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Produced by the NASA Center for Aerospace Information (CASI)
i

ro.

November 1983

CORNELL UNIVERS11rY

Center for Radiophysics and Space Research

ITHACA, N.Y.

Z 23 42S FINAL TECHNICAL REPORT

0

NASA Grant NSG-7324

ale BOLOMETER SYSTEMS" ^°-

a_

Funded

October 1, 1978 through October 31, 1982

Principal Investigator: Prof. James R. Houck

INASA-c$- 175325) ON 3He BC.LOMETER SYSTEMS N84-15526

Final Technical Report (Cornell Univ.) 10 p

SC AU2/MF A01 CSCL 14B

Unclas

G3/35 15285
tT

1s

!r 31, 1982

lames R. Houck
ABSTRACT

During the tenure of this grant a fie cryostat was constructed to

cool a germanium bolometer for use as an "nfrared detector at submillimeter wavelengths. The system had better sensitivity than any other

then existing system for these wavelengths; however, it was clear that

the system could be greatly improved if better optical coupling could be

achieved between the bolometer and the incoming photon stream. Considerable effort was expended to improve this coupling. However, even our

best results fell short of an ideal system by a factor of nearly 5 in

coupling efficiency.
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3

CRYOSTAT

The cryostat built in this program used an adsorption pump to

reduce the vapor pressure over a small bath of die. A summary of the

system's characteristics is presented in the Table 1.

Table 1 - Cryostat Performance

Liquid N 2 hold time 12 hours

Liquid `'He hold time 14 hours

Pumped -He hold time 15 hours

Operating temperature 0.37°K

Heat load into the site 13 IN

Thermal conductivity of heat switch 45 mW/°K

Volume of 3 H pumping system 120 cm 3

Amount of 3 H used in dewar 1 liter STP

Volume of `'He bath 0.87 liters

Volume of liquid N 2 bath 0.80 liters

t

The 3 H system consists of three basic parts, a charcoal -filled adsorption pump, a "He cooled condenser and a small liquid ale chamber. In

operation the system is first cooled to about 2K using conventional die

techniques. The adsorption pump is heated, forcing the 3He gas out of

the charcoal and into the condenser. The gas then condenses and runs by

gravity feed into the 3 H chamber. The adsorption pump was then activated by closing a heat switch between it and the ``* bath. The ^e was

thus pumped back into the charcoal, resulting in a cooling of the

remaining liquid. The system had 3He hold-time of 15 hours and could be
4

recycled in about 20 minutes. It is an ideal system for conducting

tests at temperatures between -0.3 and OAK. A schematic view of the

system is shown in Figure 1.

THE BOLOMETER

The bolometer was constructed from doped germanium supplied by A.

J. Seivers of the Cornell Physics Department. Only very recently has

better material been produced using transmutation-doping. The electrical leads to the bolometer chip were soldered on using gallium/indium

solder. The leads themselves were fine brass wires. The bolometer chip

was epoxied unto the sapphire receiver chip. The sapphire was coated

with a semitransparent coating of Nicrome to improve its absorption

properties. The electrical properties of the bolometer are shown in the

table. A schematic view of the bolometer and its optical system are

shown in Figures 2 and 3. Table 2 lists its physical properties.

Table 2

Heat Capacity of Bolometer Elements

Element Volume Heat Capacity 0.43°K Heat Capac

Sapphire chip 0.476x0.476x0.127 cm 3 3.80x10- 7 T 3J/°K 8.39x10"11J/°K

Ge thermistor 0.40x0.30x0.lO cm 3 3.5x10" 6

T3J/°K 3.22x10- 11J/° K

Epoxy 1< 6x10' 

S 

cm 

3

7.00x10- `` T 3J/°K 8.60x10" "J/°K

Indium solder 1.6x10" s cm 

3

1.47x10" 4 3J/°K 1.81x10" 1'J/0K

Brass wires 3.24x10" 

6 

cm 

3

1.36x10' 4 J/°K 2.02x10- 10J/ 0K

NiCr film 1.7x10" 7 cm 3 1.50x10" 3 T J/°K 1.10z10-'OJ/°K
ORIGINAL PAGE 19

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Bolometer

0.052 It Aperture

SiO2 Lens

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770 - Block Poly

290° i o f Ion

Figure 3
8

OPTICAL COUPLING

One of the main difficulties of a system of this type is to efficiently couple the radiation to the bolometer. Our first experiments

used a quartz field lens to image the exit pupil to the source onto the

entrance aperture of the bolometer cavity. In an effort to improve the

coupling, a Winston cone was tested. Although superior- to-simple

filed lenses, it did not perform better than our best field lens.

ELECTRONICS

Microphonics is one of the continual problems with many bolometer

systems. This is because of the relatively small voltage responsivities

of these devices. In a system used for routine observations all sorts

of measures can be taken to reduce microphonic noise. Unfortunately,

these procedures are not feasible in a system intended primarily for

testing since they, in general, make rapid assembly and disassembly more

difficult. We therefore equipped the system with an internal JFET

preamplifier to reduce both the number and lengths of wires coming out

of the dewar with low-level signals. The electrical noise sources are

listed in Table 3. Table 4 gives a summary of the bolometer's

performance.

Table 3

Dewar Noise at 10 Hz, Measured at the Bolometer

Background power noise

Thermal fluctuation noise

• Bolometer Johnson noise

Load resistor Johnson noise

Excess current noise

Preamp voltage noise

Preamp current noise

5.2 nV hiz

5.0 nV Mz

5.6 nV Mz

3.2 nV +Mz

4.9 nV 4iz

4.1 nV Mz

5.4 n V rliz
9

Table 4

Measured Bolometer Electrical Performance

Time constant C/Ce - 13.9 msec

Heat capacity C - 1.50 x 10- 9 J/°k

Effective thermal conductivity Ge - 1.08 x 10-' W/°K

Thermal conductivity G - 0.91 x 10- 7 W/°K

Operating temperature Tb - 0.43°K

Load resistor RL - 36.5 Mil

CONCLUSIONS

!A- have developed a high-sensitivity ^He cooled system for submillimeter astronomy. However, thre are still large gains to be had by

improving the coupling to the bolometer chip. This system was tested in

the field on the Palomar 5-meter telescope and achieved a sensitvity of

5.8 Jy/(Hz) 1/2 . The electrical NEP of the bolometer was considerably

better an 10-15W/(Hz)1/2.