A. Zieger

180° Compton scattering by the proton below the pion threshold

Abstract Absolute cross sections for Compton scattering from protons have been determined at 180° for the backscattered photon at incident laboratory photon energies of 98 and 132 MeV. For the difference between the electric and the magnetic polarizability of the proton a value of (7.03 − 2.37 +2.49 − 2.05 +2.14 ) × 10 −4 fm 3 has been derived using the predictions from calculations based on relativistic dispersion relations.

The photodisintegration of deuterium at 0° for the outgoing protons

Abstract The absolute differential cross section for the photodisintegration of the deuteron at 0° for the outgoing protons has been measured at lab photon energies of 20, 30, 40, 75, 100 and 120 MeV using a magnetic spectrometer and a bremsstrahlung beam. The absolute cross-section values have an error of ±6% and were determined with an angular resolution of 1.5°. A serious discrepancy is found to exist between the measured 0° cross sections and those predicted by calculations using the Hamada-Johnston and the boundary condition potential models.

Photodisintegration of the deuteron at 15-25 MeV photon energy

Abstract The total cross section for the photodisintegration of the deuteron has been measured at 15, 20 and 25 MeV photon energy. The results agree with theory within the experimental errors of ± 5 % and do not confirm a 20 % discrepancy between experiment and theory as previously reported in the literature.

Polarizability of the neutron

Abstract A first measurement of quasi-free Compton scattering by the neutron bound in the deuteron has yielded a value for the polarizability of the neutron of αn=(11.7 −11.7+4.3)×10 −4fm 3. This means that a finite value for the polarizability has been obtained, though the result is still consistent with zero. The π0 decay constant Fπ 0γγ and the pion nucleon coupling constant gNNπ have opposite signs.

The forward, backward and 90° deuteron photodisintegration between 7 and 19 MeV photon energy

Abstract The 0° and 180°, as well as the 90° differential cross sections for the 2 H(γ, p)n reaction have been measured at lab photon energies between 7 and 19 MeV. Special attention was paid to the accuracy, in particular by measuring the forward Compton electron yield. For the extreme angles, the statistical error on our results amounts to 4–5% (0°) and 7–8% (180°), respectively, while the systematic uncertainty is at most 3%. The data confirm the existence of a minimum in the forward cross section and indicate beyond any doubt that the fore/aft ratio is larger than unity. Comparison shows reasonable agreement with the results from recent “conventional” theoretical approaches, including meson exchange and relativistic corrections, although the description of the c -coefficient, appearing in the Partovi expansion of the differential cross section, remains unsatisfactory. A possible source for this discrepancy could be the inadequate description of the El and/or the E2 transition operator.

Experimental values for the static nuclear polarizability

SummaryThe recent experimental determination of total photonuclear cross-sections σγT allows a model-independent calculation of static nuclear polarizabilities. The experimental values of this second-order nuclear-structure constant are compared to Migdal’s prediction of the static nuclear polarizability. This prediction is based on the assumption that the asymmetry energy taken from Weizsäcker’s mass formula is the essential parameter entering the expression for the restoring force in response to an external electric field. The experimental values of the nuclear polarizability for all elements investigated (A⩽40) were found to be roughly three times larger than Migdal’s polarizabilities and about two times larger than the values deduced from the partial cross-sections σγn even for the high-Z elements for which σγn should be very nearly equal to σγT.RiassuntoLa recente determinazione sperimentale della sezione d’urto totale fotonucleare σγT consente un calcolo delle polarizzabilità nucleari statiche indipendente dal modello. Si confrontano i valori sperimentali di questa costante della struttura nucleare di secondo ordine con la predizione di Migdal della polarizzabilità nucleare statica. Questa predizione è basata sull’ipotesi che l’energia di asimmetria dedotta dalla formula di massa di Weizsäcker è il parametro essenziale che entra nell’espressione della forza di bilanciamento in risposta ad un campo elettrico esterno. Si è trovato che i valori sperimentali della polarizzabilità nucleare per turri gli elementi analizzati (A⩽40) sono circa tre volte maggiori delle polarizzabilità di Migdal e due volte maggiori dei valori dedotti dalle sezioni d’urto parziali σγn anche per elementi con altoZ per cui σγn dovrebbe essere molto vicina a σγT.РеэюмеНедавние зкспериментальные иэмерения полных фотоядерных попе-речных сечений σγT поэволяют не эависяшим от модели обраэом определить ста-тические ядерные поляриэуемости. Экспериментальные эначения для зтой посто-янной второго порядка, характериэуюшей ядерную структуру, сравниваются с пред-скаэаниями Мигдала для статической ядерной поляриэуемости. Это предскаэание основано на предположении, что знергия асимметрии, полученная иэ массовой формулы Вейцэекера, представляет важный параметр, который входит в выра-жение для воэврашаюшей силы, воэникаюшей под действием внещнего злектри-ческого поля. Обнаружено, что зкспериментальные эначения ядерной поляриэуе-мости для всех исследованных злементов (A⩽40) почти в три раэа больще, чем поляриэуемости Мигдала, и почти в два раэа больще, чем величины, полученные иэ парциальных поперечных сечений σγn даже для злементов с больщимиZ, для которых σγn должно быть почти равно σγT.

The photodisintegration of the deuteron below the pion threshold at 180° for the outgoing proton

Abstract Absolute differential cross sections for the photodisintegration of the deuteron at 180° for the outgoing proton have been determined at photon laboratory energies between 30 and 100 MeV. The statistical uncertainty on the individual data points is about ±4%, while the overall systematical error amounts to ±3%. The new CM frame 180° cross sections do not seem to exhibit a minimum and are in good overall agreement with the predictions from modern calculations. Between 65 and 100 MeV the mean fore-aft ratio of 1.499±0.032 was obtained in excellent agreement with modern theoretical estimates.

Search for the minimum in the forward2H(γ,p)n cross section

The forward2H(γ,p)n cross section was measured at 7.2 and 8.5 MeV photon energy. The new absolute values confirm the theoretically predicted presence of a minimum in this cross section. A discrimination is made between the results from various theoretical descriptions.

Ion recombination and the response of a “P2”-quantameter

Abstract At high power densities ion recombination influences the response of an ion chamber to X-rays. This deviation from linearity is usually called “saturation”. In a National Bureau of Standards P2-type ionization chamber saturation effects have been investigated using an intense pulsed bremsstrahlung beam. The experimental results can be well represented by a general “saturation curve”, which enables the determination of the true primary ionization.

Forward photodisintegration of the deuteron between 20 and 100 MeV photon energy

Abstract Absolute differential cross sections for the forward deuteron disintegration have been determined at photon laboratory energies between 20 and 100 MeV. The statistical uncertainty on the individual results is about ±2.5%, while the overall systematical error amounts to 3%; the angular resolution equals 1.5°. The new CM-frame cross sections are in fair agreement with the predictions from recent theoretical calculations.