D. Vinciguerra

Energy dependence of the quasi-free9Be(3He, αα)4He reaction near the coulomb barrier

SummaryThe coincidence detection of two α-particles emitted in the9Be(3He, αα)4He reaction shows evidence for quasi-free processes even for low incident energies. At 2.5 and 2.7 MeV the impulse distribution of the α-cluster in9Be can be deduced in the PWIA. The excitation function has been obtained at θc.m. −90° together with that of the sequential reaction proceeding through the 16.6 and 16.9 MeV levels of8Be. The behaviour of the excitation functions for energies around the Coulomb barrier is also discussed.RiassuntoGli spettri bidimensionali della reazione9Be(3He, αα)4He rivelano la presenza di processi quasi liberi anche a bassa energia incidente. Dai dati ottenuti alle energie di 2.5 e 2.7 MeV è stata dedotta, in PWIA, la distribuzione di impulsi del cluster α nel9Be. La funzione di eccitazione del processo quasi libero, ottenuta aθc.m.=90°, è stata confrontata con quella relativa al processo sequenziale attraverso i livelli a 16.6 e 16.9 MeV del8Be.РеэюмеДетектирование совпадений двух альфа-частиц, испушенных в реакции9Ве(3Не, αα)4Не, укаэывает на кваэи-свободные процессы даже для ниэких начальных знергий. При 2.5 и 2.7 МзВ импульсное распределение альфа-кластера в9Ве может быть выведено в РWIА. Получается функция воэбуждения при ϑцентр масс =90° с Учетом того. что последуюшая реакция происходит череэ8Ве уровни 16.6 и 16.9 МзВ. Также обсуждается поведение функций воэбуждения для знергий вблиэи кулоновского барьера.

GRAAL: a polarized γ-ray beam at ESRF☆

The GRAAL facility produces a highly polarized gamma-ray beam by Compton scattering of laser photons on the electrons of the European Synchrotron Radiation Facility (ESRF) at Grenoble. Preliminary results have been obtained with the LAGRANGE detector showing its excellent performances.

The quasi-free 9Be(3He, αα)4He reaction between 3 and 12 MeV

The 9Be(3He, αα)4He reaction was studied in a kinematically complete experiment in the energy range E = 3–12 MeV in order to extract information about the energy dependence of the quasi-free process proceeding through the virtual 5He(3He, α)4He reaction. The excitation function of this process was extracted from the data and the momentum distribution of the 4He-5He motion in 9Be was deduced. The behaviour of the excitation function around and above the Coulomb barrier is discussed.

A procedure for the analysis of the data of a three body nuclear reaction

Abstract In a three body nuclear reaction, usually only two of the three reaction products are detected and identified. Complete information on the reaction is deduced by conservation laws. This procedures cannot be used when the reaction takes place on unknown target nuclei, e.g. on target contamination. A data treatment procedure is presented here which allows for the determination of the mass of the undetected particle and of the Q -value of the reaction.

Search for quasi-free effects in the9Be(d, tα)4He reaction at low momentum transfer

SummaryThe existence of quasi-free effects in reactions with low momentum transfer was tested by studying the9Be(d, tα)4He reaction at 2.5 MeV. In contrast to previous findings observed in similar reactions at higher momentum transfer, no definite evidence for quasi-free effects has been found.The alpha-clustering in light nuclei has been widely studied in recent years, in particular through quasi-free (QF) processes (1–20). In this framework we have studied QF effects in several reactions (10–19) on light nuclei and in particular on9Be.

Resonant effects in the12C+12C?8Be gs +16O gs reaction aroundE cm=32.5 MeV

The12C+12C→8Begs+116Ogs reaction has been studied at c.m. energies of 27.9, 32.5 and 35.0 MeV. The energy dependence of the cross section is consistent with a resonant behaviour like the one observed for two12C nuclei excited in a very deformed state in the final channel. The angular distributions for the two reactions show also a similar oscillatory behaviour. The analysis suggests that the reaction proceeds through the formation of an intermediate state with a complex nature, involving largely deformed configurations as well as almost spherical ones.

Investigation of the reaction mechanisms in the11B+p → 3α Reaction at 20 MeV

SummaryThe three-body reaction11B(p, αα)4He has been studied at 20 MeV incident energy and at different angular configurations with the aim of investigating the different reaction mechanisms. Coincidence measurements between two of the α-particles have been performed at the 73°/73°, 73°/83° and 23°/137° configurations in a planar kinematics. The results show the contribution of the sequential two-step process11B(p, α)8Be* → α + α proceeding through the8Be ground state and its first excited levels and the presence of a quasi-free process involving the virtual7Li(p, α)4He reaction in the framework of the α-7Li cluster structure of11B.RiassuntoLa reaziono a 3 corpi11B(p, αα)4He è stata studiata ad un’energia incidente di 20 MeV ed in diverse configurazioni angolari per investigate i possibili meccanismi di reazione. Sono state effettuate misure di coincidenza α-α nelle configurazioni 73°/73°, 73°/83° e 23°/137° in cinematica planare. I risultati mostrano il contributo del processo sequenziale11B(p, α)8Be → α + α clle procede attraverso la formazione del8Be nel suo stato fondamentale o nei suoi primi stati eccitati nonché la presenza di un processo quasi libero che avviene tramite la reazione virtuale7Iii(p, α)4He descrivibile nel contesto della struttura ad ammasso possibile α-7Li del11B.РезюмеИсследуется трехчастичная реакция11В(р, аа)4He при энергии падающего протона 20 МэВ и при различных угловых конфигурациях с целью изучения различных механизмов реакции. Проводятся измерения совпадений между двумя а-частицами при конфигурациях 73°/73°, 73°/83° и 237137° в плоской кинематике. Полученные результаты обнаруживают вклад последовательного двухступенчатого процесса11В(р, α)8Ве* → а+α, происходящего через основное состояние8Be и его первые возбужденные уровни, и наличие квазисвободного процесса, включающего виртуальную реакцию7Li(p, α)4He в рамках α-7Li кластерной структуры11В.

Excitation function of quasi-free processes in the6Li+6Li → 3α reaction

The6Li+6Li → 3α reaction was studied at energies around the Coulomb barrier. Excitation functions of the quasi-free processes6Li(d,α)4 He with an α-particle spectator either in the target or in the beam were measured.

The6Li(3He, pα)4He reaction at 13 MeV and the αd momentum distribution

SummaryThe6Li(3He, pα)4He reaction has been studied at 13 MeV incident energy. The detection configuration was chosen in such a way to deduce within the impulse approximation the momentum distribution of the ad motion in6Li. The angular-correlation method has been used. The FWHM of the momentum distribution is (55 ± 6) MeV/c. This result has been compared with the trend of the experimental widths obtained in quasi-free reactions, as a function of the transferred momentum.RiassuntoLa reazione6Li(3He, pα)4He è stata studiata a 13MeV, mediante una scelta opportuna delle conflgurazioni di rivelazione per dedurre, in approssimazione impulsiva, la distribuzione d’impulsi del moto ad nel6Li. È stato utilizzato il metodo della correlazione angolare. La FWHM della distribuzione d’impulsi ottenuta e di (55 ± 6) MeV/c. II risultato è stato confrontato con l’ andamento della FWHM sperimentale ottenuta, in reazioni quasi libere, in funzione del momento trasferito.РезюмеИсследуется реакция6Li(3He, рα)4Не при энергии падающих частиц 13 МэВ. Конфигурация детектирования выбирается таким образом, чтобы в рамках импульсного приближения вывести импульсное распределение движения α-d в6Li. Используется метод угловых корреляций. FWHM для импульсного распределения составляет (5 5 ±6) МэВ/с. Этот результат сравнивается с экспериментальными ширинами, полученными в квази-свободных реакциях, как функции переданного импульса.

Momentum distribution of alpha clusters in light nuclei from (α, 2α) reactions at high energy

SummaryThe data of the (α, 2α) quasi-free reactions measured (5) on light nuclei at 700 MeV have been reanalysed and fitted with calculations performed in the PWIA. The results are in agreement with previous information on the momentum distribution of α-particles in light nuclei. The conditions for the validity of the PWIA in calculating quasifree cross-sections at high energy and the criteria concerning the proper choice of the wave function for the intercluster motion ar also discussed.RiassuntoSono stati rianalizzati e confrontati con calcoli in PWIA i risultati di reazioni quasi libere del tipo (α, 2α) su nuclei leggeri a 700 MeV. Si è trovato che tali risultati sono in accordo con precedenti informazioni relative alla distribuzione di impulsi delle particelle α nei nuclei leggeri. Sono discusse anche le condizioni per la validità della PWIA nel calcolo della sezione d’urto per reazioni quasi libere ad energie elevate ed i criteri di scelta della funzione d’onda che descrive il moto relativo intercluster.РезюмеАнализируются данные для (α, 2α) квази-свободных реакций, измеренные на легких ядрах при 700 МэВ. Полученные данные сравниваются с вычислениями, проведенными в рамках PWIA. Полученные результаты согласуются с имеющейся информацией об импульсном распределении альфа-частиц в легких ядрах. Также обсуждаются условия применимости PWIA при вычислении квазисвободных поперечных сочений при высоких энергиях и критерии, касающиеся выбора волновой функции для внутрикластерного движения.