Compton Polarimeter Research Articles

Accurate measurement of the electron beam polarization in JLab Hall A using Compton polarimetry

A major advance in accurate electron beam polarization measurement has been achieved at Jlab Hall A with a Compton polarimeter based on a Fabry–Perot cavity photon beam amplifier. At an electron energy of 4.6 GeV and a beam current of 40 μ A, a total relative uncertainty of 1.5% is typically achieved within 40 min of data taking. Under the same conditions monitoring of the polarization is accurate at a level of 1%. These unprecedented results make Compton polarimetry an essential tool for modern parity-violation experiments, which require very accurate electron beam polarization measurements.

The transmission Compton polarimeter of the A4 experiment

A transmission Compton polarimeter as a simultaneous, relative monitor for the longitudinal polarisation of a stabilised, polarised electron beam (polarisation degree ∼ 80 %) has been constructed in a new design. It is located in the vacuum between the target and the beam dump of the A4 parity violation experiment at MAMI. The analysing power is ∼ 80 ppm at 854 MeV and ∼ 115 ppm at 570 MeV. The measurement precision for the polarimeter asymmetry (which is proportional to the longitudinal beam polarisation degree) is ∼ 3 ppm within 5 min. at 854 MeV and ∼ 7 ppm within 5 min. at 570 MeV and the systematical error is about 0.5 ppm + 1 – 2 %. This simple polarimeter consists mainly of two graphite scatterers, a water-cooled Samarium-Cobalt permanent magnet, an aluminum secondary electron emission monitor, an understructure and electronics.

Gamma‐Ray Burst Polarization: Limits fromRHESSIMeasurements

Using the RHESSI satellite as a Compton polarimeter, a recent study claimed that the prompt emission of GRB021206 was almost fully linearly polarized. This was challenged by a subsequent reanalysis. We present an novel approach, applying our method to the same data. We identify Compton scattering candidates by carefully filtering events in energy, time, and scattering geometry. Our polarization search is based on time dependent scattering rates in perpendicular directions, thus optimally excluding systematic errors. We perform simulations to obtain the instrument's polarimetric sensitivity, and these simulations include photon polarization. For GRB021206, we formally find a linear polarization degree of 41% (+57% -44%), concluding that the data quality is insufficient to constrain the polarization degree in this case. We further applied our analysis to GRB030519B and found again a null result.

Hard X-ray polarization measured with a Compton polarimeter at synchrotron radiation facility

A hard X-ray polarimeter with CdTe detectors has been developed for measurement of the degree of X-ray polarization at synchrotron radiation facilities. It utilizes 90° Compton scattering from the low Z targets. Measurements were performed at both facilities of the beamline BL38B1 in SPring-8 and the beamline BL14A in KEK-PF. The degrees of X-ray polarization for 20keV X-rays are 99% and 82% at the BL38B1 in SPring-8 and BL14A in KEK-PF, respectively. The polarization degrees in the energy range of 15 and 40keV correspond to 99.6±0.2% and 96.1±0.2% at the beamline BL38B1 in SPring-8. The analyzing power of the polarimeter was estimated by the Monte Carlo simulation with EGS4. The synchrotron radiation facilities provide highly polarized X-ray beams at energies above 15keV.

Precision measurement with the transverse polarimeter at HERA II

The HERA ep–collider provides a longitudinally polarised positron beam to the three experiments HERMES, H1 and ZEUS. The degree of polarisation is determined with two Compton polarimeters, one measuring the transverse polarisation that naturally builds up in a sufficiently planar ring via the Sokolov–Ternov effect, the other one measuring the longitudinal polarisation between the HERMES spin rotators. The future physics program of the collider experiments requires polarimetry with a precision of better than 1%. To meet this goal, the Transverse Polarimeter has been upgraded in 2001 with a new data acquisition system and a silicon strip detector complementing the main calorimeter. A new analysis, which allows an in–situ calibration in parallel to the polarisation measurement without relying on testbeam results obtained under different conditions, is presented in this paper. The new method uses a fit of the double–differential Compton cross section (folded with detector effects) to the measured two–dimensional spectra in energy and position of the scattered photon. In March 2003, HERA succeeded to deliver up to 50% of longitudinal polarisation to all three experiments as well as collisions for H1 and ZEUS. Some first results of the new analysis on polarised HERA II data are shown. PACS: 29.27.Hj Polarized beams – 13.88.+e Polarization in interactions and scattering

Polarization sensitivity and efficiency for a planar-type segmented germanium detector as a Compton polarimeter

We have investigated performance of a planar-type germanium detector with 25 segments as a Compton polarimeter to measure the linear polarization of gamma rays. We considered three different configurations in sampling horizontal and vertical Compton-scattering events for linearly polarized gamma rays incident on the detector. The experimental polarization sensitivity Q, coincidence efficiency ϵ coin, and figure of merit F for each configuration have been extracted for gamma rays of interest below 600 keV found in our previous in-beam spectroscopy on 155 Gd . We also obtained theoretical values for these three quantities through a Monte Carlo simulation. Experimental values were in good accord with simulation results for all the three configurations. The third configuration, sampling pairs between the segment of incidence and any off-diagonal segments in coincidence, turned out to have the largest Q as well as ϵ coin value of all the three configurations, thereby resulting in the best figure of merit F.

Parity Measurements with Completely Polarized Photon Beam

Photon scattering experiments on 52Cr and 138Ba were performed to resolve ambiguous parity assignments below neutron threshold energy by using completely linear-polarized photon beam generated by Laser-Compton backscattering technique. The parity of 5544 keV level in 52Cr was confirmed to be negative. The Ml transition was identified at 5098 keV in 52Cr. The 5644 keV level in 138Ba, which was tentatively assigned as Ml transition by former Compton polarimeter measurement, was clearly assigned as El transition. A parity measurement with a completely linear-polarized photon beam was demonstrated to be simple and straightforward method.

Calibration of Compton polarimeters

The formula for the angular distribution of γ-ray linear polarization is written in a more transparent form than in terms of the more usual Legendre and associated Legendre polynomials. This form emphasises the special nature of pure dipole, 1→0 and pure quadrupole 2→0,γ-ray transitions, which produce full linear polarization (| P|=1) over the complete angular range, when the initial states are fully aligned in their m=0 substates. Some of the reactions used previously to calibrate Compton polarimeters cannot be extended to encompass low-energy γ-ray transitions for the new generation of multi-pixel strip detectors and/or tracking detectors with their potentially high-solid angles. Other possible calibration methods are noted.

First electron beam polarization measurements with a Compton polarimeter at Jefferson Laboratory

A Compton polarimeter has been installed in Hall A at Jefferson Laboratory. This Letter reports on the first electron beam polarization measurements performed during the HAPPEX experiment at an electron energy of 3.3 GeV and an average current of 40 μA. The heart of this device is a Fabry–Perot cavity which increased the luminosity for Compton scattering in the interaction region so much that a 1.4% statistical accuracy could be obtained within one hour, with a 3.3% total error.

Polarization sensitivity of a segmented HPGe detector up to [formula omitted

Linear γ-ray polarization can be measured using segmented germanium detectors. The polarization sensitivity of the Compton scattering process leads to asymmetries in the signals of a segmented detector. We have measured the polarization sensitivity of a four-fold segmented large volume germanium detector up to photon energies of approximately 10 MeV for the first time. The detector and its performance are compared to smaller Compton polarimeters which have been analyzed in previous work. A possible application of the described Compton polarimeter will be parity assignments in photon scattering experiments.

New measurement of parity violation in elastic electron–proton scattering and implications for strange form factors

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from the proton. The result is A = -15.05 +- 0.98(stat) +- 0.56(syst) ppm at the kinematic point theta_lab = 12.3 degrees and Q^2 = 0.477 (GeV/c)^2. The measurement implies that the value for the strange form factor (G_E^s + 0.392 G_M^s) = 0.025 +- 0.020 +- 0.014, where the first error is experimental and the second arises from the uncertainties in electromagnetic form factors. This measurement is the first fixed-target parity violation experiment that used either a `strained' GaAs photocathode to produce highly polarized electrons or a Compton polarimeter to continuously monitor the electron beam polarization.

Low-lying dipole excitations in the stable Cd isotopes: A systematics

Low-lying dipole excitations in the medium-weight vibrational nuclei of the Cd isotopic chain were investigated by means of nuclear resonance fluorescence experiments performed at the bremsstrahlung beam of the Stuttgart Dynamitron accelerator (endpoint energy 4.1 MeV). Detailed information has been obtained on excitation energies, spins, decay widths, and transition probabilities of numerous excited states in 110–114,116Cd. Additionally, the use of two Compton polarimeters enabled model-independent parity assignments for excitations in the even-even isotopes. Strongly excited Jπ = 1− states are found in all even-even Cd nuclei at excitation energies near the sumof the energies of the first 2+ and 3− states. These excitations are interpreted as the 1− member of the quadrupole-octupole coupled quintuplet (2+⊗3−). The fragmented strength observed in the odd isotopes 111,113Cd is compared with the strength distributions in the neighboring even-even Cd isotopes.

Compton scattering off polarized electrons with a high-finesse Fabry–Pérot Cavity at JLab

We built and commissioned a new type of Compton polarimeter to measure the electron beam polarization at the Thomas Jefferson National Accelerator Facility (Virginia, USA). The heart of this polarimeter is a high-finesse monolithic Fabry–Pérot cavity. Its purpose is to amplify a primary 300 mW laser beam in order to improve the signal to noise ratio of the polarimeter. It is the first time that a high-finesse Fabry–Pérot cavity is enclosed in the vacuum of a particle accelerator to monitor the beam polarization by Compton polarimetry. The measured finesse and amplification gain of the cavity are F=26 000 and G=7300. The electron beam crosses this high-power photon source at an angle of 23 mrad in the middle of the cavity where the photon beam power density is estimated to be 0.85 MW/ cm 2 . We have used this facility during the HAPPEX experiment (April–July 1999) and we give a preliminary measurement of Compton scattering asymmetry.

A photon calorimeter using lead tungstate crystals for the CEBAF Hall A Compton polarimeter

A new Compton polarimeter is built on the CEBAF Hall A electron beam line. Performances of 10% resolution and 1% calibration are required for the photon calorimeter of this polarimeter. This calorimeter is built with lead tungstate scintillators coming from the CMS electromagnetic calorimeter R&D. Beam tests of this detector have been made using the tagged photon beam line at MAMI, Mainz, and a resolution of 1.76%⊕2.75%/ E ⊕0.41%/E has been measured.

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.

Parity of the band head at 3710 keV in 99Rh using clover detector as compton polarimeter

Clover detector has been used as a Compton polarimeter to measure the linear polarization of γ-rays produced in heavy ion fusion reaction. The polarization sensitivity of the clover detector has been measured over γ-ray energies ranging from 386 to 1368 ke V. The E1 multipolarity of the 1117 keV transition in 99Rh has been established using this polarimeter. This has resulted in the assignment of negative parity to the band head at 3710 keV in 99Rh.

Is the 4.742 MeV state in 88Sr the 1− two-phonon state?

A nuclear resonance fluorescence experiment on 88Sr has been performed with bremsstrahlung of 6.7 MeV endpoint energy. The γ-ray linear polarisation has been measured with a EUROBALL CLUSTER detector used as a Compton polarimeter. The results indicate positive parity for the J= 1 state at 4.742 MeV in 88Sr, in contrast to the previous interpretation as a 1− two-phonon (2+1⊗ 3−1) state and in conflict with the predictions of the quasiparticle-phonon model. On the basis of such calculations the 1+ state at 3.486 MeV may be considered as the 1+1 one-phonon state and the very strong 1+1→ 0+1 deexcitation as proton spin-flip 2p1/2→ 2p3/2 transition.

Resolved dipole strength below theE1giant resonance in138Ba

The electromagnetic dipole response of ${}^{138}\mathrm{Ba}$ was measured up to 6.7 MeV excitation energy in a photon scattering experiment. Two Euroball Cluster detectors were used to detect the scattered photons under 94 and 132 degrees. The Cluster at 94 degrees served as a Compton polarimeter. The total observed dipole cross section is in good agreement with previous tagged photon data, but in the present experiment the transition strength and the electric character of most of the transitions has been determined on a state-by-state basis. The data show a concentration of $E1$ strength around 6 MeV. For one of the strongest excitations $M1$ character is suggested. The results of model calculations using the quasiparticle phonon model (QPM) agree with the observed electric dipole strength distribution.

Gamma ray polarimetry using a position sensitive germanium detector

Imaging gamma-ray detectors make sensitive polarimeters in the Compton energy regime by measuring the scatter direction of gamma rays. The principle is to capitalize on the angular dependence of the Compton scattering cross section to polarized gamma rays and measure the distribution of scatter directions within the detector. This technique is effective in a double-sided germanium detector between roughly 50 keV and 1 MeV. This paper reviews device characteristics important to the optimization of a Compton polarimeter, and summarizes measurements we have made using a device with a 5×5 cm active area, 1 cm thickness, and strip-electrodes on a 2 mm pitch.

Parity measurements in odd-odd86Nb

${}^{86}\mathrm{Nb}$ was produced using the ${}^{58}\mathrm{Ni}{(}^{32}\mathrm{S},3pn)$ reaction at 130 MeV. $\ensuremath{\gamma}$-ray linear polarizations were measured with a four-crystal ``clover'' Compton polarimeter. Results from this and an angular distribution measurement have led to spin-parity assignments for the ${6}^{\ensuremath{-}},$ ${7}^{+}$, and ${8}^{+}$ low-lying states.