Gamma-Rays and Blowfish. What are we doing? Why are we doing it? How are we doing it?

Transcription

1 Gamma-Rays and Blowfish What are we doing? Why are we doing it? How are we doing it?

2 International Collaboration University of Saskatchewan: R. Pywell, Ward Wurtz, Octavian Mavrichi, Brian Bewer, Daron Chabot, Matt Wessel, Robin Wilson, Ru Igarashi University of Virginia: Blaine Norum, Serpil Kucucker Duke University: Henry Weller, Mohammad Ahmed, Luke Myers Jefferson Laboratory/Temple University: Brad Sawatzky George Washington University: Jerry Feldman Lund University (Sweden): Kevin Fissum NSC Kharkov Institute of Physics and Technology (Ukaine): Yu.P.Lyakhno, A.Yu.Buki, S.S.Zub, B.I.Shramenko

3 Nucleon-Nucleon Interaction One aspect of our research is focused on understanding the details of the Nucleon-Nucleon Interaction. The force between nucleon (neutrons and protons) within the nucleus. We know that the nucleons are made up of quarks and that the fundamental force between quarks is the Strong interaction described by Quantum Chromodynamics (QCD). However, theorists are unable to describe the properties of nuclei and nuclear reactions by starting with QCD.

4 Nucleon-Nucleon Interaction Therefore we resort to using a semiphenomenological model for the force between nucleons. This force is the residual strong force between the, 3-quark nucleons. To first order this N-N force is the result of the exchange of 2-quark mesons. So, in principle, the use of an N-N interaction makes it possible to calculate the properties of nuclei and the results of nuclear reaction. Especially important in cases where it is not possible to make direct measurements such as some reactions occurring inside stars. Therefore it is important to understand the properties of the N-N interaction.

5 Deuterium An important testing ground for properties of the N-N interaction is the study of Deuterium. Deuterium is the simplest nucleus consisting of one neutron and one proton. We have made measurements on deuterium using the techniques I will describe in this talk and more are planned. Not the focus of this talk. These measurements hint at anomalies not described by the best calculations using the best form of the N-N interaction.

6 Few-Body Photodisintegration We are testing models of the N-N interaction using nuclear reactions initiated by a high energy gamma-ray e.g. or 2He2He n He(, n) We measure the Cross Section (or probability) of such reactions as a function of various parameters such as Gamma-ray energy Angle of emitted particle Polarization of incident photon

7 Few-Body Photodisintegration We compare our measurements to the results of theoretical calculations using various N-N interaction models. Unfortunately, such calculations have traditionally been very difficult. Have only been attempted for nuclei with few nucleons in them. Have included various approximations that have masked the difference between N-N models. Interest in photo-nuclear reactions has waned since the 1970 s. However, there have been recent advances in theoretical techniques that now allow accurate calculations.

8 Lorentz Integral Transform A technique pioneered by Orlandini, Leidemann, Efros, Bacca and others. The calculation is of the cross section for a transition from an initial state with nucleons bound within a nucleus to a final state with free, unbound, nucleons. The use of the Lorentz Integral Transform (LIT) turns the problem into one that is effectively a transition between two bound states. Such a problem can be solved using known techniques. The use of the technique removes the need for approximations that could mask the differences between N-N models.

9 Lorentz Integral Transform The calculations are time-consuming. Have been applied nuclei as heavy as beryllium. Initially only total photon absorption cross sections were calculated. 6 e.g. Li anything But now calculations are being extended to calculate the cross section for individual reaction channels. e.g. 6 5 Li Li n The calculations are complex, so theorists have little incentive to do them if the experimental data is of insufficient precision. I will focus on one example: 4 He.

10 4 He Photodisintegration Significant differences between potential models 4 He Total V UCOM AV18+UIX - -JISP Bacca, Phys. Rev. C 75 (2007)044001

11 4 He Photodisintegration X Berman et al. (1980) CBD Evaluation (1983) 4 He(,n) Berman et al. PRC 22 (1980) 2273

12 Photon Energy (MeV) 4 He Photodisintegration Cross Section (mb) He(γ,n) CBD Evaluation 1983 Berman et al Shima et al Nilsson et al Quaglioni et al Halderson 2004

13 Precision Absolute Cross Sections There is no value in making new measurements unless systematic uncertainties are shown to be under control. We need measurement with systematic uncertainties < 5%. We are now in a position to make such measurements.

14 Measuring a Cross Section N Target t Detector NObserved n = number density N Observed N nt Detector Efficiency Number of Reactions Cross Section

15 Measuring a Cross Section N Target t Detector NObserved n = number density N Observed N nt Statistical uncertainty. Can be made as small as practical Detector Efficiency Target Parameters. Can be very well known Number of Gamma Rays

16 Gamma Rays Traditionally created via Bremsstrahlung Electron beam, Energy E e Photons, E Problems: Not monoenergetic. Total flux difficult to measure. Difference methods can be used to extract yields as a function of energy, but these methods are prone to systematic errors. log scale Intensity E e E

17 Positron Annihilation in Flight Enhances number of photons at a particular energy. Still requires a difference to be taken Still requires the use of ion chambers for total gamma-ray flux measurement.

18 Tagged Photons Technique used at Saskatchewan Accelerator Laboratory (SAL) and now at Lund. Accurate counting of photons (in principle). Requires a tagging efficiency to be measured. Random coincidences need to be subtracted. E E 0 Ee

19 High Intensity Gamma Source (HIS) Duke University Free Electron Laser Facility

20 HIGS Operation

21 HIGS Operating Mode High-flux, quasi-cw operation Micro-pulses with sub-ns durations at 5.58MHz. (180 ns apart) Typical energy spread (FWHM): <5% Gamma-ray fluxes: ~0.5 x 10 8 s 1 Energy spread and flux depend on collimation of gamma-ray beam Smaller collimator better energy spread. Linear and circularly polarized photon beams

22 Photon Flux Monitor HIGS beam is not tagged. Pulsed at 5.58 MHz (180 ns between bunches) A direct counting photon detector with an efficiency known to better than 2% has been designed and commissioned. Low efficiency 1 2 % Very stable efficiency Insensitive to small changes in gain Wide energy range MeV Wide photon flux range up to 10 8 photons/s

23 Photon Flux Monitor Detects recoil Radiator electrons and positrons from a radiator. Described well with Photon a GEANT4 Beam simulation. Gains can be monitored by sampling paddle spectra. Scintillators Discriminators Recoil e + or e Monitor Output Veto

24 Photon Flux Monitor Data compared to GEANT4 simulation Coincidence of paddles 0, 1 and 2. Used for determining gain and threshold of paddle 1. Coincidence of paddles 2, 3 and 4 in anticoincidence with paddle 1. Black Measured Red Simulation Threshold

25 Photon Flux Monitor Inter-calibrated with a large NaI detector. Regularly during a measurement. N f Calib N Monitor Can determine N to better than 2%. (Pywell et at. NIM A 606 (2009) 517)

26 Blowfish Large solid angle neutron detector 88 BC-505 liquid scintillators Spherically arranged on a 16 inch radius. Covers ¼ of 4 sr. Pulse shape discrimination.

27 Blowfish GEANT4 Simulation

28 GEANT4 Simulation Simulation for Blowfish has been built using the GEANT4 toolkit (C++) Tracks particles through materials taking into account all possible interactions. Can determine: e.g. if particles are detected the energy deposited in detectors. Vital to the process of determining the detector efficiency.

29 Detectors n p BC-505 Liquid Scintillator Light Guide (Lucite) Photomultiplier tube n Neutrons are detected by recoil charged particles in the BC-505 liquid scintillator (mostly protons).

30 Detectors e - BC-505 Liquid Scintillator Light Guide (Lucite) Photomultiplier tube The detectors are also sensitive to gamma-ray photons through Compton scattering. Useful: when we want to calibrate the detector with a source of known energy -rays. Not Useful: when we want to measure neutrons against a background of -rays.

31 Pulse Shape Discrimination Fortunately the is a way to tell the difference between recoil protons (neutrons) and recoil electrons (photons). Because of the different way electrons and protons deposit energy in the BC-505, the resulting scintillation light has a different time structure. Signal from the photomultiplier: time neutron Long gate gamma PMT QDC QDC Short gate

32 Pulse Shape Discrimination (Long gate) (Short gate) PSD parameter Gammas Neutrons Detector Threshold

33 Time of Flight Most of the gamma rays that hit our detectors are beam photons that have been Compton scattered from the target. beam target 16 inches n The gamma beam from the accelerator comes in pulses which are 180 ns apart. The Compton scattered gamma rays reach a detector long before the slower neutrons.

34 Time of Flight 16 d(, n) O(, n) Time of flight of the neutrons from the target can be measured. From this the neutron energy can be found. e.g. With a D 2 O target the expected neutron energy can be calculated from the incident photon energy and the kinematics of deuterium breakup. the difference between the measured neutron energy and the expected neutron energy is plotted.

35 Detector Efficiency n p BC-505 Liquid Scintillator Light Guide (Lucite) Photomultiplier tube n For a given neutron energy there is a distribution of recoil proton energies up to the neutron energy.

36 Detector Efficiency Energy Deposited n p BC-505 Liquid Scintillator Light Guide (Lucite) Photomultiplier tube n Light Output The relationship between energy deposited and light output is not linear. Depends on particle type.

37 Light Output Response of BC-505 The scintillation light output is in general not simply proportional to the particle energy. Understanding the light output is vitally important to simulating the detector response accurately so that the efficiency can be calculated. Light output spectrum from 9.8 MeV tagged neutrons from the p n reaction, measured at TRIUMF. Experiment Simulation

38 Light Output Response of BC-505 Excellent fits to measurements have been obtained using the Chou parameterization. Light output for a particle of energy E stopping in a material with range R. L( E) f de dx ( de dx min min ) 0 R f de dx dx With, dl dx f de dx de dx 1 kb de dx C 2 de dx 1 Pywell et al. NIM A 565 (2006) 725

39 Detector Efficiency The detector efficiency is determined by integrating the light output spectrum from a hardware discriminator threshold (or software threshold) It is therefore vitally important to know the gain of a detector. Can be measured using a radioactive source. But drifts can occur during a measurement period.

40 Gain Monitoring System LED light pulser with a Fiber optic light distribution system. Monitored with a GSO scintillator and radioactive source. Does not depend on the stability of any components. Radioactive Source Pulser Monitor Detector Fibers LED & Fiber Distribution box Neutron Detector Cell Disc. Disc. Monitor ADC Cell ADC Data Acquisition System

41 45 Target for 4 He(,n) Experiment Currently being designed and constructed by our Ukraine collaborators He cell is a stainless steel can inside a H 2 filled tube Designed with a safety factor of 3 Gamma 1 2 Foil 10 mkm 3 4 H H 50 atm 2 50atm 4 He 50 atm L1 L2 L3 Vacuum H2 48 4He

42 Experience with 6 Li and 7 Li Complicated by numerous reaction channels Systematic uncertainties are introduced by the fitting Ward Wurtz PhD thesis Li

43 Experience with 6 Li and 7 Li Error bars include systematic uncertainties Total uncertainty between 3 5% 6 Li

44 Conclusion Precision photoneutron measurements are now possible Aiming for 3% systematic uncertainties Will address theoretical needs to evaluate Nucleon-Nucleon Interaction model. Approved for beam time at HIGS within this year.