Measurement of the Absolute np Scattering Differential Cross Section at 194 MeV

1. Measurement of the Absolute np Scattering Differential Cross Section at 194 MeV M. Sarsour Indiana University Cyclotron Facility M. Sarsour et al., PRL 94, 082303 (2005) Texas A&M University May 10, 2005

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 • NN scattering is a very important process • Testing of strong interaction theories • An input for microscopic nuclear calculations • np data base in less desirable state: • Experimental inconsistencies (Strong disagreements in shape among different medium-energy experiments) • cross section normalization difficulties : -- Data sets taken with mag. Spectrometer – Must move apparatus for different angles bites -- Both normalized against other exp’ts, or against PWA’s and total np cross sections • 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 (Bugg and Machleidt, Phys. Rev. C52, 1203 (1995).)

3. Basic Concept (New Approach): • Produced 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 Primary beam Tagged n • Build multiple internal cross-checks into data analysis procedures.  Absolute cross section below 2%  Address previous discrepancies!

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  200 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  allow monitoring of tagged n flux

6. Particle Identification (PID) • The backing detectors were not included in the trigger. • 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 measurement of np and pp scattering with same target, 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)

7. 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 time distribution, between both protons in the tagger, has FWHM of ~7 ns. • This resolution was adequate for coincidences at the level required for an ~1% cross section determination (at primary beam luminosity of ~ 1-2  1031/cm2/sec). The distribution of DSSD arrival time difference between the two recoil protons in tagged neutron event.

8. … Reconstructs 4-Momentum and Origin of the Tagged Neutron (or Proton): • Energy + angles of both recoil protons  Reconstructing neutron’s path. • Energy & angles are intertwined : • Using a bisection method to find where p(z)=0. • The vertex reconstruction was done iteratively. • Energy loss correction in the dead layers depends on the angle of incidence. • Angles depends on the primary vertex. • Extended gas jet target has differential pumping tails • z of n production determined event-by-event with z  2 mm • Tagging measures En, n with E  60 keV,   2 mrad, n position on 2ndary target with x,y  few mm

9. 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  Quality of tagged neutron trajectory reconstruction Zmin-resolution deteriorates with decreasing proton scattering angle. 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!

10. Forward Setup • The secondary target was a CH2 target (placed ~1 m downstream of the GJT) of 20x20 cm2 transverse to neutron beam direction and 2.46 cm thickness, a C target, with the same transverse dimensions and # C atoms/cm2, was for background subtraction. • Forward Detectors: A stack of detectors used to measure the forward scattered proton emerging from secondary target. • The forward detectors include plastic scintillators for triggering and energy information and a set of multi-wire-proportional counters (MWPCs) to provide position information. • Two charged particle veto scintillators (LUV & SUV) located upstream of the secondary target. • The rest of the forward detectors (MWPCs, E, veto2, and Scintillator Hodoscope) were located after the secondary target. • Hodoscope thickness was sufficient to stop forward 200 MeV protons, and to provide ~20% efficiency for neutrons.

11. Total flux (N1+N2+N3 event streams)  “N1” : neutrons don’t scatter in the target nor interact in forward detectors “N2” : np scattering candidates “N3” : neutrons don’t scatter in the target nor interact in forward detectors except for the hodoscope N2() (ci) (d/d)lab = {N1()+N2()+N3()}tH|dcos(plab)|a

12. Neutron Flux Properties : (Tow Sub-samples to compare) • Total flux (1+2+3) 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 “2-punch” Ignored, since tagged n energy typically much lower • 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

13. Missing Backing Detector Information 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. Identifying Free np Elastic Scattering pscat = 12-15 pscat = 15-18 CH2  C CH2  C • Rely on C subtraction to remove background from quasifree np scattering from protons bound in C nuclei and other sources • Normalize C to CH2 data via pd elastic yield  subtraction accurate to ~ 0.4%, judged from removal of known background features Tagged n vertical position on secondary target (cm) Pulse height in E scintillator • 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) Proton energy deposition in rear hodoscope

15. Calibrating Acceptance pscat = 33-36 2-stop events • Proton ray-tracing  measure pscat distribution 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! pscat = 42-45 2-stop acceptance • Acceptance systematic errors typically < 0.1%,  1.5% at c.m. 90 pscat = 42-45 2-stop events

16. Systematic Errors : Background subtraction 2.ΔE Detector • Real ΔE peak within (ΔEmean±4σ) •  out the real peak range after background subtraction/ before background subtraction = Background survives subtraction. • Weighted average  (2.97±0.24)x10-3 • 1. Secondary target • Judged from the extent to which the aluminum frame peak is successfully removed from the Ytag spectrum. • Ratio of the events in the Gaussian distribution (after to before) subtraction (1.94±0.54)x10-3 Counts Counts 2000 Gaus+Fermi+Poly fit Angle bin = 6-9 1500 After back ground subtraction 1000 500 AL support frame 0 0 200 400 600 800 Ytag (cm) E PH

17. Systematic Errors : Software cuts (E cut) • Efficiency of E cut judged by looking at : • R = • The overall weighted average (weighted by the statistical contribution of each data set)  0.01 ± 0.005 d/d( outside E cut) d/d( inside E cut) 1-punch events sample E E Cut Boundaries XS(.NOT.E)/XS(E) LAB (deg.) CM (deg.)

18. y(tracking) – y(tagging) on CH2 (0.1 mm) Systematic Errors : Sequential Reaction in the secondary target • Events where the neutrons undergo scattering or reaction before the one that gives rise to the observed forward proton. • Distorted events! Incorrect neutron’s incidence angle or energy • Sequential reaction events eliminated by only including events that fall within ±3 narrow peak of zero in both Xtrack-Xtag and Ytrack-Ytag in the cross section. • For event streams 1&3, the tagged neutron yields simply reduced by the same factor for events stream 2. • The correction to the neutron flux due to these events  1.063±0.010

19. Systematic Error Budget 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.

20. Results N2() (ci) (d/d)lab = {N1()+N2()+N3()}tH|dcos(plab)|a 2 0.813

21. 2 1.189 2 0.736 • Cross sections for all 3 data samples are completely consistent within statistical uncertainties • Data samples come from complementary regions of the tagged beam spatial and energy profiles  comparison supports the reliability of the experiment and analysis. • Cross sections also extracted for different time periods within the run production  consistent with uncertainties.

22. Results (Averaged over all three data samples) • 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 • Overall normalization uncertainty in present data is < 1.5%

23. Conclusions • The tagged neutron facility at IUCF 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%.