Fotoprotoni dall’ossigeno

Riassunto

Si studia lo spettro dei fotoprotoni dall’Ossigeno esponendo targhette gassose ai raggi γ del betatrone di 31 MeV dell’Istituto di Pisica dell’Università di Torino. Sono stati eseguiti irraggiamenti ad energia massima di 23, 26, 30 MeV. L’incertezza nell’energia dei protoni per quanto riguarda lo spessore della targhetta va da 0.15 MeV per protoni di 2 MeV a 0.02 MeV per protoni di 15 MeV. Gli spettri a 23 e 26 MeV mostrano una struttura fine con della risonanze in assorbimento per E_γ= 18.8, 19.3, 20.6 e 22,2 MeV, in buon accordo con le risonanze trovate da altri autori. Gli spettri a 30 MeV mettono inoltre in evidenza altre risonanze a 24 (e 25.5) MeV. La resa/mole/roentgen fino a 30 MeV risulta ∼ il 20% più alta della resa fino a 26 MeV. Si dà l’andamento della sezione d’urto per protoni che lasciano¹⁵N solo nello stato fondamentale; si mette in evidenza il contributo dei protoni che lasciano¹⁵N in livelli eccitati, e si dà l’andamento della sezione d’urto tenendo conto anche di questi protoni.

Summary

The¹⁶0(γ, p) reaction has been studied by exposition of an oxygen gas target to the γ-rays of the B.B.C. Betatron of 31 MeV maximum energy. Expositions at 23, 26 and 30 MeV maximum energy have been made with the experimental arrangement shown in Fig. 1a. Good definition in proton energy (about 0.1 MeV) is obtained. The uncertainty for the half thickness of the target (1 mg/cm² equivalent to about 4 μm of nuclear C₂ emulsion) goes from 0.15 MeV for 2 MeV protons to 0.02 MeV for 15 MeV protons. Only tracks having 0 angle 90°≤ϕ≤ 115° with respect to the γ-rays (Fig. 16) have been examined. The background (< 2%) was ≤ 0.5% for protons having energyE _p> 5 MeV. The spectra at 23, 26 and 30 MeV (Fig. 2, 3 and 4) show the same proton groupsB, G, D, E, F, found by Cohen and others (3) by irradiation with bremsstrahlung of E_γmax = 25 MeV. Besides our 30 MeV spectrum shows the proton groupsG andH in the region of higher energy. As the first excited level of the residual nucleus¹⁵N isE ^x1= 5.3 MeV (Fig. 10) the protons having energyE _p > 15/16(E_{γ max}-E _s-E_x1), - whereE _γmaxis, the energy of irradiation andE _s,= 12.1 MeV the threshold of the O(γ,p) process leave¹⁵N in the fundamental state. Taking account only of these protons, from the experimental spectra (Fig. 2, 3 and 4) the cross-section for decay in the fundamental state of +¹⁵N-shown in Fig. 7A-is obtained and a giant resonance in photon absorption at E &#x03B3; = 22 MeV is found. The spectra show a fine structure with well resolved peaks at 18.8; 19.3 (=D);20.6 (=E); 22,2( = F); 24(=G) and 25.5 (=H) MeV. After normalization to the bremsstrahlung spectrum (Fig. 5 and 6) the difference (26 MeV spectrum - 23 MeV spectrum) (Fig. 8) and (30 MeV. spectrum - 26 MeV spectrum) (Fig. 9) shows a large contribution of photoprotons leaving¹⁵N in excited states. Taking account also of these protons, the mean cross-section in the interval 23/26 MeV and 26/30 MeV shown in Fig. 7B is derived. The shape of the crosssection (Fig. 70) is also derived taking account of the excited states of¹⁵N in approximate way. The yield (protons/mole/iöntgen) up to 30 MeV is about 20% higher than the yield below 26 MeV. In the E_γ=(26/30) MeV region the majority of the photoprotons leave₁₅N in excited states (Fig. 10). Contribution of protons leaving₁₅N in the low energy excited states and omitted by absorption of photons in the (23/26) MeV region is shown from difference of the spectra 26 - 23n (Fig. 8).