G. S. Pappalardo

Fission of Bi, Pb, Au and Pt induced by a coherent photon beam from 1000 MeV electrons

SummaryThe photofission yields of Bi, Pb, Au and Pt induced by a coherent bremsstrahlung beam from 1000 MeV electrons striking a diamond single crystal have been measured. The experiment has been performed at sixteen different energies of the main peak of the photon spectrum, in the energy range between 220 MeV and 500 MeV, by detecting the fission fragments with glass sandwiches. All the photon spectra have been measured simultaneously to the exposures of the fissionable samples by means of a magnetic pair spectrometer and a real-time acquisition system with an on-line computer. An appropriate unfolding method was developed in order to deduce the behaviour of the photofission cross-section from the experimental yields. The estimated cross-section curves clearly show a first resonance at a photon energy of about 320 MeV with a FWHM ≈125 MeV, whilst there is only a hint of a second broad resonance centred atk≈750 MeV. The very good agreement between the position of the first resonance put in evidence in the present experiment and the energy of the first baryon resonance in the pion photoproduction is attributed to a predominance of the photomesonic mechanism in the photofission process of the investigated elements.RiassuntoSono state misurate le rese di fotofissione di Bi, Pb, Au e Pt indotta da un fascio di fotoni coerenti ottenuti per frenamento di elettroni da 1000 MeV su un monocristallo di diamante. I frammenti di fissione sono stati rivelati mediante sandwich di vetro. Il fascio di fotoni coerenti adoperato è caratterizzato da un picco principale «quasi monocromatico» sovrapposto ad uno spettro continuo. L’esperimento è stato eseguito a sedici differenti energie del picco principale nell’intervallo energetico compreso tra 220 MeV e 500 MeV. Poichè la posizione del picco principale di fotoni dipende criticamente dall’orientazione del monocristallo, ogni spettro è stato misurato contemporaneamente all’esposizione dei campioni fissionabili. Tale misura è stata eseguita mediante uno spettrometro magnetico e un sistema di acquisizione automatica in tempo reale con un calcolatore in linea. Allo scopo di dedurre l’andamento della sezione d’urto di fotofissione dalle rese sperimentali è stato sviluppato un appropriato metodo di deconvoluzione. Le curve ottenute per la sezione d’urto mostrano chiaramente una prima risonanza ad energia di fotoni di circa 320 MeV con una FWHM ≈ 125 MeV, mentre vi è soltanto l’accenno di una seconda risonanza centrata ad un’energia di circa 750 MeV. Si mette in evidenza come l’uso di un fascio di fotoni coerenti, adoperato per la prima volta in misure di fotofissione, ed il particolare metodo di deconvoluzione sviluppato hanno consentito di dedurre l’andamento risonante della sezione d’urto di fotofissione con maggiore attendibilità rispetto ai risultati della letteratura ottenuti con fasci di fotoni da targhette amorfe. L’ottimo accordo tra l’energia della prima risonanza, messa in evidenza nel presente esperimento, e l’energia della prima risonanza barionica nella fotoproduzione di pioni è ritenuto un’indicazione del prevalere del meccanismo fotomesonico nel processo di fotofissione per gli elementi studiati.

Fission of238U from 100 MeV to 1000 MeV by a coherent photon beam

SummaryThe cross-section per equivalent quantum in the fission induced in238U by a quasi-monochromatic photon beam has been measured. The photon beam was obtained by coherent bremsstrahlung from 1000 MeV electrons striking a diamond single crystal. In the experiment the energies of the main peak of the photon spectrum assumed sixteen different values in the range from 220 MeV to 500 MeV. The fission fragments have been detected by means of glass sandwiches containing thin targets of UF4. The cross-section per photon from 100 MeV to 1000 MeV has been deduced from the experimental yields applying an appropriate unfolding method after having subtracted from the yields the contribution due to photons with energies below 100 MeV. The obtained cross-section clearly shows a resonance centred at a photon energy 320 MeV with a FWHM ⋍125 MeV and a peak value of 70 mb. This resonance is in good agreement with the first baryon resonance in pion photoproduction. The energy dependence of the photofission cross-section for238U can be explained by both assuming that the photomesonic absorption mechanism prevails over the quasi-deuteron interaction in producing fission above 150 MeV and that the nuclear fissility increases with the energy.RiassuntoÈ stata misurata la sezione d’urto per quanto equivalente nella fissione dell’238U indotta da un fascio di fotoni quasi monocromatici ottenuti per bremsstrahlung coerente di elettroni da 1000 MeV con un monocristallo di diamante. Le misure sono state eseguite a 16 differenti valori del picco principale dello spettro di fotoni nella regione tra 220 e 500 MeV. I frammenti di fissione sono stati rivelati mediante sandwich di vetro contenenti bersagli sottili di UF4. La sezione d’urto per fotone è stata dedotta applicando un opportuno metodo di deconvoluzione alle rese sperimentali, corrette per il contributo dovuto all’assorbimento di fotoni con energia minore di 100 MeV. La sezione d’urto ottenuta mostra chiaramente una risonanza centrata ad un’energia dei fotoni di 320 MeV con una FWHM ⋍ 125 MeV ed un valore di picco di 70 mb. Questa risonanza risulta in buon accordo con la prima risonanza barionica nella fotoproduzione di pioni. L’andamento della sezione d’urto di fotofissione dell’238U può essere spiegato assumendo che il meccanismo di assorbimento fotomesonico prevalga sull’interazione a quasi-deutone nel processo di fissione sopra 150 MeV e che la fissilità nucleare cresca con l’energia.РеэюмеБыло иэмерено поперечное сечение деления, индуцированного в238U кваэи-монохроматиче ским фотонным пучком. Фотонный пучок получается с помошью когерентного тормоэного иэлучения 1000 МзВ злектронов, падаюших на монокристалл алмаэа. Предполагается, что в зксперименте знергии основного пика фотонного спектра имеют щестнадцать раэличных эначений с области от 220 МзВ до 500 МзВ. Фрагменты деления детектируются с помошью стеклянных сзндвичей, содержаших тонкие мищени UF4. Поперечное сечение на один фотон с знергией в области от 100 МзВ до 1000 МзВ выводится иэ зкспериментальных выходов после вычитания иэ них вклада, обусловленного фотонами с знергиями ниже 100 МзВ. Полученное поперечное сечение ясно покаэывает на наличие реэонанса, расположенного при знергии фотона 320 МзВ с ЕУНМ ⋍ 125 МзВ и величиной пика 70 мб. Этот реэонанс хорощо согласуется с первым барионным реэонансом при фоторождении пионов. Энергетическая эависимость фотоделения для238U может быть общяснена с помошью предположения, что механиэм фото-меэонного поглошения превалирует при кваэи-дейтронном вэаимодействии в процессе деления выще 150 МзВ и что делимость ядер увеличивается с знергией.

Light-Fragment Emission in Heavy-Ion Reactions Producing Pions and Protons in 16O + 27Al Collisions at 94 MeV/nucleon

Angular and multiplicity distributions of light-charged fragments (1 ≤ Z ≤ 8) have been measured at 94 MeV/nucleon incident energy: these distributions are rather similar when triggered either by a pion or by a high-energy proton. In both cases, they show drastic differences with inclusive-fragment production: higher multiplicities and flatter angular distributions as compared to the inclusive ones suggest that pion emission occurs through a violent collision process.

Evidence of a two-source emission for light charged particles in coincidence with pions produced in 16O + 27Al collisions at 94 MeV/nucleon

Abstract H and He ions have been detected in coincidence with charged pions in the reaction induced by 16O on 27Al target at 94 MeV/u incident energy. We analyse velocity spectra and cross sections of He ions emitted in the angular range 4°–150° in coincidence with charged pions detected at 90°. A two-source emission mechanism of the helium particles and a pion statistical production from an equilibrated participant zone is stressed. The absolute yields at different angles are compared with results of a theoretical model for medium energy heavy-ion reactions in the framework of a participant-spectator picture.

A study for the use of the LNS crystal ball multidetector as a neutral pions spectrometer

Abstract The response of the large BaF2 multidetector Crystall Ball to high energy photons and neutral pions was studied by simulations carried out with the code GEANT3. The properties of the detector relevant to π0 detection, such as resolution, linearity and efficiency are discussed. The performance of the system can be improved by considering a cluster of 8 detectors around the illuminated one to follow the development of the electromagnetic shower. As a π0 spectrometer the Crystal Ball covers the whole angular range and permits the detection of π0s over 100 MeV kinetic energy. Estimated π0 efficiencies vary between 60% for pio emitted at rest to 35% at 100 MeV kinetic energy.

16O breakup on Al, Ni and Au targets at 94 MeV/nucleon

Abstract The HeHeHeHe disassembly channel of the 16O on 27Al, 58Ni and 197Au targets has been studied at 94 MeV/nucleon. Charged particles have been detected by a multielement array of plastic scintillators covering the zenithal angular domain between 3° and 30° and the total azimuthal one. Standard relativistic kinematics has been used to reconstruct the excitation energy of the primary projectile-like nucleus. Mean values of the excitation energy are found independent of the target mass. The comparison with data taken at lower bombarding energies is compatible with a saturation of the mean excitation energy.

Light fragment emission correlated with large transverse momentum protons in 94 mev/u 16O induced reactions

Abstract Angular, multiplicity and velocity distributions as well as azimuthai asymmetries of light fragments (Z = 1 and 2) correlated with large transverse momentum protons detected at 90° have been measured in 16O + 27Al collisions at 94 MeV/u. Data are compared with a model based on the standard high-energy fireball geometry coupled with the Weisskopf theory of evaporation. Reasonable agreement is achieved with the exception of some discrepancies which could arise either from the absence of specific intermediate-energy corrections or from a non-statistical process.

Fission of Bi, Pb, Au, Pt, Be, W and Ta induced by photons from coherent and incoherent bremsstrahlung of electrons up to 1000 MeV

SummaryThe photofission yields of Bi, Pb, Au, Pt, Re, W and Ta have been measured by using a bremsstrahlung photon beam at maximum energies from 300 MeV to 1000 MeV. The fission fragments have been detected by means of glass sandwiches. The behaviour of the photofission cross-section has been deduced by applying an appropriate unfolding method to the results of the present experiment and to the data of previous measurements performed by a coherent photon beam. The cross-sections obtained for all the seven studied elements clearly show a first resonance at a photon energyk ≃ 340 keV and a second one centred atk ≃ 720 MeV, in good agreement with the position of the baryon resonances in the pion photoproduction. The resonant behaviour of the photofission cross-section is explained by assuming a photomesonic model of the photofission process and a nuclear fissility increasing with photon energy.RiassuntoSono state misurate le rese di fotoflssione di Bi, Pb, Au, Pt, Ee, W e Ta usando un fascio di fotoni da bremsstrahlung con energie massime fra 300 MeV e 1000 MeV. I frammenti di fissione sono stati rivelati mediante sandwich di vetro. L’andamento della sezione d’urto di fotoflssione è stato dedotto applicando un opportuno metodo di deconvoluzione ai risultati del presente esperimento e ai dati di precedenti misure eseguite con fotoni quasi monocromatici da bremsstrahlung coerente. Le sezioni d’urto ottenute per tutti gli elementi studiati mostrano chiaramente una prima risonanza ad energia dei fotonik ≃ 340 MeV ed una seconda risonanza centrata ad energiak ≃ 720 MeV, in buon accordo con la posizione delle risonanze barioniclie nella fotoproduzione di pioni da nucleoni. L’andamento risonante della sezione d’urto di fotofissione è spiegato ammettendo un modello fotomesonico del processo di fotoflssione ed una fissilità crescente con l’energia.РезюмеИзмеряется выход фотоделения Bi, Pb, Au, Pt, Re, W и Та, используя пучок тормозного излучения при максимальных энергиях от 300 МэВ до 1000 МэВ. Фрагменты деления детектируются с помощью стеклянных сэндвичей. Поведение поперечного сечения фотоделения выводятся посредством применения соответствующего метода развертывания к результатам настоящего эксперимента и к данным предыдущих измерений, проведенных с помощью когерентного фотонного пучка. Поперечные сечения, полученные для всех семи исследованных элементов, обнаруживают первый резонанс при энергии фотона k ≏ 340 МэВ и второй резонанс с центром при k ≏ 720 МэВ, что хорошо согласуется с положением барионных резонансов при фоторождении пионов. Резонансное поведение поперечного сечения фотоделения объяснается с помощью фотомезонной модели для процесса фотоделения и с помощью увеличения ядерной делимости с энергией фотона.

Fission cross-section of Re, W and Ta by a coherent photon beam from 1000 MeV electrons

SummaryThe photofission yields of Re, W and Ta induced by a coherent bremsstrahlung beam from 1000 MeV electrons striking a diamond single crystal have been measured. The experiment has been performed at eighteen different energies of the main peak of the photon spectrum, in the energy range between 220 MeV and 550 MeV, by detecting the fission fragments with glass sandwiches. The behaviour of the photo-fission cross-section has been deduced from the experimental yields by using an appropriate unfolding method. The obtained curves clearly show a first resonance centred at a photon energyk≃350 MeV with a FWHM≃145 MeV, while there is a hint of a second resonance atk≃750 MeV. Information on the energy dependence of the nuclear fissility from 100 MeV to 1000 MeV has been deduced from the comparison of the estimated photofission cross-section with the total photon interaction cross-section. It has been found that the photomesonic model of the fission process permits to explain the energy dependence of our photofission cross-sections if a nuclear fissility increasing with photon energy is assumed.RiassuntoSono state misurate le rese di fotofissione di Re, W e Ta indotta da un fascio di fotoni coerenti ottenuti per frenamento di elettroni da 1000 MeV su un monocristallo di diamante. Il fascio di fotoni è caratterizzato da un picco principale sovrapposto ad uno spettro continuo. Le misure sono state eseguite a 18 differenti energie del picco principale nell’intervallo energetico compreso tra 220 MeV e 550 MeV, rivelando i frammenti di fissione mediante sandwich di vetro. L’andamento della sezione d’urto di fotofissione è stato dedotto dalle rese sperimentali adoperando un appropriato metodo di deconvoluzione. Le curve ottenute mostrano chiaramente una prima risonanza centrata ad energie di fotonik≃350 MeV con una FWHM≃145 MeV, mentre vi è l’accenno di una seconda risonanza ad un’energia di ≃750 MeV. Dalla conoscenza della sezione d’urto di fotofissione sono state dedotte informazioni sulla dipendenza energetica della fissilità nucleare fra 100 MeV e 1000 MeV. È stato osservato che il modello fotomesonico del processo di fotofissione permette di spiegare la dipendenza energetica della sezione d’urto di fotofissione se si assume una fissilità crescente con l’energia.РезюмеЪыл измерен выход фотоделения Re, W и Ta когерентным тормозным фотонным пучком, образованным в результате взаимодействия электронов с энергией 1000 МэВ с монокристаллом алмаза. Эксперимент проведен при восемнадцати различных энергиях главного пика фотонного спектра, в области энергий от 220 МэВ до 550 МэВ, причем детектирование фрагментов деления производилось с помощью стеклянных сэндвичей. Из экспериментальных результатов определяется поведение поперечного сечения деления. Полученные кривые ясно указывают на первый резонанс при энергии фотонаk≃350 МэВ с FWHM≃≃145 МэВ, тогда как имеется лишь намек на второй резонанс приk≃750 МэВ. Из сравнения поперечного сечения фотоделения с полным поперечным сечением взаимодействия фотона получена информация об энергетической зависимости делимости ядер в области от 100 МэВ до 1000 МэВ. Установлено, что фотомезонная модель процесса деления позволяет объяснить энергетическю зависимость нашего поперечного сечения фотоделения, если предположить, что делимость ядер возрастает с энергией фотона.

The40Ar(d, p)41Ar reaction at deuteron energies below the Coulomb barrier

SummarySpectroscopic factors are obtained from stripping reactions at energies below the Coulomb barrier. Compound-nucleus effects are taken into account. The dependence of spectroscopic factors on the energy of the incident particle, on the nuclear optical parameters and on the geometrical parameters of the captured-neutron potential well is investigated in detail.RiassuntoSi sono misurate le sezioni d’urto differenziali delle reazioni40Ar(d, d)40Ar e40Ar(d, p)41Ar nell’intervallo energetico ΔEd=(1.5÷2.5) MeV. In queste condizioni si sono ottenuti i fattori spettroscopici per energie dei deutoni inferiori alle barriere coulombiane, tenendo conto anche degli effetti di nucleo composto. Si è studiata dettagliatamente la dipendenza dei fattori spettroscopici sia dall’energia delle particelle incidenti che dai parametri ottici dei canali di ingresso e d’uscita e dai parametri geometrici della buca di potenziale del neutrone catturato.РеэюмеОпределяются спектроскопические факторы иэ реакций стриппинга при знергиях ниже кулоновского барьера. Учитываются зффекты компаунд-ядер. Подробно исследуется эависимость спектроскопических факторов от знергии падаюшей частицы, от ядерных оптических параметов и от геометруческих параметров потенциальной ямы эахваченного нейтрона.