Ripristino rivelatori HpGe ( focus on Ortec PopTop)
Inviato: 26/03/2022, 20:49
Una breve descrizione per ripristinare i dewar per i rivelatori HpGe e anche i HpGe stessi (Streamline e PopTop).
Il testo e' tutto in inglese, proveniente da vari documenti vintage, risalendo ai tempi quando la gente poteva ancora mettere le mani sui rivelatori. Oggi queste procedure sono piuttosto riservate alle dite che vendono gli strumenti, a prezzi di manutenzione fuori dalla disponibilità dei comuni mortali. Per esempio un comune ripristino puo arrivare anche a 10k Euro. Anche se tutto il testo e' scritto per i rivelatori Ortec, basta cambiare la valvola Richardson e' la procedura potrebbe essere estesa a qualsiasi tipo di rivelatore.
Detector Vacuum: Pump-Out and Bake-Out
Germanium crystal spectrometers must be manufactured and maintained at high purity and operated at low temperatures to minimize :free electron currents when the bias voltage is applied across the p-n junction. In order to achieve and maintain satisfactory temperatures, it is necessary to minimize heat gain by the crystal through any process of conduction, convection, or radiation. When the crystal has been mounted properly and the supporting electronics installed, the cryostat is evacuated to a pressure of about 10-5 to 10-6 atm in order to inhibit heat gain by convection. To help maintain this low vacuum, a small amount of "getter" material has also been installed in the cryostat to absorb air and other impurities which may enter the vacuum.
A common failure mechanism for germanium detectors is the loss of vacuum in the cryostat and the consequent inability to maintain the germanium crystal at liquid nitrogen temperature. A valve port is installed on the preamplifier side of the detector segment for attaching to a vacuum system. A special valve, called a Richardson valve, has been fabricated for opening and closing the main valve port while the system is under vacuum. If the detector has lost vacuum, simply pumping it down to operating vacuum may be adequate to restore operation. However, if the detector surface has become contaminated with non-volatile materials, it may also be necessary to heat the detector assembly to assist in driving off these impurities from both the Ge crystal and the getter. Accumulation of these impurities on the surface of the germanium crystal can result in electron transport, and hence, dark currents, on the detector which will adversely affect the resolution. For this reason it is important that air not be allowed into the vacuum chamber while the detector is cold, because these impurities may become irreversibly attached to the crystal surface. Pumping the detector requires removing the three screws that hold the bellows to the preamplifier end of segment 2. Carefully pull the bellows away from the detector segment (Figure G 1 ), and install the short safety harness between segments 2 and 3 to prevent breaking any of the wires going through the bellows. Mount the Richardson valve, which has been connected to a suitable vacuum system, using two #40 machine screws through the ears of the valve by carefully screwing into the holes on the flat plate on either side of the main valve port.
Figure G2 shows the Richardson valve . Screw the threaded pushrod of the Richardson valve into the main valve port plug. It may be advantageous to thread the pushrod into the plug before.
An oil-free type of vacuum pump should be used such as a helium leak detector system which has a pumping system along with a system to measure He gas. First obtain a good vacuum between the pump and the Richardson valve, and then pull the main valve port plug open with the attached rod. Generally speaking, a satisfactory vacuum can be re-established by pumping overnight. If it is suspected that the detector surface may have become contaminated, then the detector system needs to be baked at -100 °C for 24-72 hours while pumping. This is most easily accomplished by wrapping a heat tape connected to a Variac for temperature control around the detector and Dewar assembly. A digital thermometer can be used to monitor the temperature.
When a satisfactory vacuum has been obtained, seal the port by pushing the plug back into the main valve opening. Carefully unscrew the pushrod so it can be backed out of the plug, then vent the hose and carefully remove the Richardson valve. Remove the safety tether and reinstall the bellows.
Ultimately, if satisfactory detector specifications cannot be achieved, it may be necessary for the manufacturer to remove the germanium crystal and acid etch it (sometimes even a slight trim is required) to remove the surface contaminants from the crystal and re-establish optimum detector performance.
Il testo e' tutto in inglese, proveniente da vari documenti vintage, risalendo ai tempi quando la gente poteva ancora mettere le mani sui rivelatori. Oggi queste procedure sono piuttosto riservate alle dite che vendono gli strumenti, a prezzi di manutenzione fuori dalla disponibilità dei comuni mortali. Per esempio un comune ripristino puo arrivare anche a 10k Euro. Anche se tutto il testo e' scritto per i rivelatori Ortec, basta cambiare la valvola Richardson e' la procedura potrebbe essere estesa a qualsiasi tipo di rivelatore.
Detector Vacuum: Pump-Out and Bake-Out
Germanium crystal spectrometers must be manufactured and maintained at high purity and operated at low temperatures to minimize :free electron currents when the bias voltage is applied across the p-n junction. In order to achieve and maintain satisfactory temperatures, it is necessary to minimize heat gain by the crystal through any process of conduction, convection, or radiation. When the crystal has been mounted properly and the supporting electronics installed, the cryostat is evacuated to a pressure of about 10-5 to 10-6 atm in order to inhibit heat gain by convection. To help maintain this low vacuum, a small amount of "getter" material has also been installed in the cryostat to absorb air and other impurities which may enter the vacuum.
A common failure mechanism for germanium detectors is the loss of vacuum in the cryostat and the consequent inability to maintain the germanium crystal at liquid nitrogen temperature. A valve port is installed on the preamplifier side of the detector segment for attaching to a vacuum system. A special valve, called a Richardson valve, has been fabricated for opening and closing the main valve port while the system is under vacuum. If the detector has lost vacuum, simply pumping it down to operating vacuum may be adequate to restore operation. However, if the detector surface has become contaminated with non-volatile materials, it may also be necessary to heat the detector assembly to assist in driving off these impurities from both the Ge crystal and the getter. Accumulation of these impurities on the surface of the germanium crystal can result in electron transport, and hence, dark currents, on the detector which will adversely affect the resolution. For this reason it is important that air not be allowed into the vacuum chamber while the detector is cold, because these impurities may become irreversibly attached to the crystal surface. Pumping the detector requires removing the three screws that hold the bellows to the preamplifier end of segment 2. Carefully pull the bellows away from the detector segment (Figure G 1 ), and install the short safety harness between segments 2 and 3 to prevent breaking any of the wires going through the bellows. Mount the Richardson valve, which has been connected to a suitable vacuum system, using two #40 machine screws through the ears of the valve by carefully screwing into the holes on the flat plate on either side of the main valve port.
Figure G2 shows the Richardson valve . Screw the threaded pushrod of the Richardson valve into the main valve port plug. It may be advantageous to thread the pushrod into the plug before.
An oil-free type of vacuum pump should be used such as a helium leak detector system which has a pumping system along with a system to measure He gas. First obtain a good vacuum between the pump and the Richardson valve, and then pull the main valve port plug open with the attached rod. Generally speaking, a satisfactory vacuum can be re-established by pumping overnight. If it is suspected that the detector surface may have become contaminated, then the detector system needs to be baked at -100 °C for 24-72 hours while pumping. This is most easily accomplished by wrapping a heat tape connected to a Variac for temperature control around the detector and Dewar assembly. A digital thermometer can be used to monitor the temperature.
When a satisfactory vacuum has been obtained, seal the port by pushing the plug back into the main valve opening. Carefully unscrew the pushrod so it can be backed out of the plug, then vent the hose and carefully remove the Richardson valve. Remove the safety tether and reinstall the bellows.
Ultimately, if satisfactory detector specifications cannot be achieved, it may be necessary for the manufacturer to remove the germanium crystal and acid etch it (sometimes even a slight trim is required) to remove the surface contaminants from the crystal and re-establish optimum detector performance.