David A. Whitehouse

Development of a NaI(Tl) detector with superior photon energy resolution for use above 100 MeV

Abstract We have designed and tested a new high resolution NaI(Tl) total absorption scintillation counter. The detector is a cylinder composed of a 26.7 cm diameter by 55.9 cm long NaI core with a concentric 10.8 cm thick NaI annulus that is divided into quadrants. The NaI detector is surrounded by a 12.7 cm thick plastic scintillator to veto both cosmic rays and events with significant shower leakage from the NaI. High uniformity of light production and collection throughout the detector is required for superior resolution. The detector has a measured resolution of 1.3% and 1.7% FWHM for 130 MeV photons and 330 MeV electrons, respectively. Computer simulations to account for loss of resolution due to pileup and energy spread of the beam indicate that the ultimate experimental resolutions at these energies are 1.2±0.1% and 1.3±0.1%. The resolutions at these two energies are at least a factor of 2 better than that of any other total absorption scintillation counter available today. Based on shower simulations, the detector is expected to have a resolution of approximately 1.3% for collimated 130–2000 MeV photons.

["Measurement of the Λ→n+γ branching ratio"]

The branching ratio for the {Lambda} weak radiative decay has been measured to be {ital B}({Lambda}{r arrow}{ital n}+{gamma})/({Lambda}{r arrow}anything) =(1.78{plus minus}0.24(stat){plus minus}{sub 0.16}{sup 0.14}(syst)){times}10{sup {minus}3}. A low-energy kaon beam was used to produce the {Lambda} hyperons via the reaction {ital K}{sup {minus}}+{ital p}{r arrow}{Lambda}+{pi}{sup 0} at rest. Photons from the signal channel and {pi}{sup 0} decay were detected with a NaI(Tl) array. The final spectrum contains 287 events after background subtraction, an order of magnitude more events than from the only previous measurement.

Measurement of the. Lambda. r arrow n +. gamma. branching ratio

The branching ratio for the {Lambda} weak radiative decay has been measured to be {ital B}({Lambda}{r arrow}{ital n}+{gamma})/({Lambda}{r arrow}anything) =(1.78{plus minus}0.24(stat){plus minus}{sub 0.16}{sup 0.14}(syst)){times}10{sup {minus}3}. A low-energy kaon beam was used to produce the {Lambda} hyperons via the reaction {ital K}{sup {minus}}+{ital p}{r arrow}{Lambda}+{pi}{sup 0} at rest. Photons from the signal channel and {pi}{sup 0} decay were detected with a NaI(Tl) array. The final spectrum contains 287 events after background subtraction, an order of magnitude more events than from the only previous measurement.

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 measurement of the branching ratio for the∑+ →pγ decay

We have measured the weak radiative decay branching ratio (∑+ →pγ)/(∑+ →pπ0). The experiment was carried out at Brookhaven National Laboratory using a low-energy separatedK− beam stopping in liquid hydrogen. The∑+ was tagged by observing the pion fromK−K+p→∑+π− in a range telescope, and the gamma was detected in a segmented NaI detector. The result is (2.81±0.39−0.43+0.21)×10−3, which is consistent with previous measurements. This corresponds to a branching ratio for (∑+ →pγ)/(∑+ → all) of (1.45±0.20−0.22+0.11)×10−3, where the quoted errors are statistical and systematic, respectively. A total of 408 signal events were observed, doubling the previous world total.

Radiative hyperon processes

SummaryWeak as well as pure electromagnetic radiative decays of hyperons are discussed. The radiative decays of excited hyperon states can provide information on the quark wave functions inside the hyperons. The weak (strangeness-changing) decays represent the last low-q2 frontier of weak-interaction physics. The theoretical motivation of these studies is reviewed, as well as the existing experiments. Special emphasis is placed on recent experiments at the Brookhaven AGS and at Fermilab. Radiative K− at rest on the deuteron is also discussed, and new results from Brookhaven are presented.RiassuntoSi discutono i decadimenti radiativi elettromagnetici puri nonché quelli deboli. I decadimenti radiativi degli stati eccitati degli iperoni possono fornire informazioni sulle funzioni di onda dei quark all’interno degli iperoni. I decadimenti deboli (che cambiano la stranezza) rappresentano l’ultima frontiera a bassoq2 della fisica delle interazioni deboli. Si passano in rassegna la motivazione teorica di questi studi e gli esperimenti esistenti. Si pone particolare enfasi sui recenti esperimenti all’AGS di Brookhaven e a Fermilab. Si discute anche la cattura di K− radiativa a riposo sul deuterone e si presentano nuovi risultati di Brookhaven.РеэюмеОбсуждаются слабые и чисто злектромагнитные радиационные распады гиперонов. Радиационные распады воэбужденных гиперонных состояний могут дать информацию о волновых функциях кварков внутри гиперонов. Слабые (с иэменением странности) распады представляют последнюю нижнююq2 границу для фиэики слабых вэаимодействий. Предлагается теоретическая мотивация зтих исследований и сушествуюших зкспериментов. Особое внимание уделяется последним зкспериментам в Брукхейвене AGS и в лаборатории Ферми. Также обсуждается радиационный K−-эахват в покое на дейтроне и приводятся новые реэультаты, полученные в Брукхейвене.

Weak radiative hyperon decays

New measurements of the Σ+ and Λ weak radiative decays are discussed. The hyperons were produced at rest by the reaction K−p → Yπ where Y = Σ+ or Λ. The monoenergetic pion was used to tag the hyperon production, and the branching ratios were determined from the relative amplitudes of Σ+ → pγ to Σ+ → pπ0 and Λ → nγ to Λ → nπ0. The photons from weak radiative decays and from π0 decays were detected with modular NaI arrays.

Status of E811: Hyperon weak radiative decay

We report the status of branching ratio measurements of Σ+→p+γ and Λ→n+γ. For the Σ+ decay a preliminary result is presented.

Radiative kaon capture

Negative kaons were stopped in liquid hydrogen and liquid deuterium targets to study radiative capture rates. Branching ratios for the radiative kaon capture reactions K−p→Λγ and K−p→Σ0γ provide information about the quark structure of the Λ(1405). In the reaction K−d→Λnγ, the shape of the photon spectrum in the endpoint region provides information on the final state Λ‐n interaction. Superior photon resolution is needed to distinguish the signal photons from the background of π0 decay photons. Such resolution is provided by the B.U. NaI detector, which has a resolution of 1.3% (FWHM) near 300 MeV. A description of the experimental technique and data reduction is presented, along with preliminary branching ratio results.