Photon scattering experiments are performed in Darmstadt at the DHIP-setup. In these experiments mainly magnetic and electric dipole excitations are observed by measuring de-exciting gamma-rays with HPGe detectors. For an unambiguous scientific interpretation, e.g. in the context of the pygmy dipole response, the electric excitation mode has to be distinguished from the magnetic one. Since the parity of the ground is state is known, therewith the parity of the corresponding excited nuclear state is measured by observing the linear polarization state of the depopulating gamma-rays (~4-10 MeV) of the nuclear level of interest.
The method of Compton polarimetry to determine the linear polarization of gamma-rays uses the angle-dependence of the Compton scattering process on the orientation of the electric field vector of the incident photons, according to the Klein-Nishina cross-section. Electric multipole radiation can be distinguished from magnetic radiation by measuring the according characteristic asymmetry-curve of occuring Compton scattering angles.
A conventional technique is to use a gamma-ray polarimeter consisting out of one central detector serving as a Compton scatterer and a pair of detectors, absorbing the scattered photons. The latter detect the scattered photons typically at angles phi = 0° and phi = 90° with respect to the plane defined by the photon beam direction and its polarization orientation.
Such a setup can reach maximum polarization sensitivity at the expense of detection efficiency due to insignificant solid angle coverage of the analyzing detectors. Efficiency can be increased by converging scattering and analyzing detectors, but then the resolution of the Compton scattering angle is low. Here the concept of polarimetry using a highly segmented, and therefore position sensitive detector becomes obvious: by measuring precisely the location of gamma-ray interaction points, Compton scattering angles can be observed continously.
Important quantities to describe the quality of the polarimeter are the polarization sensitivity and the figure of merit, which includes the detection efficiency as well.
With this concept the DArmstadt GAmma ray Tracking Assembly DAGATA was funded within the collaborative research center SFB 634 to measure the polarization of high energetic gamma-rays, where the analyzing power of the Compton effect is too low for the use of conventional Compton polarimeters.
Currently one detector of the AGATA-type is mounted within a triple-cryostate, the other two detectors are funded and ordered.
In a calibration measurement we were able to prove that the polarization sensitivity of the one-detector DAGATA is significantly larger in comparison with conventional four-fold polarimeters.
Details can be found in the according publication:
B.Alikhani, et al,
NIM A 675(2012)144
Next steps of the project are:
- to implement the other two detectors in DAGATA
- to repeat the calibration measurement with the full triple detector
- to integrate the detector within the NRF setup DHIPS
- to use pulse shape analysis and improved gamma-ray tracking techniques
- to perform first in-beam measurements.