South Hall Ring Research Articles

THE BLAST EXPERIMENT: POLARIZED ELECTRON SCATTERING FROM HYDROGEN AND DEUTERIUM

At the MIT-Bates Linear Accelerator Center, the nucleon form factors have been measured by scattering polarized electrons from vector-polarized hydrogen and deuterium. The experiment used the longitudinally polarized electron beam stored in the MIT-Bates South Hall Ring along with an isotopically pure, highly vector-polarized internal atomic hydrogen and deuterium target provided by an atomic beam source. The measurements were carried out with the symmetric Bates Large Acceptance Spectrometer Toroid (BLAST). Results are presented for the proton form factor ratio, [Formula: see text], and for the charge form factor of the neutron, [Formula: see text]. Both results are more precise than previous data in the corresponding Q2 ranges.

NEW RESULTS FROM THE BATES LARGE ACCEPTANCE SPECTROMETER TOROID (BLAST)

An experiment using the novel technique of scattering a longitudinally polarized electron beam from polarized internal hydrogen/deuterium gas targets was carried out in the South Hall Ring at the MIT-Bates Accelerator Center. The scattered particles were detected by the Bates Large Acceptance Spectrometer Toroid (BLAST) detector. The proton electric to magnetic form factor ratio, [Formula: see text] at Q 2 = 0.1 - 0.65 ( GeV/c )2 has been determined from the experiment by measuring the spin-dependent ep elastic scattering asymmetry in the two symmetric sectors of the BLAST simultaneously for the first time. The neutron electric form factor [Formula: see text] in the same Q2 range has been extracted by measuring the spin-dependent asymmetry from the [Formula: see text] process with a vector polarized deuterium target. These results on the nucleon form factors from the BLAST experiment are presented.

Nucleon form factors and the BLAST experiment

Measurements of the charge and magnetic form factors of the nucleon present a sensitive test of nucleon models and QCD-inspired theories. A precise knowledge of the neutron form factors at low Q 2 is also essential to reduce the systematic errors of parity-violation experiments. At the MIT-Bates Linear Accelerator Center, the nucleon form factors have been measured by means of scattering of polarized electrons from vector-polarized hydrogen and deuterium. The experiment used the longitudinally polarized stored electron beam of the MIT-Bates South Hall Ring along with an isotopically pure, highly vector-polarized internal atomic hydrogen and deuterium target provided by an atomic beam source. The measurements have been carried out with the symmetric Bates Large-Acceptance Spectrometer Toroid (BLAST) with enhanced neutron detection capability.

Measurement of the Proton’s Electric to Magnetic Form Factor Ratio fromH→1(e→,e′p)

We report the first precision measurement of the proton electric to magnetic form factor ratio from spin-dependent elastic scattering of longitudinally polarized electrons from a polarized hydrogen internal gas target. The measurement was performed at the MIT-Bates South Hall Ring over a range of four-momentum transfer squared Q2 from 0.15 to 0.65 (GeV/c)(2). Significantly improved results on the proton electric and magnetic form factors are obtained in combination with existing cross-section data on elastic electron-proton scattering in the same Q2 region.

Coherent THz Synchrotron Radiation from a Storage Ring with High-Frequency RF System

The generation of brilliant, stable, and broadband coherent synchrotron radiation (CSR) in electron storage rings depends strongly on ring rf system properties such as frequency and gap voltage. We have observed intense coherent radiation at frequencies approaching the THz regime produced by the MIT-Bates South Hall Ring, which employs a high-frequency S-band rf system. The measured CSR spectral intensity enhancement with 2 mA stored current was up to 10,000 times above background for wave numbers near 3 cm(-1). The measurements also uncovered strong beam instabilities that must be suppressed if such a very high rf frequency electron storage ring is to become a viable coherent THz source.

A highly polarized hydrogen/deuterium internal gas target embedded in a toroidal magnetic spectrometer

A polarized hydrogen/deuterium internal gas target has been constructed and operated at the internal target region of the South Hall Ring (SHR) of the MIT-Bates Linear Accelerator Center to carry out measurements of spin-dependent electron scattering at 850 MeV. The target used an Atomic Beam Source (ABS) to inject a flux of highly polarized atoms into a thin-walled, coated storage cell. The polarization of the electron beam was determined using a Compton laser backscattering polarimeter. The target polarization was determined using well-known nuclear reactions. The ABS and storage cell were embedded in the Bates Large Acceptance Toroidal Spectrometer (BLAST), which was used to detect scattered particles from the electron–target interactions. The target has been designed to rapidly ( ∼ 8 h ) switch operation from hydrogen to deuterium. Further, this target was the first to be operated inside a magnetic spectrometer in the presence of a magnetic field exceeding 2 kG. An ABS intensity 2.5 × 10 16 at / s and a high polarization ( ≈ 70 % ) inside the storage cell have been achieved. The details of the target design and construction are described here and the performance over an 18 month period is reported.

The MIT-Bates large acceptance spectrometer toroid

The Bates Large Acceptance Spectrometer Toroid (BLAST) is a detector designed to study the spin-dependent electromagnetic response of few-body nuclei at momentum transfers up to 1 (GeV/c) 2 at the MIT-Bates Linear Accelerator Center's South Hall Ring (SHR). The BLAST detector consists of an eight-sector copper coil array producing a toroidal magnetic field, instrumented with two opposing wedge-shaped sectors of wire chambers, scintillation detectors, Cerenkov counters, neutron detectors, a lead-glass forward calorimeter, and recoil detectors. A status of the project is presented as well as highlights of the scientific program.

Intermediate energy nuclear physics at MIT-Bates

The MIT-Bates Linear Accelerator Center is a university-based laboratory carrying out frontier research in nuclear physics. The research program is focussed on the flavor structure, charge distribution, shape, size and polarizability of the nucleon; the spin and electromagnetic structure of light nuclei; and the origin of the elements. The Bates research program has three central thrusts: the SAMPLE experiments to probe the flavor structure of the proton using parity-violating electron scattering at back angles; the OOPS (Out-Of-plane Spectrometer System) program which uses out-of-plane detection to probe nucleon and few-body nuclear structure; and the BLAST (Bates Large Acceptance Spectrometer Toroid) program which will use a new spectrometer under construction to measure electron scattering from internal gas targets in the South Hall Ring.

The BLAST project at MIT-Bates

The Bates Large Acceptance Spectrometer Toroid (BLAST) is a detector designed to study the spin-dependent electromagnetic response of few-body nuclei at momentum transfers up to 1 (GeV/c)2 at the MIT/Bates Linear Accelerator Center's South Hall Ring (SHR). The BLAST detector consists of an eight-sector copper coil array producing a toroidal magnetic field, instrumented with two opposing wedge-shaped sectors of wire chambers, scintillation detectors, Cerenkov counters, neutron detectors, a lead-glass forward calorimeter, and recoil detectors. A status of the project is presented as well as highlights of the scientific program.

The South Hall Ring and the BLAST detector

Although the electron injection system of the infrared free electron laser FELIX permits a high electron bunch repetition frequency of 1 GHz, user experiments often require a lower frequency. At the same time, the pulses from the gun have to be sufficiently short to fit within the longitudinal acceptance of a 1-GHz prebuncher. The generation of 300 ps electron bunches with a 25 MHz repetition rate, produced by pulse modulation of a triode gun, is addressed in this paper. The modulating waveform was generated by superposition of a 1 GHz sinusoidal wave, a 25 MHz train of pulses with a duration of 2 ns and a DC bias voltage. The hardware for the adopted modulation method, which consists of two RF cavities and an adjustable delay line, is described and analysed. Measurements of the electron bunch length are presented.

Status of the BLAST project

The Bates Large Acceptance Spectrometer Toroid (BLAST) is a detector designed to study the spin-dependent electromagnetic response of few-body nuclei at momentum transfers up to 1 (GeV/c)2 at the MIT/Bates Linear Accelerator Center's South Hall Ring (SHR). The BLAST detector consists of an eight-sector copper coil array producing a toroidal magnetic field, instrumented with two opposing wedge-shaped sectors of wire chambers, scintillation detectors, Cerenkov counters, neutron detectors, a lead-glass fo ward calorimeter, and recoil detectors. Presently, BLAST is under construction and it is on schedule to be done in summer 2001. A status of the project is presented as well as highlights of the scientific program.

Stored electron beam oscillations excited by trapped ions

The excitation of oscillations in a stored electron beam by trapped rest gas ions is calculated numerically for a cylindrically Gaussian beam profile and a set of basic oscillation modes. Conditions for start-up and maximal amplitudes for such oscillations in the presence of exponential synchrotron damping are derived and compared with measurements on the MIT/Bates South Hall Ring.

A Compton polarimeter for the MIT/Bates South Hall Ring

A Compton polarimeter is being developed for the MIT/Bates South Hall Storage Ring. This polarimeter will be used to make measurements of the polarization of stored electron beams with energies below 1 GeV during internal target experiments.

A precise electron beam energy calibration for use in nuclear physics

An accurate calibration of the MIT-Bates electron beam energy has been achieved. The electron beam was injected into the MIT-Bates South Hall Ring and the spin-precession frequency, Ω s , was measured. From the spin-precession frequency the equilibrium energy of the ring was determined to be 370.3115±0.0021±0.0004 MeV, where the first error is statistical and the second is systematic. The energy of the beam delivered by the linac was determined to be 369.954±0.023±0.300 MeV. This provides a rigorous check on the accuracy of the facility's energy analyzing magnets. The first moment of the energy distribution was also measured. By observing the time constant of the spin diffusion with zero power in the South Hall Ring RF cavity, we obtained a value of σ γ γ = (3.7±0.5) × 10 −4 . We provide a lower limit to the polarization lifetime. At a beam energy of 370 MeV no measurable decrease in the polarization was observed up to 2 ms after injection.