C. Cattadori

Cryogenic Performance of a Low-Noise JFET-CMOS Preamplifier for HPGe Detectors

Cryogenic low-noise charge sensitive preamplifiers have been developed and realized for the GERmanium Detector Array (GERDA). An integrated JFET-CMOS preamplifier, which is fully functional at cryogenic temperatures, has been tested in conjunction with an unsegmented p-type HPGe detector. Both the crystal and the preamplifier were operated inside a liquid nitrogen dewar at 77 K. The detector capacitance was ~60 pF. An optimum resolution of 1.6 keV FWHM has been obtained for the pulser line at 6 ¿s shaping time. A resolution of 2.1 keV FWHM has been achieved for the 1.332 MeV line from a 60Co source. A wide bandwidth (rise time of ~16 ns) permits use of pulse-shape analysis techniques to localize the position of the photon interactions inside the crystal. A low power consumption (~23 mW) makes the preamplifier suitable for a multi-channel array of germanium detectors.

Low-noise amplification of /spl gamma/-ray detector signals in hostile environments

We designed and realized a low-noise charge preamplifier for HPGe (High Purity Germanium) /spl gamma/-ray detectors, able to operate at a distance of 3 m to 6 m from the detector. One transistor only is placed in close proximity to the detector. Such a setup is required in applications where the detector works in hostile environments that could damage or destroy the electronic circuitry. Using 3 m RG62 cables and a 23 pF detector capacitance we obtained a noise of /spl sim/1.07 keV fwhm at 2 /spl mu/s shaping time, so fully compatible with /spl gamma/-spectroscopy requirements. By compensating the preamplifier so as to completely eliminate the ringing in its response function we obtained a rise-time of /spl sim/46 ns with 3 m cables and of /spl sim/80 ns with 5 m cables. With a different approach, or using a lower compensation capacitance and eliminating the ringing through a numerical post filter, we obtained a faster rise time of /spl sim/33 ns, with a detector-preamplifier distance of 5 m, while maintaining the low-noise performance. This latter setup is adequate for spectroscopy and tracking of /spl gamma/-rays with segmented HPGe detectors.

Fabrication and low-temperature characterization of Si-implanted thermistors

We have fabricated and characterized Si-implanted thermistors to be used as phonon sensors in very-low-temperature, single-quantum detectors. After a short review of the required thermistor properties for this type of application, a detailed description of the production process and of the characterization experimental techniques is reported. The data show that the resistivity-temperature behaviour of all of the devices follows the prediction of the variable range hopping conduction model in the investigated temperature range (4.2-0.03 K). Phonon-electron decoupling and excess low-frequency noise show up at low temperatures, reducing the thermistor sensitivity. These phenomena are discussed and conveniently parametrized in view of a complete detector optimization.

A cryogenic low-noise JFET-CMOS preamplifier for the HPGe detectors of GERDA

Cryogenic low-noise charge sensitive preamplifiers have been realized and tested for the GERmanium Detector Array (GERDA). In the search of neutrino-less double-beta decay of 76Ge at LNGS, GERDA will operate bare segmented germanium detectors immersed in liquid argon. The front-end electronics will operate in the cryogenic liquid too. An integrated JFET-CMOS preamplifier, which is fully functional at cryogenic temperatures, has been developed and realized. It has been tested in conjunction with an unsegmented p-type HPGe detector. Both the crystal and the preamplifier were operated inside a liquid nitrogen dewar at 77 K. The detector capacitance was ∼ 60 pF. An optimum resolution of 1.6 keV fwhm was obtained on the pulser line at 6 μs shaping time. The obtained resolution for the 1.332 MeV line from a 60Co source was of 2.2 keV fwhm. No peak shifts or line broadenings were seen during long-term acquisitions, thanks also to the extremely high preamplifier loop gain which yields a very high closed-loop gain stability. A wide bandwidth (rise time of 16 ns) permits use of pulse-shape analysis techniques to localize the position of the photon interactions inside the detector. A low power consumption (23.4 mW) makes the preamplifier suitable for the foreseen multi-channel array of germanium detectors.

Characterization of broad energy germanium detector (BEGe) as a candidate for the GERDA experiment

Broad Energy Germanium detectors (BEGe) offer an excellent discrimination power for pulse shape analysis of signals induced by interactions in the active volume of the detector. Such a feature makes them potential candidates for double beta decay experiments. In fact, analysis of time development of pulses allows to reject multi sites events (MSE) for which ionization takes place in more than one position, from single-site events (SSE) that release all the energy within a small volume. Double beta decay events, characterized by the interactions of the two electrons emitted, belong to the latter category of events, while MSE, mainly γ-rays interactions, constitute the background that has to be rejected. BEGe are currently considered as potential candidates the Phase II of GERDA experiment, looking for 76Ge double beta decay at the INFN Underground Gran Sasso National Laboratory (LNGS). Characterization of a commercial BEGe from Canberra is presented together with the results of the pulse shape analysis. Moreover, a full detector model (electric field and pulse generation), developed to understand the pulse shape discrimination power of the detector and validated with dedicated measurements, is presented and discussed.

GeFRO, a new front-end approach for the phase II of the GERDA experiment

A novel approach was developed for the readout of ionization detectors in cryogenic environments. It was proposed for the phase II of the GERDA experiment at LNGS, and it is currently under test by the GERDA collaboration. The readout scheme consists of a JFET and a diode operated at cold, close to the detector for the best noise performances, and of a remote second stage of amplification located at room temperature. The second stage provides a slow feedback loop through the diode to discharge the input node after each event. The fact that only two components are located at cold allows to minimize the amount of mass contributed by the electronics close to the detector: this is a very remarkable aspect when radiopurity issues are concerned. The first protoype was named GeFRO (Germanium FROnt-end), and it was succesfully tested with a coaxial High Purity Germanium detector of 20 pF capacitance from GERDA phase I, obtaining a zero-energy resolution of 1.1 keV, and with a Broad Energy Germanium detector of about 1 pF from GERDA phase II, where the zero energy resolution improved to 0.95 keV thanks to the reduced detector capacitance.

Spectroscopic performances of the GERDA cryogenic Charge Sensitive Amplifier based on JFET-CMOS ASIC, coupled to germanium detectors

In the GERDA (GERmanium Detector Array) double-beta-decay experiment, it is planned to operate in liquid Argon (LAr) germanium detectors, organized in three fold strings. In this application the use of cryogenic front-end (FE) electronics is mandatory. Two versions of Charge Sensitive Amplifier (CSA), namely a 1-channel (1-ch) and a 3-channel (3-ch), based on JFET-CMOS circuits, have been realized and tested. The 3-chs CSA are designed to serve the detector string. While in the reference test the 1-ch circuit and a custom encapsulated germanium (Ge) detector (SUB) were operated both submerged in liquid Nitrogen (LN2), in the naked detector test both the 1-ch circuit and the naked unsegmented Ge detector were submerged in liquid Argon (LAr). A resolution of 3.2 keV FWHM at 1.332 MeV 60Co has been obtained in the latter configuration to be compared to 2.2 keV obtained in the reference test. The 3-ch CSA, based on three JFETs connected to three channels of the CMOS ASIC, mounted on a Cuflon PCB, has been tested both coupled to the reference SUB and to a naked prototype detector. The obtained resolution for the 1.332 MeV line of 60Co was 2.4 keV with the CSA coupled to the SUB, and 2.9 keV with the naked Ge detector. The spectroscopic performances have been measured connecting the CSA output to an acquisition system through 10 m long cryogenic cables to simulate the real FE connection in the GERDA environment.

Setup of cryogenic front-end electronic systems for germanium detectors read-out

Front-end electronic devices for the read-out of ionizing radiation detectors must operate in many cases at cryogenic temperatures. In this work we focus in particular on front-end read-out systems for High-Purity Germanium (HPGe) detectors, which are usually operated at Liquid Nitrogen (LN) temperature. We analyze the strong effects that the changed characteristics of the electronic active and passive devices have on the charge preamplifier performance when operated in LN, while taking into account the particularly challenging requirements that the circuit has to meet: radio-purity, physical reliability under thermal cycling, low noise (0.1–0.2% resolutions) and fast rise time (~20 ns) needed for pulse shape analysis applications. The developed circuit consists of an external silicon JFET (Junction Field Effect Transistor), an external feedback network, and an ASIC (Application Specific Integrated Circuit) realized in a 5V 0.8μm CMOS technology. This work has been carried on in the framework of the GERDA experiment (GERmanium Detector Array). We will focus in particular on the effects that this challenging cryogenic setup has on the preamplifier performances.

Low noise integrated CMOS front-end electronics for gamma rays spectroscopy

We present a new integrated CMOS front-end electronics for gamma rays spectroscopy designed to cope with HPGe detectors with or without segmentation. It consists of two key elements: i) a low noise preamplifier with integrated input transistors and fully differential interior circuit topology, connected to ii) a fully differential operational amplifier with complete rail to rail output signals capability for light differential loads, limited to 500 mV from the rails levels for 200 Ohm differential load. Among the key feature of the front-end electronics are: i) lower power consumption (< 50 mW) and much smaller dimensions per channel compared to equivalent hybrid preamplifiers; ii) possibility of direct connection also to low impedance long differential cables, thanks to the integrated differential output driver; iii) possibility of in-line tuning of the fundamental bias currents of both preamplifier and differential line driver, which allows either to lightly adjust the front shape of the output signals or even to compensate for much larger bias current changes induced by deep temperature shifts; iv) possibility of operating at any temperature level, from room temperature down to cryogenic temperatures (77 K), with almost the same performances in terms of signal front rise/fall time and usually slightly lower input equivalent charge noise at low temperatures. The front-end electronics has been designed in the AMS HV CMOS 0.8 um CXZ technology.

GeFRO: A New Charge Sensitive Amplifier Design for Wide Bandwidth and Closed-Loop Stability Over Long Distances

A new approach was developed for the design of front-end circuits for semiconductor radiation detectors. The readout scheme is a charge sensitive amplifier, split between a very front-end stage (input transistor, feedback resistor and capacitor) located close to the detector and a remote second stage located far from the detector. The element of novelty, with respect to similar configurations, is the fact that the connecting links between the very front-end and the second stage are made with transmission lines. As a result, wide bandwidth and closed-loop stability are maintained even if the distance between the very front-end and the second stage is much larger than usual, up to tens of meters. The circuit was named GeFRO for Germanium front-end, and was tested with a BEGe detector from Canberra. Timing resolutions of 20 ns (open loop) and 185 ns (closed loop with 60 ° phase margin) were obtained with 10 m long cables between the very front-end and the second stage. The noise of the circuit after a 10 μs Gaussian shaping was close to 160 e- RMS with an input capacitance of 26 pF.