A Micro-Strip Germanium Detector for Position Sensitive X-Ray Spectroscopy

D. Protic¹, Th. Stöhlker², H.F. Beyer², J. Bojowald¹, G. Borchert¹, A. Gumberidze², A. Hamacher¹, C. Kozhuharov², X. Ma², I. Mohos¹

¹Forschungszentrum Jülich GmbH, Institut für Kernphysik, 52425 Jülich, Germany
²GSI-Darmstadt, Planckstraße 1, 64291 Darmstadt, Germany

ATOMIC PHYSICS DIVISION

For the current x-ray spectroscopy program at the ESR storage ring (GSI-Darmstadt) a position sensitive germanium detector system has been completed. Along with a new kind of x-ray spectrometer this detector may play a keyrole for the next step of high precision x-ray experiments, aiming on a precise test of quantum electrodynamics in the heaviest one-electron systems such as a hydrogenlike uranium. The position sensitive structure of the detector has been realized on an area of 47x23.4 mm² by an array of 200 strips (200 mm wide and 23.4 mm long) separated by 35 μm wide grooves etched through boron implanted contact.

The micro-strip detector

At the beginning a 4.1 mm thick germanium diode was produced with boron implanted front contact and about 0.6 mm thick Li-diffused rear contact. The position sensitive structure of the detector has been realized by an array of 200 strips, each of them 200 mm wide and 23.4 mm long, separated by 35 mm wide grooves etched through the boron implanted contact. This means that the position information can be obtained from an area of 47 x 23.4 mm² (1100 mm²). A guard-ring which surrounds this area enables also relatively simple assembling. Each strip has been joined to a preamplifier placed outside of the cryostat with printed leads inside the flexible Kapton-foil. All connections between the strips and printed leads have been performed through bonded Al-wires. The Kapton-foil pressed between two Viton-seals serves at the same time as vacuum feedthrough. A 2 l/s Varian ion-pump maintains the pressure inside the cryostat below 5x10^-7 mbar. Thorough laboratory tests were performed using ²⁴¹Am γ-rays. A lot of effort is needed to avoid deterioration of the spectra caused by ground loops, microphonics, different pick-ups and high-frequency oscillations. By proper reducing of such effects the energy resolutions of the strips were found to be in the range of 1.6 to 1.9 keV [FWHM] for 60 keV γ-rays using a commercial shaping amplifier with 6 ms time constant. The main contributions to this resolution, as experimentally determined testing the system without germanium detector, are caused by Kapton-foil itself (inter-lead capacitance and dielectric loss effects) and its capacity to the grounded cryostat. Only slightly improved resolutions could be obtained by very careful layout of the connection elements (Kapton-foil and vacuum feedthrough), especially for higher number of strip elements. Simultaneously with measuring the signals from the strips there is possible to process the signals obtained from the common rear contact.

Figure 1: The main part of the detector system without the cryostat-cap and the cover for the electronics. 200 commercially available low dissipation (50 mW) charge sensitive preamplifiers (CSPA 02.04 from KFKI-Budapest) are placed on both sides of the printed board outside the cryostat. 100 of them are visible on the picture. The octagonal housing for the preamplifiers has a diameter of 39 cm and 12 cm in depth. No ventilation of the housing is needed since the total thermal dissipation of the preamplifiers amounts to only 10 W. The consumption of liquid nitrogen amounts to about 3 l/day.

Energy resolution 1.8 keV @ 60 keV		Position resolution 200 μm		Time resolution 70 ns
Figure 2: A sample photon spectrum recorded with one individual strip. For this purpose, a standard 241Am γ-ray source has been used. The energy resolution achieved corresponds to 1.8 keV [FWHM] at 60 keV.		Figure 3: Position spectra obtained with a collimated γ-ray source (60 keV, 241Am) at three different positions.		Figure 4: A sample time spectrum observed between two neighbouring segments of the micro-strip detector. Here, a time resolution of 70 ns [FWHM] has been achieved.

Compton scattering and charge splitting Figure 5: Reconstruction of one photon events leading to coincidences between two-neighbouring strips. b) and c) the coincident spectra associated to the time spectrum displayed in Fig. 3. a) Energy spectrum produced by adding the spectra b) and c) event by event.

For illustration we depict in Fig. 2 a sample photon spectrum recorded with one individual strip. Here, a standard ²⁴¹Am γ-ray source has been used. The energy resolution achieved corresponds to 1.8 keV [FWHM] at 60 keV whereby a shaping amplifier with 3 μs time constant has been applied. Within this first test, the individual amplifiers for in total 32 strips were connected to 4K CAMAC ADC modules. In addition, the time signals of each strip were recorded by using CF threshold discriminators connected to 4K TDC CAMAC modules. Event-list readout of all strips (energy and timing) is triggered by a signal from any strip above a threshold of approximately 5 keV. This technique allowed us to study the coincident photon spectra between neighbouring strips as well as to investigate the timing characteristics of the detector system used. As example, we display in Fig. 3 a sample time spectrum observed between two strips. Here, a time resolution of 70 ns [FWHM] is observed. Note, that for the planned experiments with heavy ion beams, a good time resolution for the x-ray detection system is required since the photons will be exclusively detected in coincidence with down-charged ions [3]. Since, the latter will be accomplished by fast scintillator or multi-wire proportional counters, a time resolution of better than 50 ns should be achievable with the current micro-strip detector. This is by far sufficient to meet the criteria of the current x-ray spectroscopy program at the storage ring ESR at GSI.

Experiment

For the next generation of Lamb-Shift experiments on hydrogenlike high-Z ions at the ESR storage ring at GSI, a new kind of a transmission x-ray spectrometer [4,5] has been designed. This instrument exploids the focusing compensated asymmetric Laue geometry (FOCAL) which focuses the radiation of a certain wavelength from an extended source to two single lines on the Rowland-circle. For a certain wavelength, these Bragg reflections arising from a cylindrically curved Si-crystal with a bending radius of 2 m. The design is optimized for photon energies in the range between 50 and 100 keV.

Figure 6: First preliminary result obtained with the mirco-strip detector mounted at the FOCAL spectrometer.in comparison with an x-ray spectrum recorded with a standard Ge(i) detector (resolution at 60 keV: about 500 eV). The intensity pattern as function of the position (energy) identifies well resolved the two components of the Kα-doublet of Tm as well as those of the Yb.

In order to achieve the desired efficiency of 10^-7, a position sensitive micro-strip Ge detector appears to be the most promising photon counting system. Very recently, the micro-strip Ge detector has been tested in combination with the FOCAL spectrometer using an intense radiactive ¹⁶⁹Yb source. Even without any strict conditions on the photon energies for the individual strips, the intensity pattern observed with the strip-detector as function of the position (i.e. strip-number) indentifies clearly the two x-ray lines of the K_α-doublet of Tm and of Yb which are separated by approximatley 970 eV and 1030 eV, respectively. This demonstrates, that in combination with the FOCAL spectrometer, a energy resolution of close to 100 eV is achieved along with a high detection efficiency.