Chopped Beams Research Articles

Proton-neutron multiplet states and isomers in the odd-odd nucleus I122

The level structure of the odd-odd iodine nucleus I122 has been studied by the Sn120 (Li7, 5n)I122 reaction at Elab=58 MeV. Two chopped beams from the 14UD (Unit Doubled) tandem heavy-ion accelerator at the Australian National University were used; one was a nanosecond (ns) pulsed-beam with separation of 1.7 microseconds (μs) and the second one was a 60-μs beam separated by 880μs. Through in-beam and out-of-beam γ-ray spectroscopy, the complex proton-neutron multiplet states, including isomers with half-lives of a few tens of ns to a few tens of μs, have been identified. Collective states based on the proton h11/2 orbital exhibit two kinds of structure depending upon the coupled neutron orbital: a quadrupole vibration when involving the h11/2 orbital and a γ-soft deformed rotation when involving the g7/2 orbital. With spherical shell model and deformed shell-model calculations, the observed level structures are explained.

Investigation of the optical absorption of a magnetic colloid from the thermal to the electronic time-scale regime: measurement of the free-carrier absorption cross-section

The free-carrier absorption cross-section σ of a magnetic colloid composed of magnetite nanoparticles dispersed in oil is obtained by using the Z-scan technique in different experimental conditions of the laser beam. We show that it is possible to obtain σ with picosecond pulsed and millisecond chopped beams with pulse frequencies smaller than about 30 Hz. For higher pulse frequencies, the heating of the colloidal system triggers the appearance of the Soret effect. This effect artificially increases the value of σ calculated from the experimental results. The limits of the different experimental setups are discussed.

Two-quasiparticle isomer,E1hindrances and residual interactions inTm172

The structure of the neutron-rich nucleus {sup 172}Tm has been studied using incomplete fusion of {sup 7}Li on an {sup 170}Er target at 30 MeV. A 190-{mu}s isomer at an excitation energy of 476 keV was identified using chopped beams and {gamma}-ray spectroscopy. The isomer decays with very inhibited E1 transitions to the rotational bands based on the parallel and antiparallel couplings of the {nu}5/2{sup -}[512] x {pi}1/2{sup +}[411] configuration, the latter (K{sup {pi}}=2{sup -}) being the ground state. The isomeric state has been assigned J{sup {pi}}=6{sup +}, arising from the energetically favored (parallel) coupling of the {nu}5/2{sup -}[512] x {pi}7/2{sup -}[523] configuration. The proton-neutron residual interaction was deduced for the configuration of the isomeric state and is found to agree with previous empirical studies.

Simple technique for obtaining photoacoustic spectra corrected for the spectral variation of the source in single scan

A simple technique is presented to obtain normalized photoacoustic (PA) spectra corrected for the spectral variation of the source in a single scan. The input light beam is passed through the center of a dual slot chopper, which splits it into two chopped output beams at two different frequencies at a fixed ratio. The beams fall on the sample and the reference kept side-by-side in the same sample chamber. The PA signals are detected by a microphone and processed by two lock-in amplifiers tuned at two different frequencies. The technique is tested by recording the PA spectra of standard samples.

Negative hydrogen ion sources for accelerators

A variety of H/sup -/ ion sources are in use at accelerator laboratories around the world. A list of these ion sources includes surface plasma sources with magnetron, Penning and surface converter geometries as well as magnetic-multipole volume sources with and without cesium. Just as varied is the means of igniting and maintaining magnetically confined plasmas. Hot and cold cathodes, radio frequency, and microwave power are all in use, as well as electron tandem source ignition. The extraction systems of accelerator H/sup o/n sources are highly specialized utilizing magnetic and electric fields in their low energy beam transport systems to produce direct current, as well as pulsed and/or chopped beams with a variety of time structures. Within this paper, specific ion sources utilized at accelerator laboratories shall be reviewed along with the physics of surface and volume H/sup -/ production in regard to source emittance. Current research trends including aperture modeling, thermal modeling, surface conditioning, and laser diagnostics will also be discussed.

Design study on a medium-energy beam-transport line for the JHF proton linac

The JHF/JAERI linac is an injector of a 3-GeV ring. It consists of an H − ion source, a 3-MeV radio-frequency quadrupole linac, a 50-MeV drift-tube linac, a 200-MeV separated-type drift-tube linac and a 400-MeV ACS accelerator in its normal conducting section. As an intense beam linac with an average current of 0.2 mA (first stage) and higher in the second stage, the issue of decreasing beam losses is of prime importance. The MEBT between the RFQ and the DTL has been designed with a modified TRACE3-D code, aimed at perfect matching, clear chopping, a small emittance growth and sufficient space for beam measurements. The line consists of eight quadrupoles, two bunchers and two choppers with a total length of about 3 m. Multiparticle codes PARMILA and LINSAC were used to check the beam dynamics of the design. Mismatch due to errors in the elements of the MEBT or the input beam was also studied. A radio-frequency deflector (RFD) is adopted in the chopper. It is characterized by a high deflecting field and compactness. The RF-deflector cavity was designed using the MAFIA code and the electromagnetic field distribution, including fringe field, is applied in TRACE3-D simulation. It illustrates the effect of the beam tube in the cavity on large deflection. A deflection initiated by the RF-deflector is amplified more than twice by the following quadrupole. Owing to this merit, the required RF field of 1.6 MV/m is rather low to generate an adequate separation of 4.3 mm between chopped and unchopped beams at the beam stopper. By means of rapid changes in the RF amplitude and phase during the rise and fall time, the chopper has an equivalent rise and fall time of less than 3.1 ns. Beam losses during the transient time of the chopper are less than 0.08% at the exit of the 50-MeV DTL. An analysis of unstable particles in the coupled RF-deflectors shows no obvious difference from the case of an independent cavity.

High intensity muon/pion beam with time structure at PSI

A new initiative is presented to develop a high intensity muon/pion beam with a time structure optimized to the muon lifetime. Such a facility would provide exciting physics opportunities for dramatically improved fundamental experiments, e.g., in the field of muon capture, muon lifetime and muonium spectroscopy. The high primary beam intensity at PSI allows intense chopped muon beams by installing a fast electrostatic kicker in a secondary channel. Two modes of operation are foreseen: a muon-on-request scheme, which uses active feedback from a beam counter in the experimental area and a periodic pulsed mode with about 5–20% duty factor.

Using time information in μLCR spectroscopy: Older muons are wiser

ForμLCR with slow polarization transfer, the weak resonance signals can be strengthened by delaying the positron observation period, for pulsed and chopped muon beams. Furthermore, the sensitivity to drifts can be reduced or eliminated by using the ratios of late to earlye+ counts. These possibilities for using time information are discussed quantitatively.

Radiation Dosimetry by Photo-Acoustic Spectrometry

Abstract Photo-acoustics, or, in general, particle acoustics (PA) offers new potentials in various fields of radiation dosimetry. As a spectrometric method it can be used to obtain new information on material properties for a wide range of dosimetric materials including opaque and powdered samples, and to measure dose by absorption spectrometry. On the other hand, as a calorimetric method, it can be used as a direct means of measuring the absorbed dose rate of pulsed or chopped radiation beams of any kind of energetic particles. The PA spectroscopy (PAS) response of powdered LiF (TLD-100) that had been exposed to 60Co gamma rays was investigated and it was found that the absorption peaks at 380 and 444 nm provided a means for measuring absorbed dose in the 0.003-1 MGy range. PA signals generated by 4 to 5 MeV electrons from a linac were detected and the energy deposition profiles in Al, Cu and Fe were measured.

Radiation Dosimetry by Photo-Acoustic Spectrometry

Abstract Photo-acoustics, or, in general, particle acoustics (PA) offers new potentials in various fields of radiation dosimetry. As a spectrometric method it can be used to obtain new information on material properties for a wide range of dosimetric materials including opaque and powdered samples, and to measure dose by absorption spectrometry. On the other hand, as a calorimetric method, it can be used as a direct means of measuring the absorbed dose rate of pulsed or chopped radiation beams of any kind of energetic particles. The PA spectroscopy (PAS) response of powdered LiF (TLD-100) that had been exposed to 60Co gamma rays was investigated and it was found that the absorption peaks at 380 and 444 nm provided a means for measuring absorbed dose in the 0.003-1 MGy range. PA signals generated by 4 to 5 MeV electrons from a linac were detected and the energy deposition profiles in Al, Cu and Fe were measured.

Direct-inelastic and trapping-desorption scattering of N 2 and CH 4 from Pt(111)

Angle and velocity distributions for supersonic chopped beams of N 2 and CH 4 scattered from clean close-packed Pt(111) surfaces are reported. For specular direct-inelastic scattering N 2 and CH 4 velocity distributions can be characterized by empirical relationships used for Ar scattering. For instance, for specular scattering the following relation is found for Ar, N 2 and CH 4: 〈KE f 〉 = A(KE i) + B(2 kT s), where 〈KE f〉 is the average final kinetic energy, KE i is the incident kinetic energy and T s is the surface temperature. The beam and surface temperature independent coefficients A and B are, respectively: Ar 0.87, 0.17; N 2 0.79, 0.19 and CH 4 0.84, 0.25. Unlike Ar, N 2 desorbs from Pt with a Maxwell-Boltzmann velocity distribution near the surface temperature. Qualitatively the trapping probabilities for these molecules on Pt(111) are ordered: Xe > N 2 > CH 4> Ar.

Direct-inelastic and trapping-desorption scattering of N 2 and CH 4 from Pt(111)

Abstract Angle and velocity distributions for supersonic chopped beams of N 2 and CH 4 scattered from clean close-packed Pt(111) surfaces are reported. For specular direct-inelastic scattering N 2 and CH 4 velocity distributions can be characterized by empirical relationships used for Ar scattering. For instance, for specular scattering the following relation is found for Ar, N 2 and CH 4 : 〈KE f 〉 = A (KE i ) + B (2 kT s ), where 〈KE f 〉 is the average final kinetic energy, KE i is the incident kinetic energy and T s is the surface temperature. The beam and surface temperature independent coefficients A and B are, respectively: Ar 0.87, 0.17; N 2 0.79, 0.19 and CH 4 0.84, 0.25. Unlike Ar, N 2 desorbs from Pt with a Maxwell-Boltzmann velocity distribution near the surface temperature. Qualitatively the trapping probabilities for these molecules on Pt(111) are ordered: Xe > N 2 > CH 4 > Ar.

Phase measurements in chopped particle beams

It is shown that phase and amplitude of a chopped particle beam as measured with a lock−in amplifier are given by the Fourier transform of the time−of−flight spectrum, independent of the exact form of the chopper. The information extractable from a measurement of the phase as a function of the chopping frequency is discussed.

An investigation of proton-rich nuclei and other products from the bombardment of 24Mg, 28Si AND 32S by 16O ions

Abstract The β- and γ-ray spectra have been studied resulting from the bombardment of 24Mg, 28Si and 32S with continuous and chopped beams of 16O ions in the energy-range of 22–33 MeV. Compound-nucleus formation is found to predominate in the reactions. Protonrich nuclei are observed via the (16O, 2n) reaction in each case. In particular the mass excess of the previously unobserved nucleus 46Cr is found to be −29.46 ± 0.03 MeV; its half-life is 0.26 ±0.06 s. The systematics of the Coulomb displacement energies of the mass triplets up to A = 46 are discussed.