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Sceince advance, splitting.
    a  r   X   i  v  :  n  u  c   l  -  e  x   /   0   6   0   9   0   0   1  v   1   1   S  e  p   2   0   0   6 A. A. Vasenko, N. D. Galanina, K. E. Gusev, V. S. Demidov,E. V. Demidova, I. V. Kirpichnikov, A. Yu. Sokolov,A. S. Starostin, and N. A. Khaldeeva Formation of   24 Mg ∗ in the Splitting of   28 Si Nuclei by1-GeV Protons Institute of Theoretical and Experimental Physics,ul. Bol’shaya Cheremushkinskaya 25, Moscow, 117218 Russia The  28 Si(p,p ′ γ  0 X) 24 Mg reaction has been studied at the ITEP accelerator by thehadron-gamma coincidence method for a proton energy of 1 GeV. Two reaction prod-ucts are detected: a 1368.6-keV  γ  -ray photon accompanying the transition of the  24 Mg ∗ nucleus from the first excited state to the ground state and a proton p ′ whose momentumis measured in a magnetic spectrometer. The measured distribution in the energy lost bythe proton in interaction is attributed to five processes: the direct knockout of a nuclear α  cluster, the knockout of four nucleons with a total charge number of 2, the formation of the  ∆ Si isobaric nucleus, the formation of the ∆ isobar in the interaction of the incidentproton with a nuclear nucleon, and the production of a  π  meson, which is at rest in thenuclear reference frame. The last process likely corresponds to the reaction of the forma-tion of a deeply bound pion state in the  28 P nucleus. Such states were previously observedonly on heavy nuclei. The cross sections for the listed processes have been estimated.  1. INTRODUCTION. Stodolsky [1] proposed to use the coherent properties of an atomic nu-cleus for the suppression or enhancement of various nuclear reaction mech-anisms. The experimental verification of theoretical predictions [1] is dif-ficult, because it is necessary to separate nuclear states with excitationenergies  ∼ MeV when incident particle energies are ∼ GeV.As shown in [1-6], the fixation of the residual nucleus state allows theeffective separation of certain reaction channels. Some reactions can betreated as semicoherent, because certain transitions are associated withthe collective excitations of a certain group of particles.In the MAG magnetic germanium spectrometer [7], used in this ex-periment, prompt  γ  -rays from the transitions of excited nuclei to a statewith a lower excitation energy or to the ground state are detected. Thismakes it possible to determine the state of the residual nucleus includingits quantum numbers.In view of this circumstance, investigation of the formation of   24 Mg inthe collision of protons with  28 Si nuclei by the hadron-gamma coincidencemethod at the MAG spectrometer is of great interest, because the processcan be treated in terms of the  α  cluster model, the  28 Si target nucleus isthe  12 C– α  – 12 C nuclear molecule [8]. In this model, the quasi-free particlecan be a target for the incident proton and provide the direct knockout of particles from the nucleus in peripheral interactions.Central interactions of protons with the nucleus can lead to collectiveexcitations whose energies are higher than the discrete levels and giant res-onances [9]. The average excitation energy of the residual nuclei formed af-ter completion of the nuclear cascade induced by 1-GeV protons in the  28 Siis  ∼ 0.06 GeV and the excitation energy distribution expands to 0.4 GeVor higher.The reaction 28 Si(  p,p ′ γ  0 X  ) 24 Mg , (1)that is more general than the quasielastic knockout of particles was in-vestigated at the MAG spectrometer. Here, p ′ is the secondary leadingproton whose momentum is measured by the magnetic spectrometer,  γ  0  isthe photon accompanying the transition of the  24 Mg nucleus from the firstexcited state to the ground state (its energy is used to identify the finalnuclear state), and  X   is any combination of secondary hadrons (nucleonsand pions) with a total atomic number of 4 and a total charge number of 2.  The direct knockout reactions correspond to  X   =  α . Competitive cascadeinteractions of the proton with quasi-free nucleons in the nucleus that leadto the emission of four nucleons ( X  =2  p 2 n ) are simultaneously detected.In this work, reaction (1) is investigated in a wide energy-transfer rangeup to 0.8  T  0  , where  T  0  is the proton beam energy. This range includes notonly the excitations of discrete levels but also  π -meson and isobar nuclearexcitations. 2. EXPERIMENTAL PROCEDURE AND DATAPROCESSING The experiment was carried out with the universal beam of the ITEPproton synchrotron for  T  0 =1.02 GeV. Reaction (1) was studied by thehadron-gamma coincidence method using the MAG magnetic germaniumspectrometer [7] The silicon target ( 28 Si–92.23%,  29 Si–4.67%,  30 Si–3.1%)has a thickness of   x =6.18 g/cm 2 . Gamma-ray photons from the target weredetected by the germanium spectrometer based on a Ge(Li) crystal and theleading secondary charged particles (predominantlyhadrons) were detectedby the magnetic spectrometer based on a wide- aperture magnet. Thetrack part of the spectrometer consists of multiwire proportional chambersplaced in front of and behind the magnet. The momenta  p  of the chargedparticles were measured in the magnetic spectrometer with the accuracy dp p  = (0 . 32  p + 0 . 57)%, where  p  is measured in GeV. Angles were measuredwith an accuracy of 0.003 rad. The germanium spectrometer was adjustedfor detecting prompt photons emitted from the target perpendicularly tothe beam direction and for measuring their energy from 50 to 3000 keV.We selected 892 596 events with a  γ  -ray photon and one charged hadronemitted from the target at angle  θ <  15 ◦ to the beam direction. In thephoton-energy distribution of the events, a number of lines correspondingto certain  γ  -transitions in the reaction product nuclei are observed againstthe background. Twenty eight  γ  -transitions are identified for 19 productnuclei [10, 11]. To analyze reaction (1) of the formation of   24 Mg*, weconsidered range  A  of the  γ  -ray energies 1350-1380 keV including the line E  γ  0 =1368.6 keV of the transition of the  24 Mg nucleus from the first excitedstate to the ground state.Two factors distort the angular distributions of secondary protons de-tected in the magnetic spectrometer. The first factor is associated withrandom events induced by beam protons that do not interact in the tar-get. Small angles  θ  are characteristic of these events. The second factor  is the dependence of the detection efficiency of the magnetic spectrometeron  θ . This factor is associated with the limited sizes of track detectors andtheir rectangular geometry. This factor is noticeable for  θ >  6 . 5 ◦ and, if necessary, is taken into account by correcting coefficients. 3. DATA ANALYSIS FOR 3 ◦ < θ <  6 . 5 ◦ . In this region, the random-particle background can be disregarded andthe efficiency of the magnetic spectrometer can be taken as ≈ 100%. Figure1 shows the fragment of the energy spectrum of photons for these eventsnear the  E  γ  0 , line whose tabulated value is indicated by the arrow. Thehistogram part corresponding to range  A  are doubly shaded. The numberof events in this range is  N  2 =1965. The right and left shaded intervals  B ,are used to determine the background under peak. The spectrum fragmentis approximated by the sum of a normal distribution with the standarddeviation equal to the resolution of the  γ  -spectrometer [7, 11] and a linearfunction describing the background. The least squares fit are shown bythe smooth curves. The number of events of reaction (1) in interval  A  is N  3 =745 ± 73 whereas the background from the continuous is  N  b =1217.Figure 2 shows the distribution of events of reaction (1)in the energy ω  =  T  0 + M   p −   M  2  p  + P  2 ( M   p  – is the proton mass and  P   – is the scattered-proton momentum measured in the spectrometer) that is transferred by theproton interacting with the target nucleus. This distribution is obtainedby subtracting the background from the  ω  distridution of   ωN  2  events ininterval  A . The background distribution in  ω  is determined in intervals  B and is normalized to  N  b  events.The peak near  ω  ≈  0 corresponds to the interaction of beam protonswith quasi-free  α  clusters. Negative values appear due to the errors of themeasurement of the momentum  P   in the magnetic spectrometer. Events inthe range  ω =0.22–0.6 GeV, where the production of mesons is energeticallypossible, are attributed to the formation of the isobar and isobaric nuclearexcitation. The shape of the distribution of events with the maximum at ω 0  ≈ 0.145 GeV is characteristic of the production of a pion that is at restin the laboratory system or in the system associated with the nucleus.In order to describe the experimental data, the following processes aresimulated:1) proton scattering on the nuclear  α  cluster with the emission of the clus-ter from the nucleus,


Jul 23, 2017
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