data; normalized by 1.093.  Uppsala’s data —PWA93 Need a resolution of experimental discrepancies!

3. New Approach: • Produce neutrons via p+d n+2pTag neutron by detection of associated recoil particles! • Used stored, cooled proton beam on ultra-thin (gas jet) production target  Enable detection of low-energy recoils, while maintaining reasonable luminosity • Used large-acceptance secondary detector array to measure np scattering over broad angle range simultaneously • Used carefully matched solid CH2 and C secondary targets, with frequent swapping, to permit accurate subtraction of quasifree background, minimize reliance on kinematic cuts P beam direction Tagged n P P  Kinematically complete double-scattering exp’t !

4. Tagged Neutron Facility IUCF Cooler Parameters: • Stored proton energy: 202.6 MeV • Proton current: up to 2.0 mA • “Coasting” (rf off) beam • Time-averaged prod’n L ~1.0 × 1031 cm-2 s-1 on D2 gas jet target • Electron cooling  p beam with small energy spread, spot size, divergence Cooler Injection Synchrotron 6° magnet Tagged Neutron “Beam” : • Central Production angle = 14º • Angle acceptance = 5º • Beam energy  185 – 198 MeV  approximate match to earlier high precision polarization data from IUCF • Tagged flux  100 Hz during Cooler flattop • Secondary target = 2.5 cm CH2   1 Hz free np scattering rate

5. Neutron Flux Properties : • Total flux (1+2+3 event streams)  restricted to neutrons whose reconstructed coordinates pass through the secondary target. • Separate analyses of 2 event samples: “2-stop”, both recoil p’s stop in DSSD’s “1-punch”, 1 of 2 p’s stop in backing detector • The two samples have quite different distributions in neutron energy and position on CH2 target  compare d/d results for the two as powerful internal consistency check on tagging technique • Ignore “2-punch” events, since tagged n energy typically much lower

6. Identifying Free np Elastic Scattering • Rely on C subtraction to remove background from other sources and quasifree np scattering from protons bound in C nuclei : • CH2 : (production target) : tH = 1.9861023 atoms/cm2 , tC = 9.93 1022atoms/cm2 C : (background target) : tC = 9.94 1022 atoms/cm2 • Normalize C to CH2 data via pd elastic yield  subtraction accurate to ~ 0.4%, judged from removal of known background features • Aluminum platform which was used to support the target • Protons from GJT that emerge above the top of the upstream veto scintillators

7. Systematic Error Budget 2-stop events y(tracking) – y(tagging) on CH2 (0.1 mm) The net systematic error at this point in absolute d/d is 1.5%, with small angle-dependence. It is dominated by uncertainties regarding sequential reactions and tagging errors.

8. Results N2() (ci) (d/d)lab = {N1()+N2()+N3()}tH|dcos(plab)|a • Error bars in plot statistical only, but statistics dominate! • 1-punch and 2-stop results agree in shape and magnitude within stat. errors (2/point  1) • Shape, magnitude both in excellent agreement with Nijmegen PWA93

9. Conclusions • Tagged neutron facility has allowed medium-energy np backscattering measurement with tight control of systematic errors in absolute d/d. • The usage of solid CH2 and C targets permitted accurate background subtraction, thus reducing the systematic errors. • The present Results are in good agreement with the Nijmegen PWA93 calculations, over the full angular range, and deviate systematically from other recent experimental results. • The net systematic error in the overall absolute cross section is 1.5%,.

10. The Tagger … • 4 silicon 6.4  6.4 cm2 double-sided strip detect-ors (DSSD) + 4 silicon large-area pad (backing) detectors • Place detectors ~10 cm from gas jet target to cover large solid angle • DSSD’s  energy, timing + 2-dim’l position (0.48 mm readout pitch) information for multiple particles • Backing detectors  energy and particle ID for recoils that punch thru DSSD’s (protons > 7 MeV) • Self-triggering readout electronics triggers on 2-particle coincidence among 64 logical pixels  allow monitoring of tagged n flux • Energy correlations between the DSSD and backing detectors provided Particle ID, distinction of protons that stop in the backing detector

11. Forward Detectors & Event Streams • 2ndary target: 20 x 20 x 2.5 cm3 CH2 (1.991023 H atoms/cm2) or C (graphite) of same transverse dimensions and C atoms/cm2 • Forward detectors: plastic scintillators- ( Ep info, timing, triggering, veto beam protons) + multi-wire chambers for p tracking • Forward array has 100% (>50%) acceptance for np scattering. from CH2 at c.m. 130 ( 95 ) • Forward hit pattern  3 mutually exclusive event streams to which we apply identical cuts: • 1  tagged n’s that don’t interact • 2  np scattering candidates • 3  n’s that convert in rear hodoscope (~20% efficiency)

12. Calibrating Acceptance pscat = 33-36 2-stop events pscat = 42-45 2-stop events • Proton ray-tracing  measure pscat distrib’n within each pscat bin • Use measured distribution of tagged n on CH2 to simulate acceptance of forward detectors, separately for 2-stop and 1-punch events pscat = 27-30 2-stop events • Allow slight adjustments from measured detector locations to optimize simultaneous fit to measured  distributions for all pscatbins • All observed  “structure” is geometric! • Acceptance systematic errors typically < 0.5%,  1.5% at c.m. 90

13. From the “Best-Laid Plans” Dept: Conspiracy/Redundancy x error for real 2-stops x error for “simulated” corrupted events • Discovered during data analysis that apparent electronics malfunction removed all backing detector E info for ~23% (random) of events  mis-ID 1-punch and 2-punch events as 2-stop with systematic error in tagged n path! • Able to accurately “simulate” all prop-erties of corrupted events by artificially setting Eback=0 in software for remaining good 1- and 2-punch events • Accurate normalization of corruption rate by comparison of “simulated” to real 2-stop events with Eback = 0, tback 0 • Subtraction removes bad events with little systematic error, but small loss of 1-punch statistics

14. Where the Neutrons Scatter: For np scattering candidates, the distance of closest approach of the tagged n and ray-traced p paths define the secondary scattering vertex in 3 dimensions  “medium-energy neutron radiography” Illustrates the power of n tagging technique, but not actually used in free np event reconstruction, since vertex z resolution (~ 7 mm) depends on np scattering angle!

18. Identifying Free np Elastic Scattering • Rely on C subtraction to remove background from other sources and quasifree np scattering from protons bound in C nuclei : • CH2 : (production target) : tH = 1.9861023 atoms/cm2 , tC = 9.93 1022atoms/cm2 C : (background target) : tC = 9.94 1022 atoms/cm2 • Normalize C to CH2 data via pd elastic yield  subtraction accurate to ~ 0.4%, judged from removal of known background features • Aluminum platform which was used to support the target • Protons from GJT that emerge above the top of the upstream veto scintillators