D. M. Skopik

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.

Asymmetry ratio in pair production and the degree of linearly polarized photons at intermediate energies

Abstract In order to initiate the experiments using linearly polarized tagged photons at intermediate energies, it is imperative to know and to monitor the degree of polarization. The relationship is re-examined between the linear polarization of photons and the asymmetry ratio in pair production by such photons. An improved method is proposed in which pairs are prohibited from entering the cone region around the incident photon beam. By restricting the directions of pairs, the asymmetry ratio is much improved.

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.

Nuclear compton scattering from pion threshold to the delta

Abstract The first measurements of elastic photon scattering on complex nuclei from 150 MeV to the Δ(1232) (Eγ = 300 MeV) are compared with theoretical calculations. Data on 4He at the Δ peak are in agreement with a Δh calculation, but substantial disagreement occurs at lower energies that is at least partly due to the approximate treatment of non-resonant parts of the elementary nucleon amplitude. An impulse approximation calculation which includes meson exchange provides an adequate description of the data at 187 MeV on 4He. Improved data on the proton at 20–30° and 120–140° at a range of energies are in agreement with a recent dispersion relation calculation. Data taken on 12C are still too preliminary to report, due to the difficulty of eliminating inelastic contributions.

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.}

Study of γp → nπ+ very close to threshold

We are planning to use the Tagger facility at the Saskatchewan Accelerator Laboratory (SAL) to measure the total cross section for the reaction γp → nπ+ very close to threshold. The purpose is to extract the electric dipole amplitude Eo+(π+) to high accuray. The experimental techniques involve the use of a liquid hydrogen target and the detection of the reaction neutrons in coincidence with tagged photons.