Improved grounding concept / design for highly segmented and high-resolution Ge detector systems

Abstract

The position of the ground leads in the ATC cryostat is characterized by a 120° symmetry that determines the position of the ground leads of the individual Ge detector systems, and all cable lengths and connections are absolutely identical.

Description

[0001] In complex (high-segmented, high-resolution) germanium (Ge) detector systems with fast signals, there is always the problem of self-generating and high-frequency oscillations (MHz) that prevent any measurement because they mask the weak event signals. Avoiding these oscillations becomes increasingly difficult the more signals are read out in a Ge detector system (for example, 111 in the ATC). The following factors contribute to this problem: 1. The desired signals are based on extremely weak charge currents (100-500 nanoamperes) which are amplified by a factor of 100 by the FETs and then directed to the warm electronics outside the cooling cryostat. 2. The signals have a fast rise time, and their Fourier analysis reveals a large number of very high frequencies. 3. The Ge detector systems and thus the 0 level grounding are connected to the cooling finger and the entire housing. 4. Due to technical limitations and poor connections in the electronics wiring, different zero-level grounds are assigned to different signal paths. Ground loops and, consequently, massive oscillations are the result. 5. Sufficiently good grounding could be achieved with large diameter wiring. However, since this connects the warm layer of the atmosphere-side electronics with the cooled layer (FETs) in a vacuum, it leads to enormous thermal bridges that cannot be compensated for by cooling.

[0002] Ideally, the Ge detector systems and the signal readout, along with the subsequent electronics, would be isolated from the cryostat's ground. However, this is not possible, as good electrical insulation would also prevent adequate cooling of the Ge detector systems. Therefore, a different solution is needed. State of the art:

[0003] In previous, more complex geometer detectors, adjusting the electronics to achieve oscillation-free operation was a lengthy task that could only be solved through trial and error and considerable experience. The more signals that needed to be read, the more difficult the task became. Inventive step:

[0004] The invention consists of aligning the electronics concept and the entire layout towards optimized grounding. This is achieved by: 1. Suitable signals are grouped in blocks (e.g., 6-signal blocks in the ATC), both in the cold and warm electronic components. 2. All cable strands have exactly the same length 3. All grounding cables and return current cables for all blocks and thus signal paths within the same detector system not only have the exact same length (down to the millimeter), but also identical connections. 4. Different elements of the cooling loop (cooling fingers coming from the Dewar) within the cryostat are bridged with thick copper conductors, thus eliminating the high-resistance indium connections between these elements. 5. The construction plane of the cold preamplifiers with their FETs is defined as the 0-level and a star-shaped grounding from there to the warm electronics is implemented. 6. The grounding connection from the "cold" plane (vacuum side) to the "warm" plane (atmosphere side) is made with sufficiently thick, but optimized thin cables. 7. The shielding of the primarily cold FETs against interference from nearest neighbors (so-called cross-talk) is also consistently kept at the defined 0-level level. 8. Massive grounding connections are soldered to each other in the cold area. 9. Any other electronics, such as temperature sensors or heating resistors, must be routed with shielded cables far away from the signal electronics. Commercial applicability:

[0005] The invention is used in the production of every asymmetric AGATA Triple cryostat and is commercially marketed. The invention is also successfully used in comparable cryostats for other applications. Summary:

[0006] The present invention for improving grounding and thus avoiding serious oscillations within a highly segmented Ge detector system is based on the consistent, clean and identical cable layout in millimeter lengths and

[0007] Micro-ohm resistance range. Adherence to this concept significantly improves reliability against spontaneously induced oscillations. Terminology ATC AGATA Triple Cryostat (abbreviation for AGATA Triple Cryostat) is the system that enables the operation of exactly three highly segmented Ge detectors. The cryostat ensures the required high vacuum, mechanical precision in detector positioning, cooling to below -180 °C, and electrical readout of a total of 111 high-resolution signal outputs. Array A detector array is a combination of many detectors that, similar to a soccer ball, form a (closed) spherical shell. This is achieved with a specific number of hexagons and pentagons that then form the front of the detectors. Dewar A Dewar is a vacuum-insulated storage vessel for liquid nitrogen, similar to a thermos flask. For more complex detector systems, which can be suspended flat or upside down in an array, so-called all-layer Dewars are used, whose internal construction prevents the liquid nitrogen from leaking out. LN2 The abbreviation stands for Liquid Nitrogen. The 2 indicates that naturally occurring nitrogen consists of two molecules. Getter A getter is a chemically reactive material used to maintain a vacuum for as long as possible. Gas molecules form a direct chemical or physical bond at the getter's surface and are thereby extracted from the volume. A standard getter is, for example, zeolite, an open-pored ceramic that, when cooled to liquid nitrogen temperature, is an effective getter. Grounding English term for grounding, or earth (electrical); the grounding is the reference level against which a measurement is taken or a signal is generated. FET Abbreviation for Field Effect Transistor, a semiconductor transistor with high sensitivity and low intrinsic noise combined with high gain. The electronic performance of these components is particularly good when cooled. List of designations

[0008] Fig. : (1) Shielded ground wire of a Ge detector system (exemplary). (2) Shielded ground wire of a Ge detector system (exemplary). (3) Contact point at the vacuum feedthrough to the warm, atmospheric side. (4) Contact point on the cold plane in a vacuum. Fig. : (1) Shielded ground wire of the Ge detector systems. (2) The angle between two mounting positions of the ground leads is 120°.

Claims

[1] Location of the ground leads in the ATC cryostat characterized by , that a 120° symmetry determines the position of the ground leads of the individual Ge detector systems, and that all cable lengths and connections are absolutely identical. [2] Material selection for the ground wires characterized by , that only shielded ground wires made of pure silver or copper are used. [3] Material construction of the grounding conductors characterized by , that the cross-sectional area is sufficiently large in relation to the identical length of all ground leads for a disturbance-free ground contact, but at the same time is so small that the thermal contact between cold and warm contact points is minimized.

Images