Γ-ray Yields Research Articles

Interference effects in 12C(p, γ) 13N and direct capture to unbound states

Excitation functions of the capture reaction 12C(p, γ 0) 13N have been obtained at θ γ = 0° and 90° and E p = 150–2500 keV. The results can be explained if a direct radiative capture process, E1(s and d → p), to the ground state in 13N is included in the analysis in addition to the two well-known resonances in this beam energy range [E p = 457( 1 2 +) and 1699 ( 3 2 −) keV] . The direct capture component is enhanced through interference effects with the two resonance amplitudes. From the observed direct capture cross section, a spectroscopic factor of C 2 S( l = 1) = 0.49 ± 0.15 has been deduced for the 1 2 − ground state in 13N. Excitation functions for the reaction 12C(p,γ 1p ∗) 12C have been obtained at θ γ = 0° and 90° and E p = 610–2700 keV. Away from the 1699 keV resonance the capture γ-ray yield is dominated by the direct capture process E1 (p → s) to the 2366 ( 1 2 +) keV unbound state. Above E p = 1 MeV, the observed excitation functions are well reproduced by the direct capture theory to unbound states (bremsstrahlung theory). Below E p = 1 MeV, i.e., E p → 457 keV, the theory diverges in contrast to observation. This discrepancy is well known in bremsstrahlung theory as the “infrared problem”. From the observed direct capture cross sections at E p ⪆ 1 MeV , a spectroscopic factor of C 2 S( l = 0) = 1.02 ± 0.15 has been found for the 2366 ( 1 2 +) keV unbound state. A search for direct capture transitions to the 3512 ( 3 2 −) and 3547 ( 5 2 +) keV unbound states resulted in upper limits of C 2 S( l = 1) ≦ 0.5 and C 2 S( l = 2) ⪅ 1.0, respectively. The results are compared with available stripping data as well as shell-model calculations. The astrophysical aspect of the 12C(p, γ 0) 13N reaction also is discussed.

Projectile coulomb excitation of 20Ne and 22Ne

The static quadrupole moments of the first excited J π = 2 + states in 20Ne and 22Ne and the reduced electric quadrupole transition probabilities of these states to the ground states were measured via projectile Coulomb excitation. The quadrupole moments were deduced from the shapes of γ-ray angular distributions. The results are: Q( 20 Ne, 2 +) = −0.20±0.05 b and Q( 22 Ne, 2 +) = −0.11±0.05 b . The transition strengths were deduced from yield measurements and by comparison with the yields of target γ-rays. The results are: B( E2; 0 + → 2 +, 20 Ne) = 0.037±0.003 e 2 · b 2 and B( E2; 0 + → 2 +, 22 Ne) = 0.025±0.002 e 2· b 2 . The results for the transition strengths are consistent with the results of accurate timing methods and resolve discrepancies between previous experiments. The results for the quadrupole moments are consistent with earlier measurements, although the mean values we obtain are slightly lower. The experimental measurements are compared with theoretical predictions and a detailed discussion is given of corrections to this type of reorientation experiment.

Spectroscopic factors from radiative capture reactions

The non-resonant γ-ray yield in radiative capture reactions is known to arise from a direct-capture process. Previous studies of this process in several light nuclei have been mainly concerned with astrophysical aspects. The present work demonstrates that this process can be in addition a useful tool in nuclear spectroscopy. The process selects states in the final nucleus which have parentage in the ground state of the target nucleus. The observed γ-ray angular distributions are characterized by the orbital angular momenta of the final states and the observed total cross sections reveal, when compared with model calculations, the spectroscopic factors for the final states. These features are similar to results derived from stripping reactions. The direct capture mechanism (electromagnetic interaction) is amenable to exact calculations in contrast to the more complex stripping reaction mechanism. The results for proton capture by 16O and 17O targets will be compared with available stripping data as well as shell-model calculations.

Gamma-decay of the lowest T = 2 state of 24Mg

The lowest T = 2 level of 24Mg was observed as a resonance in the γ-ray yield from the 23Na(p, γ) 24Mg reaction at E p = 3905±3 keV. Gamma-ray branching ratios for this J π = 0 +, T = 2 level at 15434±3 keV to the J π = 1 + , T = 1 states at 9967±2 keV and 10711±2 keV were measured to be (83±2) % and (17±2) %, respectively. The branching ratios for the decay of the T = 1 levels were also determined. The resonance strength was found to be ωγ = 1.9±0.5 eV. The M1 transition probabilities deduced are Γ γ (M1) = 0.8±0.3 Weisskopf units (W.u.) for the 15434 → 9967 keV transition and Γ γ (M1) = 0.27±0.09 W.u. for the 15434 → 10711 keV transition. A comparison is made between the Γ γ (M1) values deduced from the present experiment and the values calculated from the data on the analogue β-decay of 24Ne.

Excited States in 53Mn by the 53Cr(p,nγ)53Mn Reaction

The properties of the low-lying excited states in 53Mn were studied by measuring γ-ray angular distributions and lifetimes using the 53Cr(p,nγ)53Mn reaction. Energy levels of 53Mn have been found at 0.378, 1.288, 1.440, 1.619, 2.272, 2.405, 2.572, 2.670, 2.687, 2.705, 2.872, 2.876, 2.914, 2.947, 3.005, 3.095, 3.125, 3.183, 3.193, and 3.250 MeV. The measurement of γ-ray angular distributions and yield curves together with predictions based on the compound nuclear statistical model have enabled spin assignments to be made to the following excited states in 53Mn: 1.440 MeV (11/2), 1.619 MeV (9/2), 2.572 MeV (7/2), 2.687 MeV (7/2), 2.947 MeV (9/2), 3.005 MeV (5/2). Multipole mixing ratios and branching ratios were determined for some of the observed transitions. Lifetimes of nine excited states in 53Mn have been measured using the Doppler-shift attenuation method.

Average resonance spectroscopy in 93, 95, 97Tc

The (p, γ) reactions on the 92Mo, 94Mo and 96Mo isotopes were studied in the energy range E p = 2.4–3.0 MeV using a Ge(Li) detector incorporated into a three-crystal pair spectrometer and a Ge(Li) detector in coincidence with a NaI(Tl) detector. A three-crystal pair spectrometer was utilized to determine the average yield of γ-rays leading directly to the low-lying states in each of the three residual nuclei. These average yields were examined for a possible correlation between the intensity of high-energy γ-ray transitions and the J π of the final state. The data indicate that such a correlation exists. A two-dimensional coincidence experiment determined the decay scheme of the low-lying excited states in 93Tc 95Tc and 97Tc. The J π assignments inferred from these experiments are consistent with those for the states in 93Tc, 95Tc and 97Tc which have previously been obtained. For those states which were previously unassigned, the results of this experiment are as follows (all energies are in keV): 93 Tc-680 ( 7 2 +), 1193 ( 5 2 +), 1406 ( 1 2 −, 3 2 −), 1555 ( 1 2 −, 3 2 −), 2429 (≦ 5 2 ); 95Tc-1278 ( 3 2 +), 1619 ( 3 2 +, 5 2 −), 2556 (≧ 5 2 ); 97Tc-948 ( 1 2 −, 3 2 −), 1060 ( 5 2 ±, 7 2 +), 1138 ( 5 2 ±), 1366 ( 3 2 +), 1387 ( 3 2 +, 5 2 ∼), 1517 ( 1 2 −, 3 2 −), 1573( 3 2 +, 5 2 ±), 1676 ( 3 2 +, 5 2 ±) . The state at 2429 keV in 93Tc has not been reported previously.

Electromagnetic transition probabilities between the low-lying states of 203Tl, 205Tl and 209Bi

A high-accuracy investigation of absolute γ-ray yields and angular distributions after Coulomb excitation of 203Tl, 205Tl and 209Bi allowed the determination of B(E2) and B(M1) values in these nuclei. Some of the data obtained are compared with direct lifetime measurements and internal conversion data. The influence of deorientation effects on our results is discussed. A comparison is made between the experimental transition matrix elements and shell-model and core-coupling-model calculations. The “ l-forbidden” M1 transitions, which are caused by core-polarization effects, have strengths of ≈ 10 −3 W.u. In 209Bi the strength of the f 7 2 → h 9 2 . E2 transition is equivalent to a surprisingly large proton polarization charge of (2.8 ± 0.2) e.

Coulomb excitation of 73Ge and quasiparticle-phonon coupling theories

The excitation energies and γ-ray decay modes of several low-lying states in the nucleus 73Ge have been studied using Coulomb excitation with 2.6–4.0 MeV α-particles on an enriched 73Ge target to test the predictions of quasiparticle-phonon coupling theories, in particular to test if there are any high-spin states in the low-energy spectrum. Gamma-rays were observed with a 60 cm 3 Ge(Li) detector. Thick-target γ-ray yields were utilized to establish Coulomb excited levels, to clarify the γ-ray decay of 73Ge, and to obtain reduced upward transition probabilities, B(E2)↑, associated with the levels indicated as follows: 68.6 keV (0.073 ±0.007), 499.0 keV (0.0091±0.0005) and 825.6 keV (0.077±0.004). Angular distributions of the de-excitation γ-rays from the Coulomb excited levels were measured and various spin possibilities for each level were eliminated by the χ 2 technique. The following spinparity assignments are consistent with our γ-ray angular distribution measurements: 68.6 keV ( 5 2 + , 7 2 + , 9 2 + ), 499.0 keV ( 7 2 + ), and 825.6 ( 7 2 + , 13 2 + ). Branching ratios were extracted from the γ-ray angular distribution measurements. The εB(E2)↑ values for the first 2 + excited states of 72Ge(0.18), 74Ge(0.29), and 76Ge(0.27) were also obtained. The results for 73Ge are compared with the predictions of several quasiparticle-phonon coupling theories and some suggestions are made to improve agreement between experiment and theory.

Coulomb excitation of the excited 0 + state in 70Ge

Measurements have been made of the yield of γ-rays following the Coulomb excitation of 70Ge with 35Cl ions at energies from 60 to 83 MeV and with 32S ions at 75.7 MeV. A γ-ray of 175.9 ± 0.3 keV has been observed and has been associated with the decay of the excited 0 2 + state in 70Ge. A comparison of the yield of this γ-ray with the γ-ray de-exciting the 2 1 + state of 70Ge gives a B(E2; 2 1 + → 0 2 +) = 0.0158 ± 0.0024 e 2 · b 2, in reasonable agreement with previous lifetime measurements but in disagreement with a previous Coulomb excitation result. An attempt has been made to observe Coulomb excitation of the second excited 2 2 + state and thereby determine the latter's effect on the excitation of the 0 2 + state.

The decay of 114Ag

The decay scheme of 114Ag has been studied with Ge(Li) γ-ray detectors and a plastic β-ray detector. Of the 11 γ-rays observed in the decay, 9 were incorporated into the 114Cd level structure previously determined by conversion electron measurements made on the products from the 113Cd(n, γ)114Cd. reaction. The end-point energy of the β-decay was determined as 4.90 ± 0.26MeV; no branching was evident in the (β-spectrum. A decay scheme is proposed for which the β-branching has been deduced from the measured γ-ray yield and an extrapolated cross-section value for the 114Cd(n, p)114Ag reaction. The 114Ag half-life was determined as 4.52 ± 0.07sec; a search for a previously reported isomeric state of 114Ag was unsuccessful.

A measurement of the radiative width of the 8+ 11.948 MeV level of 20Ne

With the 4 He( 16 O,γ) 20 Ne capture reaction, the 8 + , 11.948 MeV level in 20 Ne was observed as a resonance in the yield of γ-rays corresponding to the 8 + → 6 + transition. A strength, ωγ = 104 ± 35 MeV was obtained for the resonance which gives 7.5 ± 2.5 W.u. for the 8 + → 6 + transition probability establishing the 11.948 MeV level as the 8 + member of the ground state band. The experimental E 2 transition probabilities in the ground state band of 20 Ne deviate strongly from those of the pure rotational model, but agree reasonably well with the predictions of the SU 3 shell model.

Coulomb excitation of the lead isotopes with α and 16O beams

The lead isotopes were Coulomb excited with α-particles of 15 and 18 MeV and 16O ions of 70 and 80 MeV. By measuring the absolute yields of the de-excitation γ-rays very accurately, B( Eλ) values for the first 2 + states in 204Pb and 206Pb, the 3 − states in 206Pb and 208Pb and of four states in 207Pb were obtained. The E2 transition strengths to the two first excited states of 207Pb are well described by coupling these states to a quadrupole core excitation of 208Pb. The B(E3) values for the excitation of the ( 3 − ⊗p 1 2 ) doublet show only slight deviations from the weak coupling value, yet the lifetimes of the two states, which were determined with the Doppler-shift attenuation method, are different from model predictions. The M1 transition rate of the 3 2 − state to ground as determined from the lifetime of 0.17 ps agrees with the value obtained from the B(E2) and the mixing ratio measurement.

Spectroscopy of states in 91Zr from γ-ray transitions observed in the 88Sr(α, nγ) 91Zr reaction

Levels below an excitation energy of 3.1 MeV in 91Zr and 3.6 MeV in 90Zr were studied through the prompt γ-radiation in the 87, 88Sr(α, nγ) 90, 91Zr reactions. Gamma-ray spectra were recorded in a 25 cm 3 Ge(Li) detector at 90° to the beam using 87, 88SrCO 3 targets and α-particle energies of 10.3 – 13.5 MeV. The γ-rays observed for transitions in 90Zr are consistent with the known level structure, although some more accurate energy values were measured. In 91Zr, a higher level density was observed in the 2.0 – 3.1 MeV region than had previously been revealed by charged particle reaction experiments. Spins for some of these states are suggested by comparing the variations in γ-ray yields with α-particle energy with Hauer-Feshbach calculations. A new level scheme is proposed for 91Zr which suggests that some previous interpretations of the low-lying states in terms of simple vector-coupled states of the 2d 5/2 neutron with 2 + and 3 − core states of 90Zr may need to be modified.

Couloub excitation of 197Au and a core-excitation analysis

Additional results for magnetic dipole and electric quadrupole transition moments have been obtained from the Coulomb excitation reaction on 197Au with 4He and 16O ions. The accuracy of earlier results from the Coulomb excitation reaction has been improved. The measurements included γ-ray yields, γ-ray angular distributions, and γ-γ coincidence spectra. The reduced E2 and M1 transition probabilities for all transitions among the low-lying levels 1 2 + ( ground state) , 1 2 + ( 3 2 +) 2 , 5 2 + , 7 2 + are compared with the predictions of de Shalit's core-excitation model of the nucleus. In particular, the data from the intramultiplet transitions have provided a more stringent quantitative test of the core-excitation model. The electromagnetic properties for 197Au are described rather well by the model. The model parameters for the core state are found to have values which are in accord with the experimental data for the electromagnetic properties of the 2+ state in the neighboring even- A nuclei.

Gamma-ray yields from the reaction D(p, γ)3He at low energies

Total cross sections and measurements of the small isotropic component of the γ-ray yield from the reaction D(p, γ)3He have been determined in the energy range from 57 to 1100 KeV using gas and deuterated polyethylene targets. The results are compared with some theoretical predictions.

Investigation of levels in 51Cr, 55Fe and 59Ni by means of (p, n) and (p, nγ) reactions

Abstract The level structures of 51 Cr, 55 Fe and 59 Ni have been investigated by yield and angular distribution measurements of neutrons and decay γ-rays from (p, n) reactions on 51 V, 55 Mn and 59 Co at E p ≦ 3.3 MeV. Accurate excitation energies as well as γ-decay schemes and branching ratios are presented. Several new spin assignments are made. In particular the low-lying 9 2 − and 11 22 − states predicted for odd-mass nuclei in the f 7 22 shell are identified in 51 Cr at 1165.0 ± 0.5 keV and 1479.1 ± 0.5 keV, respectively.

Time distribution of γ-rays from spontaneous fission of 252Cf

The gross time distribution and the absolute yield of γ-rays from spontaneous fission of 252Cf have been measured in the time range 0.1–120 ns after fission for γ-ray energies greater than 100 keV. The measurements have been made by the recoil distance method for the shorter times, electronically for the longer times and electronically with a shield between the source and the γ-ray detector for the intermediate times. The absolute yield at 120 ns has been found to be 11.6 ± 1.0 γ-rays per fission, of which 79 % are emitted within 0.1 ns, 11.6 % between 0.1 and 1.0 ns, 6.2 % from 1 to 10 ns and 2.9 % from 10 to 120 ns.

The static electric quadrupole moments of the first excited states of 144, 146, 148, 150Nd

Abstract From the relative yield of γ-rays deexciting the 2 + first excited states of 144, 146, 148, 150 Nd following Coulomb excitation by helium, oxygen and sulfur ions, it has been possible to measure the relative static quadrupole moments of these states. Ratios of the γ-ray yields for different projectiles were compared with the calculations of a Coulomb excitation coupled channels computer code. Absolute quadrupole moments of +0.03 ± 0.21, −0.63 ± 0.10 and −1.24 ± 0.14 e · b were obtained for 144, 146, 148 Nd, respectively, if the rotational value of −1.48 e · b is assumed for the quadrupole moment of 150 Nd. Best agreement with the quadrupole moments of 144, 146, 148 Nd measured by Crowley 1 ) is obtained assuming Q 2+ = −2.00 ± 0.51 e · b for 150 Nd. Corresponding to this value for 150 Nd, the quadrupole moments for 144, 146, 148 Nd are −0.07 ± 0.15, −0.78 ± 0.09 and −1.46 ± 0.13 e · b, respectively. A comparison with previously published data on the quadrupole moments for neighboring nuclei shows a smooth increase in the magnitude of the (negative) quadrupole moments of the first 2 + states as one adds pairs of neutrons or neutron holes to the closed neutron shell at N = 82. The static quadrupole moments for 148 Nd and 150 Nd exceed the rotor value, derived from the B (E2, 0 + → 2 + ), by about 37 %. Similar results have been observed in previous reorientation measurements on rotational nuclei and are not understood at the present time.

Static quadrupole moments of the first excited states of 58,60,62Ni

Measurements of the relative thick-target yields of γ-rays de-exciting the 2+ first excited states in 58, 60, 62Ni, following Coulomb excitation by carbon, oxygen and sulfur ions, have been used to determine the relative static quadrupole moments of these states as well as the relative reduced transition probabilities, B(E2; 01+ → 21). The measured yields were compared with calculations using a Coulomb excitation coupled channels computer code in order to extract the quadrupole moments and B(E2) values. The following results are obtained if the effects of higher excited states are ignored: Q(58Ni) − 0.94Q(60Ni) = − (0.122 ± 0.039)e · b, B(E2; 01+ → 21+)58Ni/B(E2; 01+ → 21+)60Ni = 0.783 ± 0.014, Q(62Ni − 1.11Q(60Ni) = − (0.075 ± 0.092)e . b, B(E2; 01+ → 21+)62Ni/B(E2; 01+ → 21+)60Ni = 0.964 ± 0.026. A previous experiment provided measurements of the absolute static quadrupole moment and B(E2) for 60Ni and these are used to obtain absolute values for 58Ni and 62Ni. The experimental results are compared with various shell-model calculations as well as macroscopic model predictions. Qualitative agreement between theory and experiment is obtained.

Heavy-ion reactions induced by 16O, 18O and 19F ions

Excitation curves have been determined from γ-ray yield measurements for heavyion reactions induced by E lab = 12–30 MeV 16O, 18O and 19F ions incident upon thick targets of 9Be, 10B, 11B, 12C and 23Na. The yields of radioactive decay products with half-lives greater than one second were measured; hence a large number of the outgoing reaction channels could be observed. The preponderance of heavy reaction products suggests compound-nucleus formation as the dominant reaction mechanism. Statistical-model calculations with a spin-dependent level density have been performed, in which the nuclear moment of inertia was treated as a parameter. Many of the results can be explained satisfactorily with a nuclear moment of inertia 0.55 to 0.7 of the rigid body value.