A scintillation cell operates by allowing a charged particle to impinge upon a luminescent material attached to its surface. The struck luminescent substance then emits a pulse of light, usually in the visible range, to provide an indication of the particle's presence. Measuring the total number of pulses of light thus produced gives a quantitative measurement of the radioactivity in the particular sample.
Previously, scintillation cells, especially those for alpha radiation, assumed the shape of a bell with a transparent glass glued across its open end. A stopcock or other valve attached to the neck of the bell-shaped cell and allowed for the entrance of the sample, usually a gas. A luminescent material, coated on the inside of the bell, converted the charged particles in the sample to light. This light, after passing through the glass at the end of the bell, contacted a photomultiplier tube for detection.
The article, "Improved Low-Level Alpha-Scintillation Counter For Radon" by H. F. Lucas, Review of Scientific Instruments 28, 680 (1957), indicates the acceptability of stainless steel, iron, or Kovar for the bell portion of the cell. It also indicates that Pyrex.RTM. glass produces an unacceptably high background counting rate. Kovar became the clearly preferred material since it has the same coefficient of thermal expansion as the glass, to which it attaches.
However, to reduce the expense of the item, especially when using the costly Kovar material, the bell portion of the cell generally has a very thin wall. Moreover, the thin wall presumably assists in drawing the material into the desired bell shape. The bell shape, in turn, provides the thin-walled cell with greater structural strength. Consequently, its displays less susceptability to deformations caused by externally applied forces such as impacts with other objects. Furthermore, the rounded end can also best withstand the positive and negative partial pressures produced by the introduction and withdrawal of sample fluids.
However, the bell cell suffers from various structural shortcomings. Initially, the cell's structure simply does not allow its convenient and safe placement upon a surface. The glass cover over the end of the bell represents the only flat portion of the cell. Yet, placing the glass in contact with another surface runs the risk of possibly scratching the glass and very likely soiling it. Moreover, the cell generally carries a stopcock on the rounded end, furthest removed from the glass. The stopcock generally represents a large fraction of the cell's weight. Thus, setting the cell down with the glass in contact with the supporting surface results in a center of gravity near the top of the cell where the stopcock attaches. This elevated center of gravity accordingly represents a relatively unstable situation. A slight, inadvertent touching of the cell may knock it over, possibly damaging it; the stopcock may break off, the thin wall of the bell may dent, or portions of the scintillating material may dislodge from the inside of the cell.
The thin metal wall of the cell reduces the cost of the item as well as facilitating the drawn bell shape. Yet, it also introduces undesirable features into the item. Simply grabbing the cell too tightly may deform the cell's wall. Even where only temporary, the deformation may nonetheless dislodge the scintillating material on the inside.
Furthermore, the cell generally requires an electrical connection to drain off the charge that may develop upon it. The thin-walled cell permits only a surface contact with a loose-fitting clamp to provide this desired electrical connection. The clamp, however, may loosen or the cell wall may develop a coating of metal oxide, either of which would reduce the efficiency of the electrical contact.