Summary
Energy spectra of α-particles emitted under bombardment of (20÷30) MeV protons have been studied in many nuclei of masses ranging from 150 to 210. It is found that the α-spectrum is mainly due to the pre-compound emission mechanism, except for the nuclei near lead, where a strong component of direct effect is found. As the pre-compound emission mechanism here considered assumes the pre-formation of α-like clusters in the target nucleus, the reduction of this effect in nuclei near lead is attributed to a reduction of the α cluster pre-formation coefficient. In the frame of the pre-compound theory, the α cluster pre-formation coefficient is extracted from the absolute values of the reaction cross-section and compared with other results.
Riassunto
Si presentano qui i risultati sperimentali della misura di spettri α ottenuti dal bombardamento con protoni di energia compresa tra 20 e 30 MeV di nuclei di massa tra 150 e 210. Si trova che tali spettri possono essere spiegati sulla base dell’emissione di pre-equilibrio delle particelle α, eccezione fatta per i nuclei nell’intorno del doppio magico208Pb. In questa zona risulta dominante un tipo di meccanismo con caratteristiche simili all’effetto diretto, mentre è molto piccola la parte che si può attribuire all’emissione di pre-equilibrio. Poiché nell’emissione di pre-equilibrio si suppone che la particella α sia preformata nel nucleo bersaglio, si attribuisce la riduzione dell’emissione di pre-equilibrio alla riduzione del coefficiente di preformazione delle α nei nuclei. Questo coefficiente di preformazione viene estratto dalle misure sperimentali sulla base della teoria del pre-equilibrio e confrontato con altri simili risultati.
Реэюме
Сообшаются зкспериментальные реэультаты для реакций (p, α) на ядрах в области масс 140<A<232 для раэличных знергий падаюших протонов. Доминируюший механиэм представляет обычно предкомпаунд-иэлуч ение α-частиц, в то время как для тяжелых ядер вблиэи эамкнутой нейтронной оболочкиN=126 сушественным становится прямой зффект. Вероятность обраэования α-кластера в ядрах, которая выводится иэ величин поперечных сечений для предкомпаунд-иэлуче ния, обнаруживает глубокий минимум для ядер с эамкнутой оболочкой (N=126). Обшее поведение зтой вероятности аналогично поведению, получаемому иэ аналиэа естественной α-радиоактивности.
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This work has been performed under the I.N.F.N.-CISE collaboration programme.
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Milazzo-Colli, L., Braga-Marcazzan, G.M. & Milazzo, M. Further measurements of the probability of α cluster pre-formation by means of (p, α) reactions in heavy elements. Nuov Cim A 30, 632–652 (1975). https://doi.org/10.1007/BF02730490
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DOI: https://doi.org/10.1007/BF02730490