Simple and Sensitive Low Temperature Control Apparatus for Mossbauer Spectroscopy Arthur J. Nozik a n d Morton Kaplan Department of Chemistry, Yale University, New Haven, Conn. MANYSTUDIES utilizing the Mossbauer effect require detailed measurements of the resonance absorption parameters as a function of temperature. In addition to temperature regulation requirements, a thermostat for Mossbauer experiments must be designed to permit transmission of low-energy gamma rays. We describe below a simple but very sensitive control apparatus for use in the range between room temperature and liquid nitrogen temperature. Cooling is accomplished by thermal exchange with a stream of cold gas, and a high degree of temperature regulation is obtained by means of a feedback mechanism which controls the gas-flow rate. Temperatures from 0°C to -180°C can readily be maintained within 0.005"C to 0.01 "C for extended time periods, and samples may be changed quickly and easily with minimum temperature disturbance. Devices for similar applications have been described previously (1-3) but the simplicity, sensitivity, and low cost of the present design are advantages which may be of importance in many laboratories. DESCRIPTION AND OPERATION
Cooling Block. The heart of the thermostat (see Figure 1) is a cylindrical copper block, 4-inch diameter X 1 inch high, containing a central channel in which the sample was mounted, and through which the gamma rays were transmitted. An internal spiral groove, extending in a continuous sweep around the circumference and top face of the block, was located near the outer surfaces. This geometrical configuration and large contact area of the cooling canal minimized thermal gradients within the block. In construction, the canal was actually machined as a 3/16-in~hwide by 3i16-inch deep semicylindrical groove into the exterior of the main block, and then rendered internal by enclosing the block in a tight-fitting copper shell. Prior to assembly, the shell was lined with a thin Foil of solder, and upon heating the assembled block, the canal was effectively sealed against leakage between adjacent turns. The samples for Mossbauer study, sealed in a plastic holder, were mounted in a hollow copper fitting which screwed into the central port in the cooling block. The gamma rays emitted from a source above the block passed through the port and sample t o a detector located below the block. The block was insulated o n all sides by 2 inches of Styrofoam, except a t the two ends of the transmission port. At these positions, Styrofoam plugs '/*-inch thick were used to permit a source-to-detector distance of 2'/4 inches. The plugs had 0.0005-inch AI foil pasted on their cold ends to act as thermal radiation shields. The outside of the Styrofoam insulation also had aluminum foil shielding and was sealed to prevent penetration of moisture. The samples could be readily changed, without seriously disturbing the block temperature, by removing the bottom Styrofoam plug and unscrewing the (1) B. Sharon and D. Treves, Reu. Sci. instr., 37, 1252 (1966). (2) F. W. D. Woodhams, J. S. Carlow, and R. E. Meads, J . Sci. instr.,43, 334 (1966). (3) W. Weidemann, W. A. Mundt, and D. Kullman, Cryogenics 5, 94 (1965).
854 *
ANALYTlCAL CHEMISTRY
sample holder fitting. This latter feature was particularly convenient in working with samples which must be maintained a t low temperatures between preparation and use. Cooling of the copper block was accomplished by flowing prepurified nitrogen gas through a heat exchanger and into the block via a short section of vacuum-jacketed transfer line. The heat exchanger consisted of 25 turns, 4-inch diameter, of '/*-inch copper tubing immersed in liquid nitrogen. Because of the very low thermal losses between the heat exchanger and the block, stable temperatures as low as -180°C could be obtained with a gas flow rate of 12 liters/minute. Operation of the thermostat a t 0°C required a flow rate of 0.25 liter/ minute. Continuous experimental runs of up to one week have been carried out with no difficulty, as long as adequate supplies of liquid and gaseous nitrogen are maintained. As will be described below, our apparatus contains a n emergency switch which automatically terminates the accumulation of data in the event that the fuel reserves become exhausted. Temperature Control and Measurement. The thermostat temperature was regulated by means of the circuit shown in Figure 2. A bead thermistor located in the copper block was put in series with a 1.35-volt mercury battery and a ten-turn helipot. The helipot was set equal to the resistance the thermistor would attain a t the desired temperature, and the voltage across the helipot was opposed by a "bucking voltage" equal to 0.675 volt. Under these conditions, the sensitivity of the net voltage to thermistor resistance is
338
- RT mv/ohm (1) where RP is the resistance set o n the helipot, RT is the thermistor resistance, and Rp ,- RT. The sensitivityof the net voltage t o temperature depends upon the characteristics of the thermistor :
By selecting various thermistors over different temperature ranges, the sensitivity can be maintained at about 25 mV/"C. Two thermistors were found adequate to cover the temperature range from 0°C to -180°C. One thermistor (Type RLlOX04-10K-315, Keystone Carbon Co., St. Mary's, Pa.) gives sensitivities of 30 mV/"C and 20 mV/"C a t - 180°C and -6O"C, respectively. A second thermistor (No. 24A16, Victory Engineering Co., Springfield, N. J.) gives 25 mV/"C and 20 mV/"Cat -60°C and O"C, respectively. These thermistors can both be located in the cooling block and switched into the circuit according t o the temperature desired for the particular experiment. The stability of these thermistors is excellent, and no change in their characteristics has been observed in six months. The net voltage of the thermistor circuit was applied to a multirange chart recorder (Model VOM-5, Bausch and Lornb, Rochester, N. Y . ) . The zero set (or internal bias) of the recorder was adjusted to position the recorder pen at midscale
Figure 1. Cross-sectional diagram of low-temperature thermostat, showing the location of internal cooling coils, temperature sensors, and removable sample holder
RELAY 1
r--1 0-
i
I
FROM DETECTOR
0-+4, 0-
!
N.C.
I
0 TO ANALYZER
I I I
TOORECORDER
MOVING PEN CONTACT T RI MPOT
THERMISTOR
C B a L RECORDER ‘EMER. S H U T OFF CONTACT
c N 2 TO COOLING COIL
PRESSURE REGULATOR
( P + B KCP11,2500d N2 CYLINDER
I
U
Figure 2. Schematic diagram of thermostat control system VOL 39, NO. 7 , JUNE 1967
0
855
for zero input voltage. This corresponded t o the control point, and was attained when the thermistor resistance reached the value set o n the helipot. Thermistor temperatures (resistances) other than the set value would cause a deflection of the recorder pen from the midpoint with a sensitivity of -25 mV per degree “error.” This error signal was used to operate a servo mechanism which made the necessary gasflow adjustments to restore the set temperature. The null detector and servo system were of very simple design. A nichrome contact attached t o the recorder pen would, upon deflection from the control point, meet a fixed contact, thereby closing a relay which would in turn open a solenoid valve. The opening of this valve results in a n increased gas-flow rate through the cooling block, and the temperature returns t o the set point, closing the solenoid valve as the two contacts separate. This simple system permitted such excellent temperature regulation that a method of “proportional error detection” was unnecessary. The fixed contact was made of nichrome-tipped stainless steel wire, 2 1 / a inches long, and flexible enough to permit the moving pen to slide past should the temperature increase out of control (because of loss of gaseous o r liquid nitrogen). Under such circumstances, the pen would then meet a n emergency contact and energize a relay circuit, shutting off the recorder and isolating the data-accumulation linkage to preserve existing data. The nitrogen gas flow t o the heat exchanger (cooling coils) passes through a section consisting of two parallel 1/4-inch copper lines, with the solenoid valve mounted in one of the lines. The flow rate is adjusted with manual valves and a rotameter such that the flow is insufficient to maintain the
856
ANALYTICAL CHEMISTRY
desired set temperature with the solenoid valve closed, but more than sufficient with the solenoid valve open. By careful adjustment of these flow rates, the recorder fluctuation (temperature cycle) can be held to 0.1-0.2 mV, corresponding to a temperature control of 0.005-0.01 “C. The absolute temperature of the sample was determined from calibration experiments in which the temperature of a sensor buried in the sample was related t o the readings of several thermistors and copper-constantan thermocouples located in the block and surrounding the sample. This procedure is more convenient than mounting a permanent temperature sensor in the sample, and was found to be highly reliable because the thermal gradient between the sample and block sensors was invariant with time and easily measured. This gradient was nonlinear with temperature, and varied from 0 to 4 ” over the temperature range 0°C t o -180°C. No significant uncertainty was introduced into the sample temperature from the calibrated differential. However, if good transmission geometry and ease of sample changing can be sacrificed, the temperature difference between sample and block can be considerably reduced by decreasing the diameter of the gamma-ray transmission port, increasing the insulation above and below the port, and using metal sample holders. Furthermore, keeping a temperature sensor in the sample would, of course, eliminate the necessity of a calibration. RECEIVED for review February 1, 1967. Accepted March 20, 1967. Work supported by the U.S. Atomic Energy Commission. Morton Kaplan is a n Alfred P. Sloan Foundation Fellow.
