Ripristino rivelatori HpGe ( focus on Ortec PopTop)

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cipndale
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Ripristino rivelatori HpGe ( focus on Ortec PopTop)

Messaggio da cipndale » 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.
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Ultima modifica di cipndale il 15/11/2022, 15:06, modificato 6 volte in totale.



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cipndale
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Re: Ripristino rivelatori HpGe

Messaggio da cipndale » 26/03/2022, 20:57

La seconda parte tratta il ripristino del cosi-detto streamline. La procedura puo essere utilizzata sia per il ripristino del vuoto che per ripristinare i rivelatori che hanno subito dannegio con neutroni.
STREAMLINE
The system cryostat is a streamline horizontal-dipstick type (Model CrG-N-SH-GMX) to which is a 0.5'' Viton sealed vacuum valve has been added (Fig. 3. 1 ). This allows the user to attach the cryostat to a clean. high vacuum pump.
A copper cooling rod extension is provided (Fig. 3.2). At the beginning of the repair procedure this cooling rod extension must be clamped to the end of the copper cooling rod which extends from the bottom of the dipstick. A piece of indium foil is supplied which is to be positioned between the clamp and the cooling rod to ensure good thermal contact. When the dipstick and the cooling rod extension are wrapped with heater tape, it is possible to heat the detector element to the desired temperature.
MODEL 496 TEMPERATURE CONTROLLER
The temperature controller is a thermostat-type controller which uses a J-type thermocouple as a sensor element (Fig. 3.3). The controller plugs directly into a 110-V ac outlet, and the heater tape plugs directly into the controller. (A transformer is available for 220-V ac line operation and should be requested at time of order.). The thermocouple element is positioned between the heater tape and the cooling rod extension. The controller cycles the heater tape on and off as necessary to maintain the desired temperature. To set the controller for the desired temperature use the wrench provided to loosen the socket set screw which 1s at the lower right of the temperature dial (Fig. 3.3). Set the indicator line on the adjustment knob to the desired temperature. Then tighten the socket set screw to fix the knob at that position.
HEATER TAPE
The heater tape provided with this system is to be tightly wrapped around the cooling rod extension and the cryostat dipstick, from the bottom up (Fig. 3.4). To avoid the possibility of overheating the Viton O-ring seal. do not allow the tape to be positioned closer than 3 in. to the flange.
The heating element is sealed in a heavy silicone rubber tape. The plug consists of two prongs in a split housing which can be separated for wrapping and then rejoined before plugging in.
The heater tape must be wrapped so that the tip of the thermocouple probe is held firmly between the tape and the cooling rod extension. This is necessary because the temperature-sensitive thermocouple junction is located in the tip of the probe. Failure to properly position the probe tip may result in excessive heating which can cause serious damage.
It is strongly recommended that an auxiliary thermometer also be placed between the heater tape and the cooling rod extension. If the temperature exceeds 135° C, corrective action should be taken immediately.
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cipndale
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Re: Ripristino rivelatori HpGe

Messaggio da cipndale » 27/03/2022, 12:02

Elenco attrezzi per ripristinare un HpGe.
- pompa primaria (rotativa);
- pompa secondaria (turbo);
- controller pompa turbo;
- (optional) misuratore vuoto (ideale Pirani + catodo freddo);
- (optional) quando c'è deposizione di idrocarburi al interno della capsula, riscaldatore a resistenza con controllo di temperatura (per esempio Ortec 496).
- valvola Richardson (specifica per vari modelli);
- flange varie;
- pazienza.
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cipndale
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Re: Ripristino rivelatori HpGe

Messaggio da cipndale » 10/04/2022, 20:13

Le cosi dette capsule PopTop possono perdere il vuoto dopo un lungo tempo d'utilizzo. Per ripristinare il vuoto ci vogliono altri due accessori oltre l'accessorio VVO-2, l'ultimo serve anche per ripristinare il vuoto nei DEWAR Ortec.
Qui vi presento una modalità economica per portare ad alto vuoto una capsula PopTop, eliminando il fabbisogno di tutti gli accessori consigliati da Ortec.
Comunque c'è la necessità di reperire una qualsiasi valvola Richardson a baso costo. Giusto per pura coincidenza la "soluzione" è stata costruita a torno una valvola [External Link Removed for Guests], trovata a baso costo sulla baia. Il resto si basa su pezzi trovati nella ferramenta al angolo. Pur primitiva, la soluzione permette di arrivare a 3.6x10-6 Torr, portando tranquillamente al ripristino del vuoto nel PopTop. Penso che le immagini parlano a posto mio.
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max56fe
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Re: Ripristino rivelatori HpGe

Messaggio da max56fe » 11/04/2022, 9:30

gran bel lavoro cipndale ............... complimenti !


Tutto a questo mondo ha una fine...................... solo la salciccia ne ha 2 !

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cipndale
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Re: Ripristino rivelatori HpGe

Messaggio da cipndale » 15/11/2022, 15:04


Dettagli costruzione rivelatori Ortec PopTop U.S. patent 4851684A

Abstract
In a photon detector system, such as a cryogenic gamma radiation detector cooled by a dewar, a vacuum-jacketed radiation detector is provided in a cryostat assembly. The detector can be used with any one of a plurality of cryostats without breaking the vacuum seal therearound when transferring from one cryostat or dewar to another. The detector is vacuum-jacketed and mechanically and thermally connected to an axially projecting conducting member, which member is received in a thermally conducting transition member contained in the cryostat. The transition member is thermally and mechanically coupled to the dewar and may include a fluid path completing a vacuum jacket in the dewar.
MODULAR PHOTON DETECTOR CRYOSTAT ASSEMBLY AND SYSTEM
BACKGROUND OF THE INVENTION
This invention relates to cryogenically cooled radiation detectors and more specifically to means in a cryostat for thermally and mechanically connecting a detector to a dewar.
Photon detectors or spectrometers are useful in measuring low levels of radionuclides. Examples of such measurements include monitoring of the environment and effluent discharge of nuclear power stations, measurement of food product for human intake and evaluation of the natural environment. Because of the radionuclide content of some samples, it has become standard practice to use the resolving power of germanium semiconductor detectors to identify and quantify the isotopes present. Germanium semiconductor gamma-ray spectrometers have been developed to provide optimum performance for such measurement. When x- or gamma rays impinge on a germanium detector, there is a finite probability of an interaction occurring which results in the creation of electron-hole pairs. If the impinging beam has an intensity, I, at a given energy, E,
then it is absorbed in the detector according to an exponential law:

I(X)=Io(-X/λ),

where I0 is the beam intensity at the surface of the detector and X is the distance from the surface, and 1/A. is the absorption coefficient and can be considered as the sum of three components due respectively to the photoelectric, Compton and pair-production processes. When an interaction occurs and electron-hole pairs are created, the electric field due to the bias voltage sweeps out the charge carriers resulting in an induced current pulse which is integrated at the input of a charge-sensitive preamplifier. Both charge carriers, the electrons and the holes, contribute to the current pulse. In order to provide for this operation, the germanium detector must be vacuum-jacketed and cryogenically cooled. Normally, such detectors are cooled to below 100K. A nominal operating temperature is 77K.
The germanium detector is incorporated in a cryostat. The cryostat comprises an evacuated housing surrounding the detector, the detector itself, a cryogenically cooled field effect transistor preamplifier in the housing, electronics circuit boards exterior to the housing mounted on support means coaxial with and extending from the detector, and means for communicating the detector with a cryogenic cooling source such as liquid nitrogen. Cryostats are provided in various sizes and configurations for various applications. Similarly, different dewars are utilized for varying applications.
One form of cryostat may be connected to a dewar having a handle projecting therefrom which is balanced for maintaining the cryostat in a hand-held horizontal disposition. Different dewars may be provided to provide for eight hour or twenty-four hour holding time.
Other dewars may be provided which have cryostats protecting from a top portion, a bottom portion or a side thereof. For different applications, nominal dewars may come in sizes of 0.4, 1.2, 7.5, 15, 25 or 30 liters. Different specific applications require different combinations of dewar and cryostat. The conventional means for connecting cooling devices to the detector has resulted in construction of a closure in which vacuum jacket
surrounding the detector is the same vacuum jacket that must surround the portion of the cryostat extending from the detector into the dewar and in some cases the dewar vacuum jacket. This widely successfully used and conventional design does not permit modularity of assembly between detector heads and different cryostats or different dewars.
The experience in the art is that in reworking of detectors to move them from one cryostat assembly to another, yield rate is low. Consequently, an expensive inventory of cryostats must be maintained for a number of applications.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a cryogenically cooled photon detector system comprising a dewar and a cryostat which is modular in construction.
It is also an object of the present invention to provide a detector which may be included in a modular cryostat assembly. It is a more specific object of the present invention to provide a detector head for inclusion in a system of the type described with its own vacuum jacket.
It is a further specific object of the present invention to provide a detector head of the type described including a thermally isolated and vacuum jacketed conduction means for providing thermal communication between the detector and a cooling source.
Briefly stated, there are provided in accordance with the present invention, a spectrometer including a cryostat and a modular germanium photon detector head assembly for inclusion therein. A detector is included in a vacuum-jacketed head and thermally and mechanically connected to a conducting member projecting from the vacuum chamber to the outside of the detector head assembly. The conducting member projects into a
cryostat transition member which may mate with a dewar to define a sealed, evacuatable chamber in fluid communication with a vacuum-jacket surrounding the dewar. A thermally conductive receptacle in the transition member extending from the dewar engages the conducting rod to complete the system.
BRIEF DESCRIPTION OF THE DRAWINGS
The means by which the foregoing objects and features of invention are achieved are pointed out in the claims forming the concluding portion of the specification. The invention, both as to its organization and manner of operation may be further understood by reference to the following description taken in connection with the following drawings.
Of the drawings:
FIGS. 1 through 3 are each an elevation partially in cross-sectional form illustrating a system constructed in accordance with the present invention in various stages of assembly;
FIG. 4 is an exploded view in axonometric form further illustrating the cooperation of subassemblies in the present invention;
FIGS. 5 and 6 are partial detailed views of a further form of the present invention further illustrating fluid
interfaces within an assembled system; and FIG. 7 is a cross-sectional view of a further form of transition member in a cryostat coupled to a partially illustrated dewar.
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Ultima modifica di cipndale il 16/11/2022, 22:20, modificato 10 volte in totale.



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cipndale
Messaggi: 491
Iscritto il: 16/01/2016, 18:41
Località: Napoli

Re: Ripristino rivelatori HpGe

Messaggio da cipndale » 15/11/2022, 15:04

...
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