Split-pole Spectrometer Research Articles

Sub-threshold states in Ne19 relevant to F18(p,α)O15

Classical novae result from thermonuclear explosions producing several $\gamma$-ray emitters which are prime targets for satellites observing in the MeV range. The early 511 keV gamma-ray emission depends critically on the $^{18}$F(p,$\alpha$)$^{15}$O reaction rate which, despite many experimental and theoretical efforts, still remains uncertain. One of the main uncertainties in the $^{18}$F(p,$\alpha$)$^{15}$O reaction rate is the contribution in the Gamow window of interference between sub-threshold $^{19}$Ne states and known broad states at higher energies. Therefore the goal of this work is to clarify the existence and the nature of these sub-threshold states. States in the $^{19}$Ne compound nucleus were studied at the Tandem-ALTO facility using the $^{19}$F($^3$He,t)$^{19}$Ne charge exchange reaction. Tritons were detected with an Enge Split-pole spectrometer while decaying protons or $\alpha$-particles from unbound $^{19}$Ne states were collected, in coincidence, with a double-sided silicon strip detector array. Angular correlations were extracted and constraints on the spin and parity of decaying states established. The coincidence yield at $E_x$ = 6.29 MeV was observed to be high spin, supporting the conclusion that it is indeed a doublet consisting of high spin and low spin components. Evidence for a broad, low spin state was observed around 6 MeV. Branching ratios were extracted for several states above the proton threshold and were found to be consistent with the literature. R-matrix calculations show the relative contribution of sub-threshold states to the astrophysically important energy region above the proton threshold. The levels schemes of $^{19}$Ne and $^{19}$F are still not sufficiently well known and further studies of the analogue assignments are needed. The tentative broad state at 6 MeV may only play a role if the reduced proton width is large.

Single-particle strength in neutron-richCu69from theZn70(d,He3)Cu69proton pick-up reaction

We have performed the $^{70}\mathrm{Zn}(d,^{3}\mathrm{He})^{69}\mathrm{Cu}$ proton pick-up reaction in direct kinematics using a deuteron beam at 27 MeV. The outgoing $^{3}\mathrm{He}$ particles were detected at the focal-plane detection system of an Enge split-pole spectrometer. The excitation-energy spectrum was reconstructed up to 7 MeV and spectroscopic factors were obtained after analysis of the angular distributions in the finite-range distorted-wave Born approximation. The results show three new angular distributions for which the $\ensuremath{\pi}{f}_{7/2}$ strength was measured and a lower limit of the centroid is established. State-of-the-art shell-model calculations are performed and predict a $\ensuremath{\pi}{f}_{7/2}$ strength that lies too high in energy in comparison to our experimental results.

Nucleosynthesis ofAl26in massive stars: NewAl27states aboveαand neutron emission thresholds

The ${}^{26}\mathrm{Al}$ radioisotope is of great importance for understanding the chemical and dynamical evolution of our galaxy. Among the possible stellar sources, massive stars are believed to be the main producer of this radioisotope. Understanding ${}^{26}\mathrm{Al}$ nucleosynthesis in massive stars requires estimates of the thermonuclear reaction rates of the $^{26}\mathrm{Al}$${(n,p)}^{26}$Mg, $^{26}\mathrm{Al}$${(n,\ensuremath{\alpha})}^{23}$Na, and $^{23}\mathrm{Na}$${(\ensuremath{\alpha},p)}^{26}$Mg reactions. These reaction rates depend on the spectroscopic properties of ${}^{27}\mathrm{Al}$ states above the neutron and alpha thresholds. In this context, the $^{27}\mathrm{Al}$${(p,{p}^{\ensuremath{'}})}^{27}$Al* reaction was studied at 18 MeV using a high-resolution Enge Split-Pole spectrometer. States from the ground state up to excitation energies of $\ensuremath{\approx}$14 MeV were populated. While up to the ${}^{23}\mathrm{Na}$ + $\ensuremath{\alpha}$ threshold no additional states are observed, we report for the first time 30 new levels above the ${}^{23}\mathrm{Na}$ + $\ensuremath{\alpha}$ threshold and more than 30 new states above the ${}^{26}\mathrm{Al}$ + n threshold for which excitation energies are determined with an uncertainty of 4--5 keV.

26Al g, m + p resonances studied via proton decays of 27Si

We used the Al(He,t)Si*(p)Al reaction at the Wright Nuclear Structure Laboratory to probe resonances of Al + p and Al + p for excitation energies of astrophysical interest. We used a beam of 25-MeV He ions produced by the ESTU Tandem at WNSL, and a target of 125 g/cm Al. Ejected tritons were detected at 0◦ in the gas ionization detector at the focal plane of the Enge Split-pole Spectrometer. Decay protons were detected in two position-sensitive silicon-strip detectors, located in the target chamber very close to the target, spanning an angular range of 55 − 125◦. We detected protons from the decay of states in Si as little as 600 keV above the proton threshold, populating the ground and first (metastable) and second excited states of Al. The excitation energies of states in Si that are resonances for Al + p and Al + p were determined. Work is ongoing to determine branching ratios for these decays, as well as resonance strengths, reaction rates, and consequences for stellar nucleosynthesis. The observation of Al in the galaxy [1] provided the first direct evidence for ongoing stellar nucleosynthesis. The gamma ray from the decay of Al is an observable that can be used to test astrophysical models of such nucleosynthesis sites as novae. These models use nuclear reaction rates as input. In order for the predictions made by the models to be reliable, the nuclear physics input also needs to be reliable: relevant reaction rates must be known to at least 20%. The Al(p,γ)Si reaction is important to the total amount of Al present in a star, since it is the only destruction mechanism of Al that competes with beta decay, thus preventing emission of the signature 1.8-MeV gamma ray. The rate is currently known to only about a factor of four. In addition to proton capture on the ground state, there could be proton captures on the low-energy metastable first excited state of Al. (Because this state has a very different spin from the ground state (0+ vs 5+), there is no direct communication between the two states, although at very high temperatures they can communicate via higher-lying states of intermediate spin; the states must therefore be treated as separate entities in most calculations.) No experimental information currently exists on Al(p,γ)Si. Any resonant contributions to this reaction rate could have a significant effect on the amount of Al synthesized in high-temperature astrophysical events.

Evidence for a change of structure in the heavy mercury isotopes around 200Hg

The (p, t) reaction has been performed on natural Hg and separated 196,198–202,204Hg targets, using a 25 MeV proton beam from the Orsay MP tandem accelerator and a split-pole spectrometer. In an excitation energy region below 2 MeV, new excited 0 + levels have been found. The enhancement factors of the populated 0 + states have been extracted. The resulting set of data exhibits striking discontinuities. In particular, both the energy and the (p, t) cross section of the 0 2 + level show a pronounced extremum in 200Hg. This observation, together with the previous experimental evidence, suggests a quite dramatic change in intrinsic structure of the Hg-isotopes at A=200. Microscopic calculations based on the deformed-shell-model approach suggest an explanation in terms of the Subshell closures at Z=80 and N=120 at weakly deformed oblate shapes, and a possible oblate-to-spherical shape transition. Qualitative theoretical arguments are confronted with the properties of pairing excitations calculated within a variant of the one broken-pair method. Within this approach the lowest excited 0 + states are interpreted as pairing vibrations. Energy positions of 0 + states and the pair transfer matrix elements are compared with the available experimental data.

A study of 25Ne via the 26Mg( 13C, 14O) 25Ne reaction

Abstract The mass of 25Ne has been determined from the Q-value of the reaction 26Mg( 13C, 14O) 25Ne at a 13C energy of 96 MeV. The 14O ions were momentum-analysed using an Enge split-pole spectrometer and detected at the focal plane in a multi-element, gas-filled detector. The resulting mass excess for 25Ne is −2.03±0.05 MeV. Excited states of 25Ne were observed at energies of 1.74±0.03, 3.33±0.03, 4.07±0.03 and 6.28±0.05 MeV. The widths of the states at 1.74 and 3.33 MeV suggest that they may be unresolved doublets, the levels being separated by approximately 110 and 200 keV, respectively. A qualitative discussion of the possible reaction mechanisms and of the nuclear structure of the observed states is presented and a comparison made with recent shell-model calculations.

A proton stripping study of92Nb

Levels up to energies of 4 MeV have been studied with the91Zr(3He,d)92Nb reaction with an incident particle energy of 25.5 MeV. The product deuterons were analyzed by a split-pole spectrometer. Angular distributions were obtained for many of the transitions and compared with DWBA calculations. The assigned transferl values and spectroscopic factors were used to assign shell model structures to these levels.

Direct transfer and two-step processes in the 42Ca(α, 3He)43Ca reaction

Abstract The 42 Ca(α, 3 He) 43 Ca reaction has been studied at 36 MeV incident energy. Angular distributions have been measured from 4° to 42° using a split-pole spectrometer and position sensitive Si detectors, for about 40 levels located up to 6 MeV excitation energy. A local zero-range DWBA analysis has been carried out; l = 3 and 4 assignments are tentatively proposed for levels located above 4 MeV excitation energy, indicating a strong fragmentation of the 1 f 5 2 strength between 4 and 6 MeV and the location of the main component of the 1 g 9 2 strength above 6 MeV. A number of weakly excited levels cannot be reproduced by DWBA analysis. Their angular distributions have been compared with the results of coupled-reaction-channel calculations assuming two-step excitation of weak coupling states with a [ 42 Ca ∗ ⊗ f 7 2 structure. A reasonable agreement has been obtained, confirming that the two-step process cannot be neglected in the analysis of the (α, 3 he) reaction.

Nuclear deformation in the actinide region by proton inelastic scattering

Elastic and inelastic proton differential cross sections have been measured for $^{232}\mathrm{Th}$ and $^{239}\mathrm{U}$ at incident energies of 20 and 26 MeV. Scattered protons were detected with a split-pole spectrometer using a position sensitive delay-line proportional counter. Proton groups up to the ${8}^{+}$ level in the ground state rotational band have been measured. The analysis of the data has been done using coupled-channel calculations for deformed nuclei. We have studied the sensitivity of the calculations to (a) the optical model parameters, (b) the shape of the nuclear charge distribution, (c) the coupling scheme assumed among the levels, and (d) the magnitude and sign ($\ifmmode\pm\else\textpm\fi{}{\ensuremath{\beta}}_{6}$) of the deformation parameters. The calculated multipole potential moments, obtained from the best fits to the data, agree well with the values deduced from proton measurements at other energies. They are also in good agreement with the values reported from Coulomb excitation and inelastic scattering using $\ensuremath{\alpha}$ particles and neutron probes.NUCLEAR REACTIONS $^{232}\mathrm{Th}$, $^{238}\mathrm{U} (p,p) (p{p}^{\ensuremath{'}})$, $E=26$ MeV; calculated $\ensuremath{\sigma}(\ensuremath{\theta})$; deduced deformation parameters and multipole moments of the potential. Coupled channel analysis.

Two-step processes in the 40Ca(α, 3He) 41Ca reaction

The 40Ca(α, 3He) reaction has been studied at 36 MeV incident energy. About fifty levels have been observed up to 7.1 MeV excitation energy and angular distributions were measured from 6–60° using a split-pole spectrometer. A local zero-range DWBA analysis has been carried out, and the deduced l-assignments and spectroscopic factors are compared with those obtained from previous neutron stripping experiments. Core-excited states in 41Ca with a [3 − ⊗ f 7,2], [2 + ⊗ f 7,2] and [5 − ⊗ f 7,2] component previously observed in inelastic scattering experiments, are selectively excited by the (α, 3He) reaction. Their angular distributions are compared with coupled-reaction-channel calculations, assuming a pure two-step reaction mechanism. The agreement between theory and experiment may be considered as rather satisfactory for a number of levels. In particular the 1 2 + and 3 2 + levels and the high-spin states with J π = 9 2 −, 11 2 +, 15 2 + and 17 2 + are successfully described within the framework of the weak-coupling model.

Single-particle and core-excited states in 49Sc : (I). The 48Ca( 3He, d) 49Sc reaction

The 48Ca( 3He, d) 49Sc reaction has been studied at 25 MeV incident energy. Angular distributions have been measured from 5° to 40° using a split-pole spectrometer, for about 160 levels located up to 18 MeV excitation energy. A local zero-range DWBA analysis has been carried out, using Gamow functions as form factors in the case of unbound states; l-assignments and spectroscopic factors are obtained for a large number of levels, most of them previously unknown. The summed experimental spectroscopic strengths for the T <, l = 1 and l = 3 levels are in good agreement with the shell-model sum-rule limits for 1f-2p proton states, and their energy centroids have been determined. The lg 9 2 strength in 49Sc is strongly fragmented: about 27% of the T < strength is carried by twenty-three levels located between 6.5 and 13.5 MeV. Spectroscopic factors for analog states are compared with those from previous (p, p), ( 3He, dp) and (d, p) experiments.

Single-particle and core-excited states in 49Sc : (II). The 48Ca(α, t) 49Sc reaction

The 48Ca(α, t) 49Sc reaction has been studied at 36 MeV incident energy. About eighty levels have been observed up to 7.5 MeV excitation energy and angular distributions were measured from 6° to 58°, using a split-pole spectrometer. A local zero-range DWBA analysis has been carried out, and the deduced l-assignments and spectroscopic factors are compared with those obtained from the ( 3He, d) reaction. In the other hand, a large number of angular distributions cannot be reproduced by the DWBA calculations; they have been compared with the results of coupled-reaction-channel calculations, assuming two-step excitation of weak coupling states with a [ 48 Ca ∗ ⊗ f 7 2 ] structure. Good agreement between experimental angular distributions and two-step predictions is obtained for several 49Sc levels, suggesting spin and parity assignments. Moreover, as rather large cross sections are predicted for two-step excitations, it is concluded that, generally, these processes cannot be neglected in the analysis of (α, t) reactions.

Study of the 58Ni( τ, α) 57Ni reaction at 25 MeV

The 58Ni( τ, α) 57Ni reaction has been studied at 25 MeV incident energy. Angular distributions have been measured from 5° to 50° with a split-pole spectrometer up to 13.5 MeV excitation energy. A local zero-range DWBA analysis has been carried out, allowing l-assignments for about eighty levels, most of them previously unknown. An isospin-dependent potential has been used in the calculation of the neutron form factor for both T <and T > states, and the C 2 S values deduced using this procedure are compared to those obtained with the usual separation energy method. Analog states of eleven 57Co levels have been identified and the eventuality for isospin mixing in 57Ni has been discussed. A sum rule analysis has been carried out and energy centroids of hole states have been determined. About 60% of the 1 d 5 2 and 2 s 1 2 T < strengths and the full 1 d 3 2 and 1 f 7 2 hole strengths are observed. It is shown that the two excess neutrons in the 58Ni ground state mainly populate the 2 p 3 2 , 1 f 5 2 and 2 p 1 2 orbitals, whereas the occupancy number of the 1 g 9 2 subshell is found to be smaller than 0.1%. Some non-pickup angular distributions have also been observed and a CRC analysis assuming two-step processes in the (τ, α) reaction and weak-coupling wave functions for final states has been attempted. Assignments of J π values are proposed for four 57Ni levels, based on this analysis.

Spectroscopic study of the N = 27 nuclei by means of the (τ, α) reaction

The (τ, α) reaction on 48Ca, 50Ti, 52Cr and 54Fe target nuclei has been studied at 25 MeV incident energy. Angular distributions have been measured from 5° to 40° with a split-pole spectrometer in a large range of excitation energy. A local zero-range DWBA analysis has been carried out, using an isospin-dependent potential for the calculation of the neutron form factor, in order to get a coherent set of spectroscopic factors for both T> and T< levels in different nuclei. Assignments of l-values have been done for a large number of levels, most of them previously unknown, and energy centroids of hole states have been determined. Spectroscopic factors in 39Ca, 47Ti, 49, 52, 53Cr have also been obtained for strongly excited states. A sum rule analysis has been carried out for the N = 27 nuclei: the 1d32 and 2s12T< hole strengths are generally fully exhausted by the observed levels, whereas only a fraction of the 1d52 strength has been evidenced. The 1f72, 1d32 and 2s12 analog states have been observed in all nuclei; in 53Fe, the 1d32 and 2s12 analog states appear to be split in several components. In addition, a CRC analysis has been carried out for some levels with angular distributions not accounted for by a direct pick-up process. These levels are tentatively identified with states resulting from the coupling of one f72 neutron hole with excited states of the target nucleus. In particular, the 52− and 92− members of the [2+ ⊗ f72−1] configuration have been identified in each final nucleus. Unambiguous Jπ assignments are made; and the two-step (τ, α) reaction therefo e appears as a useful spectroscopic tool, especially for investigating high spin states.

Inner hole states inPb207via thePb208(He3, α)Pb207reaction at 70 MeV

The $^{208}\mathrm{Pb}(^{3}\mathrm{He}, \ensuremath{\alpha})^{207}\mathrm{Pb}$ reaction at 70 MeV incident energy was used to populate hole states up to 28 MeV excitation energy in $^{207}\mathrm{Pb}$. A split-pole spectrometer was used for particle analysis and detection. For the excitation energy range between 0 to 4.5 MeV, about eleven well resolved states are excited. These levels contain the main part of the spectroscopic strengths of the $3{p}_{\frac{1}{2}}$, $2{f}_{\frac{5}{2}}$, $3{p}_{\frac{3}{2}}$, $2{f}_{\frac{7}{2}}$, $1{i}_{\frac{13}{2}}$, and $1{h}_{\frac{9}{2}}$ neutron subshells. Three additional regions of enhanced cross sections, centered at 5.5, 8.5, and 14 MeV excitation energy in $^{207}\mathrm{Pb}$, are also observed. The 65 keV energy resolution of the present study revealed fine structure in the peak previously observed around 8.5 MeV in $^{207}\mathrm{Pb}$. About fifteen levels or groups of levels are populated between 4.5 and 10.5 MeV, and angular distributions have been extracted for each individual peak as well as for the gross structures. A distorted-wave Born-approximation analysis of the data was carried out and shows that the missing $l=3 \mathrm{and} 5$ strengths in $^{207}\mathrm{Pb}$ are located in some strong peaks at 5.13, 5.62, and 6.37 MeV. Thus the full sum rule is obtained for the neutron subshells between $N=126$ and $N=82$. The highly fragmented peak located at 8.5 MeV excitation energy is shown to arise from $1{h}_{\frac{11}{2}}$ neutron pickup. Finally, the large structure, 5 MeV wide and centered around 14 MeV, observed for the first time in this work is tentatively interpreted as arising from neutron pickup from the $1{g}_{\frac{9}{2}}$ inner shell. The ${T}_{&gt;}$ components of the deeply-bound state are also observed between 20 to 24 MeV in $^{207}\mathrm{Pb}$ and therefore from the sum-rule analysis, centroid energies and total strengths for each inner subshell are deduced.NUCLEAR REACTION $^{208}\mathrm{Pb}(^{3}\mathrm{He}, \ensuremath{\alpha})$, $E=70$ MeV; measured $\ensuremath{\sigma}(E, \ensuremath{\theta})$. $^{207}\mathrm{Pb}$ deduced levels, ${E}_{x}$, $l$, ($J$), $\ensuremath{\pi}$, ${C}^{2}S$; inner shell spectroscopic factors; enriched target, magnetic spectrometer.

Levels ofGe70,72,74excited by the (p,t) reaction

The $^{72,74,76}\mathrm{Ge}(p,t)^{70,72,74}\mathrm{Ge}$ reactions have been studied at 26 MeV incident energy with an overall energy resolution of 10 keV using a split-pole spectrometer. Many new levels are found below 5 MeV excitation energy. Angular distributions are obtained and comparison with distorted-wave-Born-approximation calculations allow spin and parity assignments. Several new ${J}^{\ensuremath{\pi}}={0}^{+}$ levels are observed around 2.5 MeV. The positions of some ${2}^{+}$, ${3}^{\ensuremath{-}}$, ${4}^{+}$, ${5}^{\ensuremath{-}}$, and ${7}^{\ensuremath{-}}$ levels are established.NUCLEAR REACTIONS $^{72,74,76}\mathrm{Ge}(p,t)$, $E=26$ MeV; measured $\ensuremath{\sigma}=\ensuremath{\sigma}(\ensuremath{\theta})$; $^{70,72,74}\mathrm{Ge}$ deduced levels up to 4.5-5 Mev, $J$, $\ensuremath{\pi}$.

Spectroscopic study of 55Co via the 54Fe(τ, d) 55Co and 54Fe(τ, dp̃) 54Fe reactions

The 54Fe(τ, d) 55Co reaction has been studied at 25 MeV incident energy with a split-pole spectrometer. About one hundred levels have been observed in 55Co up to 10 MeV excitation energy. Angular distributions have been measured and analyzed with DWBA and Gamow functions as form factors for unbound levels. The 54Fe(τ,dp̃) 54 reaction has been investigated at 24 MeV incident energy. The angular distributions of the emitted protons were measured in coincidence using method 2 of Litherland and Ferguson, with 0 detection of deuteron groups. Spins, population parameters, branching ratios and proton partial widths for the transitions to the ground and excited states of 54Fe were determined from the analysis of the angular correlation data. The results of these two experiments provide a large number ofspectroscopic properties of unbound proton states and in particular of analog states of 55Fe low-lying levels. The IAS of the 3 2 − ground state of 55Fe is observed to be split between two individual levels. The amplitude of neutron coupling to the first 2 + excited state of 54Fe is obtained for the lg 9 2 and 2 d 5 2 low-lying parent statfes in 55Fe. summed spectroscopic factors and the centroid energies of the proton states in 55Co are obtained. A comparison is made with previous (τ, d), (d, p) and (p, p) results.

Deeply bound hole states and isobaric analog states in 89Zr

The 90Zr(τ, α) 89Zr reaction, at 39 MeV incident energy, was used to populate hole states up to 13.5 MeV excitation energy in 89Zr. A split-pole spectrometer was used for particle analysis and detection. In the excitation energy range between 0 and 3.5 MeV, about twenty well-resolved states were observed in 89Zr, many for the first time. These states contain almost all of the spectroscopic strengths of the 1g 9 2 , 2p 1 2 , 2p 3 2 and 1f 5 2 neutron Subshells. The 30–35 keV energy resolution of the present study revealed a fine structure in the bump previously observed around 4.5 MeV and tentatively identified as the main part of the 1f 7 2 . hole orbital. About thirty levels or groups of levels were excited between 3.5 and 7.0 MeV excitation energy and angular distributions have been extracted for each individual group as well as for the gross structure itself. A DWBA analysis of the data was carried out and shows that the main part of the observed fine structure is populated by l = 3 angular momentum transfer. The full sum rule strengths obtained for the states below 3.5 MeV for the pick-up of the 1g 9 2 , 2p 1 2 . 2p 3 2 and 1f 5 2 nucleon lead to the identification of this highly fragmented bump with the 1f 7 2 shell. We found between 50 to 70 % of the 1f 7 2 hole strength in this region. Four sharp and strongly excited peaks are observed at higher excitation energy (8–10 MeV) above a continuous background. They are identified with the known isobaric analog states of the first four excited levels in the 89Y parent nucleus. Around 13 MeV a weakly excited peak is observed which has an l = 3 angular distribution. This level is proposed as the IAS of the first component of the 1f 7 2 . proton hole orbital located around 5 MeV in 89Y. In order to compare the spectroscopic strengths of the IAS and of their parent states, the neutron form factor has been calculated by solving the Lane coupled-channel equations. This method is shown to produce an overall agreement between the strengths of the parent analog pairs of levels.

Levels ofGe68excited by the (p, t) reaction

The /sup 70/Ge(p,t)/sup 68/Ge reaction has been studied at 26 MeV incident energy with an overall resolution of 10 keV using a split-pole spectrometer. Forty-one /sup 68/Ge levels, among which twenty-eight are observed for the first time, are populated below 5.2 MeV excitation energy. Angular distributions are obtained, and comparison with distorted-wave Born approximation calculations allows spin and parity assignments. An interesting result is the discovery of the first excited 0/sup +/ level at 1.753 MeV as well as seven other 0/sup +/ levels above 2 MeV energy. Five J/sup ..pi../ = 2/sup +/ and four J/sup ..pi../ = 4/sup +/ levels are observed for the first time. We also establish the position of the first J/sup ..pi../ = 3/sup -/ level at 2.651 MeV and the possibility of a J/sup ..pi../ = 6/sup +/ level at 4.456 MeV excitation energy.

Increasing a spectrometer's solid angle without loss of resolution

We have increased the acceptance solid angle of an Enge split-pole spectrometer to 10 msr, maintaining good resolution by measuring θ to correct for the θ-dependent aberrations.