J.M. Vogt

The Origin and Dynamics of Soft X-Ray-Excited Optical Luminescence of ZnO

The distinct optical emission from ZnO materials, nanoneedles and microcrystallites synthesized with different sizes and morphologies by a flow deposition technique, is investigated with X-ray excited optical luminescence (XEOL) and time-resolved X-ray excited optical luminescence (TR-XEOL) from a synchrotron light source at the O K and Zn L(3,2) edges. The innovative use of XEOL, allowing site-specific chemical information and luminescence information at the same time, is fundamental to provide direct evidence for the different behaviour and the crucial role of bulk and surface defects in the origin of ZnO optical emission, including dynamics. XEOL from highly crystalline ZnO nanoneedles is characterized by a sharp band-gap emission (~380 nm) and a broad red luminescence (~680 nm) related to surface defects. Luminescence from ZnO microcrystallites is mostly dominated by green emission (~510 nm) associated with defects in the core. TR-XEOL experiments show considerably faster decay dynamics in nanoneedles compared to microcrystallites for both band-gap emission and visible luminescence. Herein we make a fundamental step forward correlating for the first time the interplay of size, crystallinity, morphology and excitation energy with luminescence from ZnO materials.

The photon tagging facility at the Saskatchewan Accelerator Laboratory

Abstract A photon tagging facility has been constructed for use with the c.w. electron beam at the Saskatchewan Accelerator Laboratory (SAL). The facility consists of a “clam-shell” type magnetic spectrometer with a 62 channel plastic scintillator focal plane detector allowing a post-bremsstrahlung electron energy resolution of 1%. Specialized focal plane and coincidence electronics have been designed and constructed at SAL and allows experiments to be performed with average tagged photon rates of greater than 1 × 108/s.

Response of a BaF2 detector to photons from 75 to 200 MeV

Abstract A large cylindrical BaF2, crystal 22.9 cm long and 12.7 cm in diameter, was calibrated with monochromatic photons from the tagged-photon facility at the Saskatchewan Accelerator Laboratory. The measured spectra for the energy deposited in the crystal from the resulting electromagnetic shower compares very well with predictions of the Monte Carlo shower simulation code EGS4. A method for establishing a transferable energy calibration using cosmic ray muons is also presented.

The 13C(γ,π−)13N reaction at Eγ≈ 184 MeV (Tπ ≈ 41 MeV)

Abstract Differential cross sections for the 13 C( γ , π − ) 13 N reaction at E γ ≈ 184 MeV ( T π ≈ 41 MeV) were measured with the Saskatoon tagged-photon facility and two range-telescope detectors. Data were obtained at pion angles of 45°, 60°, 80° and 100°, with emphasis on the forward angles, where the cross section shows a strong rise. The present data, together with existing ones at larger angles, are compared to several theoretical calculations.

SIMULTANEOUS MEASUREMENT OF THE (GAMMA ,P)/(GAMMA ,N) CROSS-SECTION RATIO IN 4HE AS A TEST OF CHARGE SYMMETRY

We have measured ([gamma],[ital p]) and ([gamma],[ital n]) differential yields at 90[degree] for the two-body photodisintegration of [sup 4]He using tagged photons of energy [ital E][sub [gamma]]=25--60 MeV. Data for both channels were obtained simultaneously using windowless [Delta][ital E]-[ital E] telescopes to detect the [sup 3]He and [sup 3]H recoils. The ration of our angle-integrated yields, which is insensitive to systematic uncertainies due to the simultaneity of the measurements, agrees with calculations employing only charge-symmetric nuclear interactions. Thus, within the present errors, our data show no evidence of a significant charge-symmetry violation in [sup 4]He in this energy range.

Igloo: a neutral pion spectrometer for low energy photoproduction studies

Abstract A π 0 spectrometer constructed from 68 lead glass Cherenkov counters has been installed on the tagged photon beam line at the Saskatchewan Accelerator Laboratory. It is being used for the investigation of neutral pion photoproduction from light nuclei within about 25 MeV of threshold. It can be configured for total cross section measurements with a large acceptance, or for angular distribution studies with a reduced acceptance.

A duty factor monitor for use with a near cw electron beam

Abstract A monitor was designed and constructed to determine the duty factor of the extracted beam from the pulse stretcher ring at the Saskatchewan Accelerator Laboratory. Two versions of the system have been built: A stand-alone duty factor monitor that is read by the accelerator control system to assess the beam quality, and a tagger duty factor monitor that is part of the photon tagger electronics. The design of this system is shown and the theory behind its operation is explained.

A Simultaneous Measurement of the (γ,n) and (γ,p) Reactions in 4He

A significant charge asymmetry in the nuclear force has been attributed to differences in the 4He(γ,n)3He and 4He(γ,p)3H cross sections below Eγ ∼ 35 MeV. To investigate this claim, we have performed a simultaneous measurement of these cross sections by measuring the recoiling 3He and 3H nuclei in the same detector system consisting of windowless ΔE proportional counters backed by silicon E detectors. We find, within our errors, that for 25 < Eγ < 35 MeV the ratio of the cross sections σ(γ,p)/σ(γ,n) is consistent with charge symmetry and with the predictions of shell-model calculations.

Measurement of the H-1 (gamma, pi0) cross-section near threshold

The cross section for the reaction {sup 1}H({gamma},{pi}{sup 0}) has been measured using tagged photons in the threshold region (144.7{endash}169.3 MeV). The total cross section, augmented by angular distribution information, is used to deduce the {ital S}-wave multipole {ital E}{sub 0+}. Extrapolation to threshold yields {ital E}{sub 0+}=({minus}1.32{plus_minus}0.05{plus_minus}0.06){times}10{sup {minus}3}/{ital m}{sub {pi}}, in disagreement with earlier estimates. Suppression of {ital E}{sub 0+} near the {pi}{sup +} threshold is confirmed, in approximate agreement with recent calculations. {copyright} {ital 1996 The American Physical Society.}

Reaction mechanisms in {sup 12}C({gamma},{ital pp}) near 200 MeV

Inclusive {sup 12}C({gamma},{ital pp}) cross sections have been measured with tagged photons in the range {ital E}{sub {gamma}}=187{endash}227 MeV using the Saskatchewan-Alberta Large Acceptance Detector (SALAD). The large angular acceptance allowed the measurement of noncoplanar {ital pp} emission. The cross sections were compared to a Monte Carlo intranuclear cascade calculation. Agreement was reasonable for the shapes of the cross sections but the calculated total cross section was 3.9 times larger than the data. {copyright} {ital 1996 The American Physical Society.}