CAPTURE REACTIONS. by P. M. ENDT*) Physisch Laboratorium, Rijksuniversiteit, Utrecht, Nederland

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1 I Endt, P. M Physica XXlI Amsterdam Nuclear Reactions Conference Synopsis CAPTURE REACTIONS by P. M. ENDT*) Physisch Laboratorium, Rijksuniversiteit, Utrecht, Nederland Capture reactions will be considered here from the viewpoint of the nuclear spectroscopist. Especially important to him are the capture of neutrons, protons, and alpha particles, which may proceed through narrow resonances, offering a well defined initial state for the subsequent deexcitation process. Each of these particles mentioned above has its own advantages and disadvantages, largely depending on particle energy and on the mass number region in which one is going to work. The y-radiation produced at a resonance may be investigated through measurements of the y-ray spectrum and its angular distribution, and through coincidence and angular correlation measurements. The theory is well established, and has been fully supported by experiment in a large number of cases. Complications in the interpretation may arise from interference between different resonances (if these are broad), and from mixing of different channel spins, of different orbital momenta of the incoming particle, and of different y-ray multipolarities. Very fruitful have proved (p, y) reactions on light elements. They offer an almost ideal opportunity for spin and parity determinations of resonance levels and lower levels. From y-ray intensities it may be possible to test isobaric spin selection rules for E 1 radiation, or, inversely, to determine isobaric, spins. Any nuclear reaction may be studied for two reasons. The first reason ot interest may be the reaction process itself, but once enough is known about the process, the reaction may also be applied as a tool to study the final nucleus. The capture process has clearly passed to the second stage. The theory has been developed to such a point that the capture reaction has become a very useful instrument in nuclear spectroscopy. We shall first go into the question which particles, particle energies and mass number regions are most advantageous in this respect. 0nly capture of protons, neutrons and alpha particles has to be considered. Capture of deuterons or tritons leads to a highly excited compound state decaying through emission of particles and not of gamma rays. Many y-ray spectra have been investigated from (n, y) reactions making use of thermal pile neutrons. The best resolution has been obtained with a magnetic pair spectrometer (Ev > 3 MeV)1)and a magnetic Compton *) Paper read at the Amsterdam Nuclear Reactions Conference on July 4th,

2 CAPTURE REACTIONS 1063 spectrometer (Ev > 0.3 MeV)2). The low-energy spectrum has also been investigated with scintillation spectrometers. The best results v~ere obtained for the lighter elements because at high Z the y-ray spectra get very complicated. The (n, y) reaction has the disadvantage that the compound state may have the spin values Ji - 1/2 or Ji + 1/2 or a mixture of these two and thus is not uniquely determined (unless Jt = 0). This obviously presents difficulties in the interpretation of the observed y-ray spectra. Apparently it would be better to work with higher energy neutron beams but so far intensity reasons have prevented the investigation of y-ray spectra at single neutron resonances. Another disadvantage of the neutron work is the fact that slow neutrons are captured in s-states, which prevents spin determinations of lower states from y-ray angular distribution measurements. Let us now consider (p, y) and (~, y) reactions. Resonances are observed in an energy region which is limited on the low-energy side by the condition that the particle has to penetrate the Coulomb barrier, while on the highenergy side one should not go above the threshold for neutron emission. The condition that resonances be resolved experimentally imposes another highenergy limit, strongly depending on A. These conditions confine the observation of (p, y) and (~, y) resonances to light nuclei up to A ~-~ 40. As radiation widths are (with a few exceptions) of the order of at most a few ev yields are generally small, which has prompted the use of scintillation spectrometers rather than that of magnetic spectrometers for y-ray detection. The y-radiation produced at a specific resonance may be investigated through measurements of the y-ray spectrum and the angular distribution of its components, and through coincidence and angular correlation measurements. To exploit fully the possibilities of the method it is of advantage to investigate several resonances. Transitions to a specific lower level, which are absent at one resonance, may be of great intensity at another resonance. Valuable information on spins, parity and isobaric spins of the resonances may be obtained by exploring not only the radiation channel, but also the channels corresponding to emission of particles. For instance the (p, y) work is greatly strengthened by an investigation of the (p, p), (p, p'y) and/or (p, ~) reactions at the same resonances. To illustrate the usefulness of angular distribution measurements for spin determinations we take the 26Mg(p, y)lya1 reaction. The 3/2- resonance at Ep = MeV is deexcited through E1 radiation to lower levels in 2~A1 with spins 1/2 +, 3/2 + and 5/2+. The y-ray angular distribution is of the form 1 + A cos2~ with A = for J1 = 1/2, A = for Js = 3/2, and A = for Jf = 5/2. These A-values differ so much that no very good statistics are needed to decide on the spin of the final level. Complications in the interpretation of the experimental material may

3 1064 P.M. ENDT arise from a number of causes, as from interference between partly overlapping resonances, and from mixing of different channel spins, of different orbital momenta of the incoming particle, and of different y-ray multipolarities. Narrow, non-interfering, resonances are found at low bombarding energies, and at not too low Z, say above neon. Mixing of channel spins may be obviated either by working with alpha particles, or, in (p, y) reactions, by bombarding zero spin initial nuclei. There are other cases where only one channel spin can contribute. E.g. a 2- resonance level in the reaction 29Si + p (with J (Si ~9) = 1/2 +) can only be produced through a channel spin with J = 1. In cases where channel spin mixing has to be considered the channel spin ratio may often be determined by measuring the angular distribution of a y-ray transition proceeding from the resonance level to a lower level with known spin. Once the channel spin ratio is known angular distribution measurements of other y-rays deexciting the same resonance can be used for the determination of unknown spins. Almost the same considerations hold for mixing of orbital momenta of the incoming particle. It is absent for zero spin initial nuclei, and often negligible in many other cases. E.g. in the 29Si + p example given above one could have mixing of p- and ]-capture, but barrier penetration at low bombarding energies might bring down the ]-contribution to one percent or less of the p-capture intensity. We now still have to consider the mixing of y-ray multipolarities. Highly excited nuclear levels chiefly decay through the emission of dipole radiation, either E1 (unmixed), or M1 with possibly a small E2 admixture. This fact may be used to determine the parity difference of initial and final state from angular distribution measurements. If the measured angular distribution does not agree with the theoretical prediction for pure dipole radiation initial and final state have the same parity. However, in practice there are all too many cases where experiment shows no deviation from the pure dipole prediction, excluding conclusions concerning the parity difference. The best way to determine parities is to observe the polarisation of the emitted y-radiation, which has been done by searching for photo-protons in irradiated deuterium loaded nuclear emulsions. The method requires long exposures and tedious track counting. One could hope to obtain information on E1 or M l character from the radiation width. If in (p, y) reactions the resonance width F is small compared to the target thickness, the radiation yield of the resonance is proportional to (2J + 1) FpF~/F, where J is the resonance spin, Fp the proton width, and Fv the radiation width. If J is known from angular distribution measurements and Fp >~/'r, the radiation yield determines F~. The uncertaintly in theoretical estimates of /'p may be a factor of I0 or 20. A survey of measured radiation widths has been made by Wilkinson 8) for A < 20. He gives I'v/E~ for some 100 cases, where the El or M1

4 CAPTURE REACTIONS 1065 character is known with fair certainty. The two groups have a cor~siderable width and overlap largely. His only safe conclusion is that if Fr (ev)/e~ (MeV) > 0.02 the transition can be classified as E1 (but only ~-~ 40% of the observed E1 transitions fulfil this condition). One can thus say that as yet there is no unique criterion to distinguish between E1 and M1 transitions from the radiation width. It would be useful to make an analogous survey of the large amount of material available for nuclei with A > 20. Such a Ep. 339 key El:,./.54 key Ep. 661 key 8~898 Ep.?23 key /2 # &0 1~ 18 =_ l I I13 3 * t?) ~ V2 (5.6) /2 ' i 16 8 i! (2) tl) 0.8/,2 : ', V2 3,ot~ s,~ i 276p I! I I t t (01 5 ''2 Fig. 1. Gamma-rays from the reaction a6mg (p, y) ~VA1. survey might also give more information on the isobaric spin selection rule for E1 radiation (A T = i 1 when Tz = 0), which, at the moment, is still very scanty.

5 1066 P.M. ENDT As illustrations of the remarks made above some examples will be given of experimental work on (p, ~) reactions of the last two years. The 94Mg(p, ~) 2SA1 reaction has been studied in much detail at some ten resonances by Litherland, Paul, Bartholomew, and Gove4). They have collected evidence, notably from measurements of ~,-ray intensities and angular distributions, for E2 transitions competing favourably with M1 transitions of higher energy. The interpretation of these anomalies in terms of collective states in ~'SA1 will be discussed more fully by the authors themselves. Ep-5OO kev Ep.622 kev Ep=675 kev Ep-760 kev Ep-77B kev 79/~ s.o~ B.o,.3?'7?/,.~ ~; 71 ~ g$s ~5 z] 11 ls 12' 2S :11 SS la, 27 ~l I'-O t" m 3.7B ~6~ ~ ~J c3~ ~ o~ J i i I I I -T, T i (½),= is :4 31p Fig. 2. Gamma-rays from the reaction a0si (p, 7) sip. The interesting nucleus 26A1 with its long:lived T = 0, J ground state and an isomeric T = 1, J first excited state at 230 kev has been investigated with the ~SMg(p, ~)26A1 reaction. From angular distribution

CAPTURE REACTIONS 1067 measurements Green, Singh and Willmott 5) could confirm the spin assignments, which had been suggested from intensity considerations by Kluyver, van der Leun, and Endt6) and by

6 CAPTURE REACTIONS 1067 measurements Green, Singh and Willmott 5) could confirm the spin assignments, which had been suggested from intensity considerations by Kluyver, van der Leun, and Endt6) and by Kavanagh, Mills, and Sherr 7). Some resonances seem,to decay almost only to T states, while other resonances feed only T states, which suggests the operation of the E1 isobaric spin selection rule. At M.I.T. Browne s) has recently succeeded in detecting an alpha-particle group from the 2sSi(d, ~)26A1 reaction corresponding to a transition to level (I), w.t~l~ transition is forbidden by isobaric spin selection rules. The intensity is generally small but at certain angles and deuteron energies it may become comparable to the groundstate transition. Analogous results were obtained for the 160(d, ~)14N reaction. From angular distribution measurements 9) at four resonances in the 26Mg(p, ~)27A1 reaction spins have been determined of eight levels in 27A1 below 4.1 MeV (see Fig. 1). The same breakdown of isobaric spin selection rules which was mentioned above for the 160(d, ~)14N and 2sSi(d,,t)~6Al reactions, has now also been observed lo) for the ~2S(d, ~)s0p reaction. Transitions to the ground state and to the first excited state at 690 kev were observed with about equal intensities, although (p, ~) angular distribution measurement~ Lll) are only consistent with an assignment of J = 1 + (and probably T = 0) to the ground state and of J = 0 + (and probably T = 1) to the first excited state. Magnetic analysis 12) of protons scattered inelastically from phosphorus has yielded the excitation energies of six levels in 31p below 3.7 MeV (see Fig. 2). Measurements at Chalk River 13), Liverpool, and Utrecht of angular distributions of ~,-rays emitted in the 30Si(p, y)31p reaction have shown that level (I) has certainly spin 3/2 while level (2) has very probably spin 5/2. Level (2) is deexcited through a E2 transition to the J = 1/2 ground state rather than by a M1 transition to level (1). The same spin order has been observed 14) for the three lowest states in ~'gsi, where also level (2) is deexcited by an E2 cross-over transition rather than by a M1 cascade. Again this suggests collective motion effects. Short communications directly /ollowing this paper were read by Green, p. I139, Gore, p Paul, p. 114o and Akhiezer, p o/this volume. Received REFERENCES l) Kinsey, Bartholomew, and Walker, Phys. Rev. B3 (1951) 519, and Phys. Rev. B5 (1952) ) Groshev, Adyasevich and Demidov, International Conference Geneva (1955), and A. M. Demidov, private communication to C. M. Braams. 3) Wilkinson, D. H., Phil. Mag. 1 (1956) 127.

7 1068 CAPTURE REACTIONS 4) Litherland, Paul, Bartholomew and Gove, Phys. Rev. 102 (1956) ) Green, Singh and Willmott, Proe. phys. Soe. A 69 (1956) ) IKluyver, Van der Leun and Endt, Physica 20 (1954) Endt, Kluyver and van der Leun, Physiea 20 (1954) ) Kavanagh, Mills and Sherr, Phys.,Rev. 87 (1955) ) Browne, C. P., Bull. Am. phys. Soc. I (1956) ) Van der Leun, Endt, Kluyver and Vrenken, unpublished. I0) Lee, L. L. and Mooring, F. P., Bull. Am. phys. Soe. I (1956) 281. II) Broude, Green, Singh and Willmott, Phys. Rev. 101 (1956) I052, and Van der Leun, Van LoeI and Muller, unpublished. 12) Paris, C. H. and Endt, P. M., unpublished. 13) Paul, Bartholomew, Gove and Litherland, Bull. Am. phys. Soc. I (1956) ) Bromley, Gore, Litherland, Paul and Almqvist, Bull. Am. phys. Soc. I (1956) 30.

THE RESONANT SCATTERING INTEGRAL; APPLICATION TO THE ANALYSIS OF ELASTIC PROTON SCATTERING by PHILIP B. SMITH

- - 1085 - Smith, Philip B. Physica XXIV 1958 1085-1091 THE RESONANT SCATTERING INTEGRAL; APPLICATION TO THE ANALYSIS OF ELASTIC PROTON SCATTERING by PHILIP B. SMITH Fysisch Laboratorium der Rijksuniversiteit,