The neutron time-of-flight facility at the Manitoba cyclotron laboratory

NUCLEAR INSTRUMENTS AND METHODS 164 (1979) 129-141; (~) NORTH-HOLLAND PUBLISHING CO.

THE NEUTRON TIME-OF-FLIGHT FACILITY AT THE MANITOBA CYCLOTRON LABORATORY* J.W. WATSON t, F.J. WILSON +, C. A. MILLER§ and D.O. WELLS

Cyclotron Laboratory, Department of Physics, The University of Manitoba, Winnipeg, Canada, R3T 2N2 Received 31 July 1978 and in revised form 15 February 1979 A time-of-flight facility for measuring the spectra of fast neutrons produced in nuclear reactions is described. Flight paths of 10 m are available for laboratory angles between 10° and 170°. The performance for (p, n) and (p, pn) reaction studies is discussed.

1. Introduction The cyclotron, because it produces a naturally pulsed beam, is an accelerator well suited for timeof-flight work. Over the past several years a timeof-flight facility for neutron energy spectrum measurements has been developed at the University of Manitoba Cyclotron Laboratory. This facility has been used primarily for the study of (p, n) and (p, pn) reactions on light nuclei. In this paper we will discuss the physical layout of the facility and some of the apparatus and capabilities we have developed for neutron time-of-flight measurements. 1.1. THE UNIVERSITYOF MANITOBA CYCLOTRON

The University of Manitoba cyclotron ~) is an isochronous, sector focused, negative ion machine. Extraction of the beam is achieved by stripping the ions from negative to positive charge states using a thin foil. The cyclotron is generally operated with a fixed rf frequency and a fixed magnetic field configuration; the beam energy is. varied by changing the radius and azimuth at which the stripping foil is positioned. To date, experiments have been performed only with proton beams. External proton beam energies between 20 and 50 MeV are available, the lower limit being fixed by extraction geometry, while the upper limit is fixed by the radius of the magnet pole tips. The cyclotron is now operated exclusively with axial injection 2) of Research supported by the Atomic Energy Control Board of Canada. t Present address: Department of Physics, Kent State University, Kent, Ohio 44242, U.S.A. + Present address: Vincent Massey High School, Winnipeg, Manitoba, Canada. § Present address: Nuclear Research Centre, University of Alberta, Edmonton, Alberta, Canada V6T lW5.

beams from external ion sources. Beam of deuterons and of polarized protons should soon be available, on a regular basis, in addition to the unpolarized proton beams now available. External beam currents for unpolarized protons are typically 1-5/~A before m o m e n t u m analysis. For proton beams, the cyclotron is operated at a fixed frequency of 28.48 MHz, hence the rf period is always 35.1 ns. Because the cyclotron is very compact ( - 5 3 cm extraction radius for 50 MeV) single turn extraction is probably impossible. Thus use of a "gated ion source ''3,4) to eliminate ( n - 1 ) out of n beam bursts would probably not succeed, in as much as single turn extraction is required for such schemes to be successful. 1.2. TIME TUNING THE CYCLOTRON

The cyclotron can be "time tuned" for a very narrow burst width, typically 500 ps or less, by fine adjustments of the magnetic field and of ion source and rf components, the procedure being largely empirical. A systerfi has been developed 5) for measuring the beam burst width and relative rf phase on target, by detecting elastically scattered protons with a fast plastic scintillator. The pulse height in the scintillator is USed to select elastic scattering events, while the time structure of the beam is determined by timing these events against the cyclotron's rf using a time-to-amplitude-converter (TAC). In addition the beam phase on target relative to the cyclotron's rf can be stabilized using computer controlled feedback to the cyclotron's main magnet power supply. The overall performance of the system has been quite satisfactory both for time tuning and for monitoring and stabilizing the beam time structure during an experiment. A complete description of the system is given in ref. 5.

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2. The neutron time-of-flight facility The Cyclotron Laboratory at the University of Manitoba is in a sub-basement annex to the building housing the physics department. This annex extends under a parking lot. Being underground imposes restrictions on the flight paths available, but given the relatively short spacing between beam bursts and the narrow beam burst widths available, this has not been a problem.

131

1.27 cm acrylic plastic face plates, and the beam pipe immediately up stream of B has a 127 a m Kapton H window 5.08 cm high by 91.4 cm long. Thus corrections for the attenuation of the neutron flux from any one of the three target positions are small.

2.1. LABORATORYFLOOR PLAN Fig. 1 shows the flOOr plan of the cyclotron laboratory. The beam line used for neutron time-offlight work is on a 15° bend to the left from the switching magnet in the cyclotron vault. The large open area between the "15 ° left beam line" and the "high resolution beam line" is used for neutron flight paths. There are three positions on the 15° left beam line where targets can be placed: a 41 cm scattering chamber (described in detail below) designed specifically for neutron physics work, and two simple targets stations " A " and " B " adapted from beam pipe junction boxes. There are three quadrupole doublet magnets on this beam line (QC1, QC2), (QC3, QC4) and (QCS, QC6) which can be used to obtain a beam focus at any one of the three target positions. In addition, if a target is placed in the 4 1 c m chamber or target position A, then (QC3, QC4) or (QCS, QC6) can be used to regather the beam and focus it into the Faraday cup. By using three target positions, flight paths of 10 m or more are available for all laboratory angles between 10° and 170 °, except for those angles blocked by the pillar supporting the ceiling between the 15° left line and the high resolution line. The Faraday cup (made of carbon, insulated with teflon), is located N4 m along a shaft bored - 7 m into the clay outside the laboratory walls. The position of the Faraday cup greatly reduces the room background neutron flux produced in stopping the beam. In addition, the 3 m of dead space past the Faraday cup decreases the probability that neutrons emitted at forward angles from the Faraday cup will scatter back along the beam line. To minimize attenuation of the neutron flux between target and detector, the 41 cm chamber has a window of 127 a m Kapton H* foil 3.81 cm high around most of the side facing the high resolution line. In addition, target boxes A and B have

2.2. PROTON BEAM ENERGY MEASUREMENT The energy of the proton beam o n the 15° left beam line can not be determined accurately from the magnetic field in the switching magnet because of the small angle of bend. We have therefore developed a time-of-flight method for determining the proton beam energy on this beam line. A small 2.5 cm by 2.5 cm cylindrical NE102* plastic scintillator mounted on a RCA 8575 phototube is positioned at a point near target box A on fig. 1. (The location of this point has been carefully surveyed.) Carbon screens are lowered into the cyclotron beam, first in a beam pipe junction box just upstream from the 41 cm scattering chamber, and then in target box B. Time-of-flight spectra relative to the cyclotron's ff are taken for the y-flashes from the two carbon screens with the pulse height threshold on the scintillator adjusted so that most of the observed ~,-rays are from the decay of the 4.44 MeV 1st excited state of 12C. The time reference from the rf is adjusted to provide a signal every second rf period, and thus a time spectrum containing two rf periods. Fig. 2 shows a typical set of spectra from such measurements. By comparing the time difference At of the 7-flashes from the two screens (called S1 and $2 in fig. 2) and knowing accurately the sides of the triangle formed by S1, $2 and the scintillator, the time-of-flight for the proton beam from S1 to $2, and hence the beam energy can be determined. The time-of-flight from S1 to $2 (which are separated by 11.02 m) varies between 3 and 5 rf periods for the proton beam energies available with the Manitoba cyclotron. Thus if the scintillator is positioned correctly to within 1 cm, a determination of At to within 100 ps will determine the beam energy to within 25 keV at 20 MeV or 100 keV at 50 MeV. The beam energy spread on this line is typically 0.5-1.0%. A measurement of the beam energy during an experimental run, using this system, typically takes 15 min, including the time to compute the beam energy from /ft.

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2.3. NEUTRON DETECTORS For our program of (p, n) and (p, pn) measurements, we have fabricated several neutron detectors of a "standard" design. Each detector consists of an l l . 4 c m diameter by 12.7cm long cylindrical capsule of NE213* liquid scintillator coupled to a 11.4 cm diameter RCA 4522 photomultiplier tube. A short acrylic plastic light pipe is used to couple the fiat glass window on the scintillator capsule to the spherical face plate of the photo tube. The inter* Supplied by Nuclear Enterprises, Inc.

ior of the scintillator capsules are coated with isotropically reflecting titanium dioxide paint. The liquid NE213 was selected because of its excellent "pulse shape discrimination" (PSD)properties, which provide a means for distinguishing events induced by neutrons from those induced by y-rays or cosmic ray muons. PSD was achieved by standard techniques, using the time difference between a fast signal derived from the anode output and the crossover time of a double delay line shaped signal from the integrated output of the 12th dynode of the phototube. The fast timing is

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usually done with a constant fraction discriminator, and this time signal is also used for the timeof-flight measurement. To obtain satisfactory PSD for a large dynamic range of pulse heights we have used two-parameter PSD, looking at both pulse height (PH) and the fast vs crossover time signal• Fig. 3 shows the performance of this technique during a recent (p, pn) experiment• Some variations in PSD performance have been observed from detector to detector but the performance has always been adequate and we have made no attempts to isolate the source of this variation. The time resolution of these detectors has proved to be significantly better than could be expected from their size. It is generally assumed that the minimum time resolution (for neutrons of a given energy) is the difference between the time it takes for a neutron to travel the length of the scintillator and the time it takes scintillation photons to travel the same distance. It is our belief that the isotropi-

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cally reflecting paint in the scintillator can helps to increase the average transit time to the photo cathode for photons from scintillations far from the photo cathode• Thus photons reaching the photo cathode from distant events are, on the average, more likely to have undergone reflections then photons from closer events• This tends to reduce the time spread due to the detectors length. The efficiency of these detectors has been calculated using a computer code 6) written by Stanton which uses a Monte Carlo technique to simulate the scattering histories in the scintillator of a large statistical sample of incident neutrons• The program uses non-relativistic kinematics and assumes that all charged particles stop in the scintillator, a reasonable assumption for a detector of this size. The program was modified by including the most recent relevant cross sections, the hydrogen to carbon ratio of NE213 and the response function for NE218 7) which should be very similar to that of NE213. Typical results are shown in fig. 4 for a number of different pulse height thresholds. The results of this calculation are estimated 6) to be correct to within 5%. Detector pulse height thresholds are usually measured using the 1.275 MeV ~,-ray from the decay of 22Na and the 4.43 MeV ~,-ray from a carbon screen lowered into the proton beam. The Compton edges of these 7-ray are at 1.067MeV and 4 . i l M e V respectively. Typical calibration spectra are shown in fig. 5. A point at two thirds of the full height of The Compton peak was chosen as the point of maximum recoil electron energy following ref. 8. Typical thresholds used for (p, pn) measurements are N $00 keV electron equivalent energy, whereas for (p, n) measurement they are 5-10 MeV electron equivalent energy, to prevent overlap. 2.4. TnE 41 cm SCATTERINGCHAMBER The 41 cm scattering chamber, shown in fig. 6, was designed specifically for charged particleneutron correlation experiments. It is fully equipped for charged particle detection with either solid state or NaI detectors, and it was designed with as low a mass as possible to minimize "in scattering" of neutrons produced in the target. To minimize attenuation of the neutron flux, the chamber has a 127/1m window of Kapton-H polyamide film on the side facing the neutron flight paths. On the other side of the chamber there is a 7 6 # m Kapton H window to permit detection of charged

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particles with an external NaI detector, for which a mount is provided. The barrel of the chamber is made of 6.35 mm aluminum. However, for a distance of 2.54cm above and below the Kapton window, the aluminum wall has been turned down to a thickness of 1.59 mm. The atmospheric load on the chamber is supported by six external pillars which can be positioned at locations which cause the least interferance with the measurement being made. The floor and lid of the chamber are made of steel; a steel rib structure in the floor makes it rigid enough to maintain alignment when under atmospheric load.

A cooled, rotatable platform for mounting solid state charged particle detectors is built into the floor of the chamber, using a rotating hub. The cooling medium is liquid nitrogen with the cooling lines passing through the hub. A small 411 storage container is soldered to the bottom of the detector platform; a level sensor in the container is used for an automatic filling system. Thermal isolation and mechanical rigidity for the detector platform are provided by a box-shaped structure of pillars and cross members machined from a single piece of nylon. The external mounting arm for NaI detectors is rigidly fastened to the bottom of the hub, so

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that it is always aligned with the detector platform inside. Thus a solid state zlE detector can conveniently be used with the NaI E detector, should this be desired. For (p, pn) measurements, neutron flight times are measured with respect to detection of the coincident proton, rather than the cyclotron's rf. The Kapton H window on the scattering chamber has proved to be extremely useful in alligning the (p, pn) coincidence circuitry. This window makes it possible for one of the neutron detectors to detect

energetic protons from p - p or p - d elastic scattering with the conjugate charged particle being detected in coincidence by the detectors in the scattering chamber. This provides a convenient source of coincident events, with characteristics similar to those of genuine (p, pn) event. 3. (p, n) measurements The Manitoba fast neutron facility is being used for the study of (p, n) reactions on light nuclei. For this work, neutron flight times are measured rela-

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difference in transit times of neutrons and photons across the scintillator is 850 ps for 40 MeV neutrons. The pulse height threshold was set just high enough to eliminate "overlap". There are relatively few other fast neutron timeof-flight facilities in existence for 20-45 MeV neutrons, against which we can compare our performance. The " b e a m swinger" facility9) at the Michigan State University Cyclotron Laboratory (MSUCL) is the notable exception. Using a detector only 1.25 cm thick, researchers at MSUCL were able to obtain a time resolution of 550 ps for 33 MeV neutrons. With a detector of comparable thickness we would expect to achieve similar time resolutions. The principle advantage of the MSUCL facility is the availability of longer flight paths. The

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4. (p, pn) measurements A substantial program of (p, pn) measurements ~°-~3) on light nuclei has been underway for several years. We describe below the features of the experimental setup which are common to all of these measurements. 4.1. DETECTORS AND TIMING

The (p, pn) measurements have all been made with targets in the 41 cm scattering chamber. Two neutron detectors were stacked one on top of the other in a "figure 8" configuration. Coplanar geometry was always used with one scintillator immediately above and one immediately below the nominal reaction plane. For these measurements, the neutron times-of-flight were measured with respect to the detection of the coincident proton. Protons were detected typically with a counter telescope consisting of three detectors: a 200/am thick Si surface barrier detector used for particle identification, a 750 m m thick Si surface barrier detector used for fast timing and particle identification and a thick E detector. The E detector was usually a 5 mm thick Si(Li) detector in the chamber, often canted at an angle of 45 ° to increase its effective thickness to - 7 m m . Occasionally the 200/am detector was omitted and a NaI (T1) E detector external to the scattering chamber was used. The proton timing signal was derived from the 750/am detector using a fast voltage sensitive preamplifier ~4) of the type developed at Argonne National Laboratory. The solid state detectors in the scattering chamber were cooled to 77 K to improve time resolution by reducing noise and charge collection time. (Charge mobility in silicon reaches its maximum near 77 K, see ref. 15.) To minimize the capacitance in parallel with the detector, the timing preamplifer was mounted inside the scattering chamber and connected to the detector by only 5 cm of cable. The preamplifier was thermally insulated from the cooled detector platform by a nylon standoff, to prevent the electronic components from falling below their operating temperature range. The time resolution of this surface barrier timing system was measured using proton-proton elastic scattering. One final state proton passed through the 750/am surface barrier while the other was detected with a small NE102 scintillator mounted on an RCA8575 phototube. The over-all time

FACILITY

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width of the p - p coincidence peak was 350 ps. Then the surface barrier detector was replaced by a second small scintillator and a much narrower time peak was observed, indicating that the 350ps previously obtained was due to our surface barrier detector timing system. Because the time reference signal for the neutrons is derived from the coincident proton, rather than the cyclotron beam, there is no problem with "overlap" in the neutron time-of-flight spectrum, and the pulse height thresholds can be set fairly low. We have generally chosen thresholds which give a dynamic range of pulse heights of ~ 20:1. This then yields efficiencies in the range of 25-35%. 4.2. ELECTRONICS A schematic diagram of the electronics used for (p, pn) measurements is shown in fig. 8. The heart of the system is the fast timing circuity for the 750 # m detector (referred to as A). The signal from the voltage sensitive (vs) preamp is passed through a fast dc-coupled wide band amplifier, a timing filter amplifier (TFA) and a constant fraction discriminator (CFD). A clipping stub with a clip time of one half the cyclotron's rf period was placed at the input to the CFD to supress ff noise picked up by the detector and cabeling, and the output impedance of the fast amplifier was matched to the 50 I2 cable attached to it so that reflections from the clipping stub arriving back at this output were completely absorbed. It was found necessary to deliberately incur a 200 ns "deadtime" following each trigger of the CFD on the fast signal from detector A. This was to completely eliminate any double pulsing, which would have invalidated the operation of the pile-up gate which was used to detect and reject pulses from the CFD occuring within 3 #s of each other. Use of the pile-up gate permitted large counting rates because the piled up events were identified. Only 7% of piled up events were not identified because of the 200 ns dead time. A sample of random coincidences was recorded simultaneously with the reaction data. Special precautions were taken because the coincidence requirements and the time-of-flight measurement are so closely related. Random coincidences were generated by causing each pulse from t h e / t discriminator to produce two pulses separated by an exact multiple of the cyclotron rf period. It was necessary that this separation be longer than the range of useful

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neutron times-of-flight and that it be such a multiple because of the possibility of a cyclical variation in the intensity of the total flux arriving at the neutron detector. Four rf periods or 140.4 ns was chosen for the delay. Relative delays between the two detector systems were arranged so that the second of the pair of pulses generated the reaction coincidences. This was because it was considered less likely that a proton rather than a neutron from a reaction would be inordinately delayed to the extent that it would erroneously appear in the sample of random coincidences. The second pulse was generated by connecting a precisely calibrated length of delay cable (which was open at the other end) to the bridged high impedance output of a logic fan-out module. A fast dc-coupled gating system using an " A N D " gate was employed to "start" the time to amplitude converter (TAC) for the neutron timeof-flight only when an event occurred with a timeof-flight in the range of interest. This not only eliminated TAC dead time but also afforded convenience in that a relatively early signal was available which indicated a valid time-of-flight, the time delay of which was fixed relative to the neutron detector signal. The input to the AND from the proton detector was in the form of two stretched pulses separated by the previously mentioned 4 rf periods. Their width determined the range of acceptable times-of-flight. If the pulse from a neutron detector timing discriminator fell within one of these gating pulses, it identified a random or reaction coincidence event, respectively. In all, six linear signals and three logic bits or "flags" were ultimately interfaced into a PDP 15/ 40 data acquisition computer. The six linear signals were e and zJ, the pulse heights in the e and zl detectors; Z" = ~ +zl + E , the hardware sum of the pulse heights from all three detectors in the proton telescope; TOF, the neutron time-of-flight measured relative to the fast signal from zf, PH, the amplified linear output from the neutron detector, and PSD, the pulse-shape-discrimination signal. The three flags identified which neutron detector was involved, whether an event fell in the "rand o m " or "reaction" time window and whether the fast A signal was piled up. 4.3. RESULTS Fig. 9 shows two "scatter plots" typical of our (p, pn) reaction data on 9Be and 13C. Displayed are the values of proton energy (Z'= e+zf + E ) v s neu-

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Fig. 9. Typical scatter plots of(p, pn) data for (a) 9Be and (b) ]3C at 46 MeV proton bombarding energy. Kinematic loci for both the ground and first excited states of the residual nuclei are clearly visible in both spectra. The upper fight hand and lower left hand portions of the scatter plots have been eliminated by software.

tron time-of-flight for coplanar, symmetric geometry suitable for observing quasi-free knockout. Note that the neutron time-of-flight axis is "reversed" in that the TAC was started with the neutron signal. The e and /1 signals have been used to identify protons. Random coincidences have been subtracted from the displayed spectra by cancelling an event in the reaction coincidence spectrum for each event in the random coincidence spectrum if the two events were within a specified "distance" of each other.

NEUTRON TIME-OF-FLIGHT FACILITY

In each spectrum, bands of events are clearly visible, corresponding to leaving the residual nucleus, SBe or 12C, in its ground state or first excited state at 2.9 MeV or 4.44 MeV respectively. Neutron flight paths were chosen to be just long enough to provide separation between the ground and first excited stai'es: - 5 m for the 9Be data and - 3 m for the ~3C data. Each spectrum represents - 1 6 hours of data taking with ~ 100 nA of beam on a 10 rag/ cm 2 target. Only one other serious program of (p, pn) measurements in this energy range has been attempted16). When binding energy spectra from our work ll) on 9Be are compared with ref. 16, we find that our resolution is somewhat better. Whether this is due to better overall time resolution or the use of longer flight paths is difficult to assess. We would like to acknowledge the support and assistance we received from the Manitoba Cyclotron Laboratory during the establishment of this facility, and especially the encouragement of Dr. K.G. Standing. The assistance of Dr. D.I. Bonbright and' Dr. D.J. Roberts in making the measurements presented in this paper is also gratefully acknowledged. References l) J.J. Bergerjon, B. Hird, F. Konopasek and K.G. Standing, IEEE Trans. Nucl. Sci. NS-13 (1966) 422. 2) A. McIlwain and S. Oh, Proc. 7th Int. Conf. on Cyclotrons and their applications, Ziirich (1975) p. 394.

141

3) R.F. Bentley, L.A. Erb, D.A. Lind, C. D Zafiratos and C.D. Zaidins, Nucl. Instr. and Meth. 83 (1970) 245. 4) R.H. Day and W. C. Parkinson, Nucl. Instr. and Meth. 111 (1973) 199. 5) j. W. Watson, C. A. Miller and F. J. Wilson, Nucl. Instr. and Meth. 133 (1976) 399. 6) N.R. Stanton, Ohio State University Preprint COO-1545-92 (1971). 7) T.G. Masterson, Nucl. Instr. and Meth. 88 (1970) 61. 8) j. R. Prescott and A. S. Pupaal, Can. J. Phys. 39 (1961) 221 ; L.E. Beghian and S. Wilensky, Nucl. Instr. and Meth. 35 (1965) 34; R. Honecker and H. Gr[issler, Nucl. Instr. and Meth. 46 (1967) 282. 9) R.K. Bhowmik, R.R. Doering, L. E. Young, S.M. Austin, A. Galonsky and S. D. Schery, Nucl. Instr. and Meth. 143 (1977) 63. 10) C. A. Miller, D. I. Bonbright, J. W. Watson and F. J. Wilson, in Few particle problems in the nuclear interaction (eds. 1. Slaus, S.A. Moszkowski, R.P. Haddock and W. T. H. van Oers, North-Holland Publ. Co., Amsterdam, 1972)-p. 731. 11) C.A. Miller, J.W. Watson, D.I. Bonbright, F. J. S. Wilson and D.O. Wells, Phys. Rev. Lett. 32 (1974) 684; C.A. Miller, Ph.D. Thesis (University of Manitoba, 1974). 12) D. I. Bonbright, J. W. Watson and D. J. Roberts, in Few body problems in nuclear and particle physics (eds. R.J. Slobodrian, B. Cujec and K. Ramavatram; Les Presses de L'Universit~ Laval, Quebec, 1975) p. 420. 13) D. I. Bonbright, D. J. Roberts, W. T. H. van Oers and J. W. Watson in Clustering phenomena in nuclei, vol. 2 (ORO-485626) (eds. D. A. Goldberg, J. B. Marion and S. J. Wallace; Nat. Tech. Info. Service Springfield, Va., 1975) p. 189; D.J. Roberts, Ph.D. Thesis (University of Manitoba, 1976). 14) I.S. Sherman, R.G. Roddick and A.J. Metz, IEEE Trans. Nucl. Sci. NS-15 (1968) 500. 15) M. Martini and T.A. McMath, Nucl. Instr. and Meth. 76 (1969) 1. 16) C.N. Waddell, E. M. Diener, R. G. Alias, L. A. Beach, R. O. Bondelid, E.L. Petersen, A.G. Pieper, R.B. Theus, C.C. Chang and N. S. Chant, Nucl. Phys. A281 (1977) 418.