Detector Dewar Construction

To understand detector dewar construction it is first necessary to understand how common detectors function and interface with the dewar. The basis for many detectors is the semiconductor P-N junction. P-type semiconductors have a surplus of “holes” or electron deficiencies in their lattice. N-type semiconductors have a surplus of conduction electrons. Both holes and conduction electrons can travel throughout the semiconductor and act as current carriers.

At the P-N junction, holes and conduction electrons spill over from their respective sides to form a negative charge on the P-side and a positive charge on the N-side. An electric field is formed at the junction and depicted by the slope of the energy bands in the band picture. The junction acts as a diode because a positive potential on the P-side and a negative on the N-side forces more of the existing holes and electrons across. Reversing the potential results in no current because the holes and electrons are pulled away from the junction and none cross it.

detector dewar construction

The junction is illustrated above. Open circles on the left side of the junction represent “holes” or deficiencies of electrons in the lattice and act like positive charge carriers. Solid circles on the right of the junction represent the conduction band electrons in the n-type dopant. There are no mobile charges in the junction region because any of them are swept away by the electric field. This region is known as the “depletion region.”

The P-N Junction is used as an electromagnetic radiation detector. Radiation of sufficient energy breaks a bond in the semi-conductor, releasing a conduction electron and forming a hole. When this occurs at the junction these two particles are swept away by the electric field forming a current pulse. Pairs formed away from the junction have a good chance of recombining and not being detected. The number of pairs formed is proportional to the energy of the incoming photons.

Radiation detection at the P-N junction is most efficient for photons with energy just above that required to form the electron/hole pair. Photons whose energy are below or very much above the pair formation energy are not absorbed or detected. The semi-conductor used for detection has to be matched to the radiation to be detected. Examples include Indium-Galium-Arsenide for infra red and silicon for X-rays.

Electron/hole pairs can also be formed by thermal energy within the semiconductor and appear as noise in the detector signal. This noise is exponentially dependent on the temperature and can be greatly reduced by cooling the detector.

The signal from the detector is small and has to be greatly amplified. The first stage in amplification is usually a field effect transistor placed next to the detector. This transistor is also affected by thermal noise and has to be kept cool.

Liquid nitrogen at 77°K (-320°F) is a cheap, reliable and readily available cooling source. It is thermally connected to the detector and transistor by a “cold finger” typically made of copper. Care must be taken in the design of this thermal connection to ensure that there are no thermal excursions or resonances. Such variations in temperature would cause variations in the number of thermally generated electron/hole pairs at the P-N junction and variations in the gain of the field effect transistor. The net result is an increase in noise in the detector signal.

The connection must also be free of microphonics. This phenomenon occurs when a mechanical vibration results in a variation in the output of an electronic device. The radiation detector and field effect transistor can both be affected. A vibration can cause both of these elements to change size which changes the energy required to form electron/hole pairs and the capacitance. A vibration can also transfer energy into vibration of the semiconductor lattice which in turn generates more electron/hole pairs. All of these effects result in signal noise.

Vibrations can be produced within a dewar when liquid nitrogen boils at isolated hot spots. Hot spots are often caused by roughness in the wall of the dewar. The Technifab dewar is manufactured with an extremely smooth interior to reduce the number of potential hot spots. Technifab craftsmen cover the interior of the dewar with a chrome conversion coating after final welding of the components. This coating covers any roughness with a very smooth surface.

The Technifab dewars are manufactured with materials of similar electronegativity in order to minimize galvanic activity within the dewar. The dewars are all 100% cold leak checked to make sure nitrogen hold time will be preserved. All dewars are helium mass spectrometer leak checked at 2 X 10-10 cc/sec sensitivity.