S.E. Vigdor, Indiana University

1. Measurement of the Absolute np Scattering Differential Cross Section with a Tagged Neutron Beam at Intermediate Energies S.E. Vigdor, Indiana University • Why a new np scattering experiment? • Approach to minimize systematic errors • Tagged neutron facility at IUCF Cooler • Experiment details • Results (paper in preparation: M. Sarsour, et al.) • Conclusions Nucl. Instrum. Meth. A 527, 432 (2004) Indiana U.-Ohio U.- Uppsala Collaboration Nuclear Data 2004, Santa Fe, 9/29/04

2. Who Needs Another np Scattering Experiment?  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). • Strong disagreements in shape among different medium-energy exp’ts • Few reliable absolute cross section standards at medium energies  normalization uncertain & inbred • Partial-wave analyses ignore most of the data! Allow normalizations to float by typically 5-10% • NN coupling constant sensitive to uncertain back-angle d/d • “Vigorous” debate in literature and at conferences • de Swart & Timmermans propose angle-dependent renormalization to “salvage” data inconsistent with PWA – YIKES! Need experimental resolution of experimental discrepancies!

3. New Experiment, New Approach: • Tag production of neutron by detection of associated recoil particles from 2H(p,n)2p  count n flux on scattering target! • Enable detection of low-energy recoils, while maintaining reasonable luminosity, by use of stored, cooled proton beam on ultra-thin (gas jet) production target. Primary beam • Use large-acceptance second-ary detector array to measure np scattering over broad angle range simultaneously. • Use carefully matched solid CH2 and C secondary targets, with frequent swapping, to permit accurate subtraction of quasifree background, minimize reliance on kinematic cuts. Tagged n • Measure acceptance of secondary detectors by proton tracking. • Build multiple internal cross-checks into data analysis procedures.  Kinematically complete double-scattering exp’t with 1st target of negligible thickness!

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 Tagged Neutron “Beam” Parameters: • 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. 6° magnet

5. 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 Position correlations between the two recoil protons reveal the band associated with the secondary scattering target.

6. … Reconstructs 4-Momentum and Origin of the Tagged Neutron (or Proton): • Extended gas jet target has differential pumping tails • z of n prod’n determined event-by-event with z  2 mm by comparing pn from energy vs. momentum conservation • Tagging measures En, n with E  60 keV,   2 mrad, n pos’n on 2ndary target with x,y  few mm • EDSSD vs. Eback  particle ID, distinction of protons that stop in DSSD or backing detector • Secondary tagged p beam available via d recoils – permits simultaneous meas’ment of np and pp scattering with same target, detectors

7. Forward Detectors & Event Streams • 2ndary tgt: 20 x 20 x 2.5 cm3 CH2 (1.991023 H atoms/cm2) or C (graphite) of same transverse dim’ns and C atoms/cm2 • Forward detectors: plastic scintil- lators ( Ep info, timing, triggering, veto beam protons) + multi-wire chambers for p ray-tracing • Forward array has 100% (>50%) acceptance for np scatt. 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)

8. 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!

9. Two Subsamples to Compare: • Perform separate analyses of 2 event samples:“2-stop”, both recoil p’s stop in DSSD’s;“1-punch”, 1 of 2 p’s stops in backing detector • The two samples have quite different distributions in neutron energy and pos’n 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

10. 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

11. Identifying Free np Elastic Scattering Tagged n vertical position on secondary target (cm) Pulse height in E scintillator pscat = 12-15 pscat = 15-18 CH2  C CH2  C Proton energy deposition in rear hodoscope • Rely on C subtraction to remove background from other sources and quasifree np scattering from protons bound in C nuclei • Normalize C to CH2 data via pd elastic yield  subtraction accurate to ~ 0.4%, judged from removal of known bkgd. features • Minimize reliance on kine-matic cuts – e.g., removes problem of “reaction tail” events in thick hodoscope, seen at left (only need to correct for reaction tail events below hodoscope hardware threshold)

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 distrib’n 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 meas-ured  distributions for all pscatbins • All observed  “structure” is geometric! • Acceptance systematic errors typically < 0.5%,  1.5% at c.m. 90

13. 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 1.9%, with small angle-dependence. It is dominated by uncertainties regarding sequential reactions and tagging errors, still under study.

14. At Long Last: Results! N2() (ci) (d/d)lab = {N1()+N2()+N3()}tH|dcos(plab)|a • Error bars in plot statistical only, but statistics dominate! • Data analyzed in En slices, each slice corrected slightly via PWA to En=194.0 0.15 MeV • 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; small deviations probably  small parameter adjustments in PWA • Systematic deviations from Uppsala (our collaborators!) data larger than can be explained by E-dependence! Overall normalization uncertainty in present data < 2%

15. Conclusions • Tagged neutron facility has allowed medium-energy np backscattering measurement with tight control of systematic errors in absolute d/d. • Results (hopefully!) resolve extensive discrepancies in np database. Excellent agreement with PWA validates “low” value of NN coupling strength and controversial data rejection criteria in PWA analyses. • Results provide new absolute standard for medium-energy neutron-induced cross sections to ~ 2%. • IUCF Cooler was elegant facility that made this experiment possible. Operations funding ceased in 2002. This was one of the final experiments performed at the facility.

16. Tagger Timing Resolution : • DSSD Timing is used to distinguish real from accidental coincidences (within the tagger +with the forward detectors). • Leading-edge front-end discriminators for DSSD’s necessitate software walk correction for every strip.  The distribution of DSSD arrival time difference between the two recoil protons in tagged neutron event.  The distributions of the arrival time difference between the first recoil proton and the Hodoscope mean time in tagged neutron event.

17. Particle Identification (PID): • The backing detectors were not included in the trigger. • The backing pulse heights were used to: 1. Measure the full energy for particles that punched through the DSSD’s. 2. Discriminate against those that also punched through the backing detectors. Energy deposited in DSSD (MeV) Energy deposited in Backing (MeV) • Figures (a&b) show PID loci for two different event streams : #1  requiring 2 particles in the tagger and no hits in the up-stream vetoes. #4  requiring 1 particle in the tagger and hits in both upstream vetoes. • The dark boundary in each figure encloses the gate used to identify protons that stop in the backing detector. Energy deposited in DSSD (MeV) Energy deposited in Backing (MeV)

18. a) b) Counts Counts Reconstructed En (MeV) Reconstructed xtag (mm)

19. 8.0 104 b) 400 2-stop events a) 300 Counts 200 6.0 103 100 0 Higher of Two DSSD Energy Depositions (MeV) 4.0 104 c) 102 103 2.0 Counts 102 Tail events: 1- + 2-punch misidentified as 2-stop 10 0.0 -300 -200 -100 0 100 xtracking  xtagging (mm) xtracking  xtagging (mm) x error for real 2-stops x error for “simulated” corrupted events -300 -250 -200 -150 -100 Higher of Two DSSD Energy Depositions (MeV) -300 -200 -100 0 100

20. 160 100 Counts Counts 80 120 60 80 40 40 20 pscat = 33-36 2-stop events 0 0 0 100 100 100 100 0 pscat = 42-45 pscat (deg.) pscat (deg.) 2-stop events