Description of the Reference Cell System

The reference cell system is comprised of three subsystems--the reference target cell, the gas handling system, and the control electronics. The reference target cell is mounted inside the target enclosure, which is located in the beam path immediately upstream of the spectrometers. The gas handling system consists of a gas manifold, which is located in the electronics rack behind the laser hut, and a series of gas lines that connect the manifold to the reference cell. A control box consisting of electronics that control the solenoid valves of the gas system is also located in the electronics rack. A remote control box, identical to the one in the hall, is located in the counting house. A schematic of the entire system is shown in figure

. The individual subsystems are depicted in figures

**Figure:** Schematic of reference cell system
$\includegraphics[scale=0.6]{ref_2.epsi}$

Under normal operating conditions, the manifold, the reference cell, and the intervening gas lines will be filled with either

He, N

, or $^{4}$ He gas at high pressure (typically $\approx 10$ atm). The system may be operated either locally (at the panel) or remotely (via switches mounted in the counting house). A switch on the gas panel itself allows one to toggle between local and remote control. All switches, local and remote, are equipped with LED displays so that one can readily determine the status of any switch.

The reference cell consists of a very thin-walled glass flask. An outlet at the opening is joined to a Copper tube by means of a glass-to-metal seal. This Copper tube is connected to a 1/8" dia. Copper tube with quick-plug-on gas fittings. High pressure

He gas bottle is mounted near the target chamber and is connected to the other end of the 1/8" Copper tube through solenoid valve V3. The rest of the gas system is connected to the 1/8" copper tube by a long 1/2" Copper tube. The reference cell is mounted at the gas fitting. A special coupling tool has been designed to facilitate the coupling/de-coupling the reference cell at the quick-plug-on gas joint.

The gas handling system consists of 8 solenoid-controlled, air-actuated valves, one hand-set needle valve, two baratron pressure gauges (0-1000 torr and 0-1000 psia), two pressure relief valves, a bottle of high-pressure

He gas, a bottle of high pressure N

gas, a bottle of high pressure $^{4}$ He gas, an oil-free turbo pump backed up by a pre-pump, and sufficient tubing and pipe fittings to connect them all together. There are 5 basic actions that the gas handling system must accomplish; pump-out, vent, fill with N

or $^{4}$ He, and fill with

He. The selection between N

and $^{4}$ He is done by a special directional solenoid valve (V8).

The reference cell control panel (shown in Fig.

) is comprised of 5 pushbutton switches, which activate the 5 possible valve setting conditions, a baratron high-pressure gauge which reads between 0-1000 psia, a baratron low-pressure gauge which reads between 0-1000 torr, two toggle switches to turn on and off the vacuum pump and a toggle switch to select local or remote operation. The valve configurations for the five actions are described in Table

Table: Action of the remote-control switch panel

Condition	Action	Valves activated (open)
C1	Evacuate	V1+V4+V5+V6
C2	Vent	V4+V5+V6+V7
C3	$\bf ^3$ He fill	V3+V4+V6
C4	N $\bf _2$ fill	V2+V4+V6
C5	$\bf ^4$ He fill	V2+V4+V6+V8

**Figure:** Valve configuration of the reference cell gas system
$\includegraphics[scale=0.6]{ref_panel.epsi}$

**Figure:** Control panel
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