For sensitive detection of astronomical continuum radiation in the 200 micron to 3 mm wavelength range, bolometers are presently the detectors of choice. In order to approach the limits imposed by photon noise in a cryogenically cooled telescope in space, bolometers must be operated at temperatures near 0.1 K. Researchers report progress in building and using bolometers that operate at these temperatures. The most sensitive bolometer had an estimated noise equivalent power (NEP) of 7 x 10(exp 017) W Hz(exp -1/2). Researchers also briefly discuss the durability of paramagnetic salts used to cool the bolometers

Lesyna, L.

Roellig, T.

Savage, M.

Werner, Michael W.

English

NASA Technical Reports Server

• i N90-21318
A Progress Report on using Bolometers Cooled by Adiabatic
Demagnetization Refrigeration
L. Lesyna (Stanford*), T. Roellig (NASA-Ames), M. Savage (Sterling Software),
M. Werner (NASA-Ames)
*Present address: Grumman Corporate Research Center
For sensitive detection of astronomical continuum radiation in the 200 #m-
3 mm wavelength range, bolometers are presently the detectors of choice. In
order to approach the limits imposed by photon noise in a cryogenically cooled
telescope in space, bolometers must be operated at temperatures near 0.1 K. We
report our progress in building and using bolometers that operate at these
temperatures. Our most sensitive bolometer had an estimated NEP of
7 x 10 -17 W Hz -1/2. We also briefly discuss the durability of paramagnetic salts
used to cool our bolometers.
Theory
The noise equivalent power (NEPoptical) of a bolometer will be the quadrature
sum of noise due to absorbed photons and noise components intrinsic to the
detector. The NEPopticaa of a bolometer limited by photon induced noise from a
thermal background is given by the following expression [1]:
NEpp2hoton = 4 A .(2 (kTb) 5 e f x4 1 + dx
q c 2 h 3 e x-- 1 e_---_-l!
The corresponding expression for the absorbed background power is:
45
https://ntrs.nasa.gov/search.jsp?R=19900012002 2020-03-19T23:17:48+00:00Z
Q= 2 A-Q "l_bk4 I x 3C2 h 3 e x- 1
--qEfdx
x_hv
kTb
where A.Q is the optical throughput, Tb is the background temperature, _ is the
background emissivity, f is the optical system transmission, q is the bolometer
absorptivity, k is the Boltzmann constant, c is the speed of light, and h is the
Planck constant.
Bolometer optimization depends strongly upon the character of the background
radiation [2]. For a bolometer exposed only to Tb = 2.7 K cosmic background in
the 400 #m - 700 I_m wavelength range by a diffraction limited telescope, and
assuming that _=1, f=0.5, q=l, we find NEPphoton = 5 x 10 -18 W Hz -1/2 and
Q=44fW. The contribution of other astrophysical backgrounds [3], and
telescope self-emission will raise both NEPphoton and Q. The Multiband Imaging
Photometer for SIRTF (MIPS) has a sensitivity goal of NEPoptical =
5 xl 0 -17 W Hz -1/2 in the 200 #m - 700 I.tm wavelength range [4]. As another
example, ground based observations in the 1.0 - 1.4 mm range (assuming _ = 0.5
and T b = 273 K) imply NEPphoton = 1.4 x 10 -15 W Hz -1/2 and Q = 870 pW.
Detector induced noise cannot be less than the noise due to thermal fluctuations;
its contribution to NEPoptical is NEPthermal = q-1 (4kT2G)l/2, where G is the thermal
conductance, and T is the bolometer temperature. We desire
NEPphoton >> NEPthermal for background limited performance. Most bolometers
heat up, and become slower and less sensitive as the background power is
increased from zero. It becomes a bolometer design challenge to maintain the
above inequality, while the bolometer absorbs the appropriate background
power. The best bolometers operating at 0.3 K have NEPoptical of about
4 x 10 -16 W Hz -1/2 [5]; to reach lower NEPs, a bolometer operating at 0.1 K is
required.
46
Experiment
Useful materials for bolometers operating at 0.1 K that meet the more stringent
low background requirements appear to exist. We have adopted a design shown
in Figure 1. Our bolometer component selections were motivated by our desire to
keep the total heat capacity, C, as low as possible. For our bolometers, the
astronomical radiation is absorbed by a 120 nm Bi film [6] deposited onto a
sapphire substrate. The temperature change induced by the heating of the Bi film
is measured with a Ge:Ga thermistor [7]. The thermistor is maintained near 0.1 K
by 25 #m brass wires, which also serve to provide electronic conduction paths to
measure the induced impedance changes.
The bolometer is mounted on the work surface of our adiabatic demagnetization
refrigerator (ADR). Our ADR system can reach temperatures of less than 0.05 K,
and can maintain temperatures of 0.1 K to within 14 #K, for periods of time in
excess of 12 hours. Additional details of the refrigerator system and its
performance are described in detail elsewhere [8,9].
We digress to discuss an important issue affecting the reliability of ADRs used for
cooling detectors: the lifetime of the magnetic refrigerant. Paramagnetic salts
useful for cooling bolometers in ADRs are often chemically active and can
deteriorate when stored at room temperature. Chemical reactions may occur that
alter their low temperature magnetic properties and degrade or render them
useless for ADR applications. We have achieved considerable success using
chromic potassium sulfate (CPS) as our working paramagnetic salt. We
encapsulate the CPS within a brass cylinder, with epoxy end caps to hermetically
seal the CPS from the atmosphere. We have used one such salt pill over a period
of three years without noticeable degradation of refrigeration performance. This
result suggests that CPS is a very suitable magnetic refrigerant for reliable long-
term use in ADRs.
Returning to our discussion of detectors, we designed our bolometers to have a
thermal conductance, G equal to 2 x 10 -8 W/K, so that they could effectively
function in the presence of background power up to 400 pW. Thermodynamic
induced noise places a minimum contribution to NEPoptical of q-1(4kT2G)1/2,
hence NEPoptical > 1 x 10 -16 W Hz -1/2. This value is higher than that required for
4"7
a cosmic background limited bolometer, but is less than NEPphoton for the
ground-based application mentior ,,1 above. In order for the bolometer to
respond to modulated power at chopping frequencies of 10 Hz and higher, we
desired a bolometer heat capacity C < 10-10J/K. This requirement was achieved,
and the estimated contributions to the heat capacity of the materials we used are
listed in Table 1.
We estimated NEPopticaIof our bolometer by measuring the current, I, and voltage
noise Vn, as a function of electrical bias across the bolometer, to infer an
electrically determined value of the bolometer NEP (NEPelec). We then applied
the relation NEPoptica_= q-1 NEPelec [1], assuming a value of q=l, as is
appropriate for a bolometer in an integrating cavity. Our bolometers were biased
through a EItek Model 106 metal film resistor, having an impedance of 250 M_ at
2 K. A cold J230 JFET source-follower circuit was coupled to the bolometer, to
match the bolometer impedance and to minimize microphonic and electrical
interference. The current-voltage relationship for our most sensitive bolometer is
shown in Figure 2. Our measured electrical parameters are listed in Table 2.
From these data, we estimate a responsivity of 6.4 x 108 V/W and NEPopticalof
6.7 x 10-17W Hz-1/2at the bias point marked by an arrow in Figure 2. This value
of the NEP is the lowest recorded for infrared and submillimeter bolometers, and
represents a significant advance in sensitivity.
We have measured the optical response of our bolometer, to modulated power
variations (Figure 3), to infer a time constant of 3 ms for the detector. This value
provides useful, fast response for chopping radiometric applications. Further
description of the bolometer and measurements made of it are reported
elsewhere [9].
The bias point was selected to be such that the electrical power dissipated in the
bolometer is greater than that of a hypothetical background power for 44 fW (as
may be appropriate for a low background spacecraft application). This choice of
electrical bias is close to optimal for this radiometric application [2]. In Table 3,
we list the theoretical contributions to the NEP; the total contribution is equal to
our measured NEPelecto within experimental error.
48
However, we have observed a discrepancy in the value of the thermal
conductance of our bolometer and the value we expected. An independent
measurement of the thermistor yields a resistance-temperature relation,
R = 2.87 D_. ° exp(4.58 K 1/2 T-1/2); we use this expression to calibrate the
temperature of the bolometer, as a function of bias from Figure 2. From this
temperature and bias power data, we infer a thermal conductance of our
bolometer, G = 1.6 x 10 -9 W/K, which is one order of magnitude lower than the
value we would expect from published brass thermal conductivity data [10]. It is
possible that boundary resistance effects induced by the electrical contacts, and
compositional differences in the brass wire [11] used account for the differences
in measured values; we do not fully understand the origin of this discrepancy. The
low G value does not affect the accuracy of the electrical self-calibration
procedure to estimate NEPelec, but does limit the maximum background power
the bolometer can accept.
It has been suggested that non-ohmic behavior of bulk thermistor material may be
partially responsible for the nonlinearity shown in Figure 2, and may affect the
accuracy of the self-calibration procedure we used to infer NEPelec [12]. We do
not believe this to be the case for our bolometers. The amount of bias power per
unit volume necessary to reduce the resistance in 0.1 K Ge by 30% is
4 x 10 -2 W/m 3 [13]. For our 0.1 K bolometer, the bias power density was
4 x 10 .4 W/m3, which suggests that non-ohmic behavior is a less than 1% effect,
and has a negligible effect on NEPelec.
Observational Tests
We have used our adiabatic demagnetization refrigerator system to perform one
millimeter continuum observations using the Hale 5 m telescope at Palomar
Mountain. During our observing run during November 1985, we used an
unoptimized bolometer operating at 0.2 K. Construction materials were similar to
our 0.1 K bolometer described earlier, except that thermistor material was
selected with a lower dopant density, to provide a bolometer resistance of 1 M_
at 0.2 K. In addition, brass wire of diameter 50 I_m was used, to design this
bolometer for the high background expected for our ground-based observing
49
location. A non-optimal Bi film thickness of 30 nm was applied to the sapphire
substrate. NEPelecof this bolometer was estimated to be 2 x 10-15W Hz-1/2.
We estimated a noise equivalent flux density (NEFD) of 10 Jy Hz-1/2for our ADR
bolometer system, based upon one millimeter wavelength measurements of the
planet Jupiter and the radio galaxy 3C84. The ADR system NEFD was higher
than the 6 Jy Hz-1/2of an optimized helium-three cooled system [14], which was
placed onto the telescope after the ADR. The relatively high NEPelec, low
absorptivity of the detector, and optical mismatch to atmospheric transmission
windows precluded the ultimate in NEFD to be realized with the ADR system. We
expect that bolometer optimization, and improved optical filtering will lower the
ADR NEFD substantially.
As bolometers are thermal detectors, we expect them to be affected by
temperature variations in the refrigeration system. In order to investigate the
magnitude of this effect, we monitored the NEFD during temperature regulation of
the ADR at the telescope. Regulation effects did not introduce significant
amounts of excess noise into our radiometric observations. This result is
consistent with our expectations, as the effect on the responsivity induced by
bolometer heat sink temperature variations is given by the following
expressions [9]:
AS _ 1 Aa
S 1_/2a a
a=l dR
G dT
where S is the responsivity, and Ao_ is calculated over the regulated temperature
range. For our 0.2 K bolometer operated at Palomar, responsivity variations were
calculated from the above equations to be less than 1%. For our 0.1 K bolometer,
we estimate AS / S = 0.2%, for 14 #K temperature variations. Thus, temperature
regulation effects would not be expected to degrade the bolometer system
sensitivity to a significant degree.
5O
Conclusions
We have successfully constructed and used bolometers cooled by ADR at an
astronomical observatory to perform sensitive radiometric measurements. We
have achieved NEPelec as low as 7 x 10 -17 W Hz -1/2, for bolometers operating at
0.1 K. We expect that bolometers can be made more sensitive by decreasing the
thermal conductance further and by cooling the load resistor below 2 K; such
changes should readily achieve NEPoptical of less than 5 x 10 -17 W Hz -1/2 as
required for SIRTF and other low background applications.
This work was supported by a NASA Cryogenics RTOP and NASA-Ames Director
Discretionary Funds.
REFERENCES AND NOTES
1. Mather, J.C. 1982, Appl. Opt. 21, 1125.
2. Griffin, M.J., and Holland, W.S. 1988, Int. J. Infrared and Millimeter Waves, 9,
1988.
3. Matsumoto, T., Hayakawa, S., Matsuo, H., Murakami, H., Sato, S., Lange, A.E.,
Richards, P.L. 1988, Ap. J. 329, 567.
4. Ramos, R., Hing, S.M., Leidich, C.A., Fazio, G., Houck, J.R., Rieke, G. 1988,
SPIE 973.
5. Downey, P.M., Jeffries, A.D., Meyer, S.S., Weiss, R., Bachner, F.J., Donnelly,
J.P., Lindley, W.T., Mountain, R.W., Silversmith, O.J. 1984, Appl. Opt. 23, 910.
6. Clarke, J., Hoffer, G.I., Richards, P.L., Yeh, N.-H. 1977, J. AppL Phys. 48, 4865.
7. Lesyna, L., Roellig, T.L., Kittel, P. 1984, Int. J. Infrared and Millimeter Waves 5,
755.
8. Lesyna, L., Roellig, T., Werner, M., Kittel, P. in Advances in Cryogenic
Engineering, 33, 955 (R. W. Fast, ed., Plenum Press, 1988).
9. Lesyna, L., Ph.D. Dissertation 1987, Stanford University (unpublished).
51
10.Lounasmaa, O.V., Experimental Principles and Methods Below 1 K (Academic
Press, 1974) p. 246.
11. Brass wire was purchased from California Fine Wire Company, and has a
70% Cu - 30% Zn composition.
12. Mather, J.C. 1984, Appl. Opt. 23, 3181.
13. McCammon, D. 1989, private communication.
14. Roellig, T.L., and Houck, J.R. 1983, Int J. Infrared and Millimeter Waves4,
299.
15. Collan H.K., Krusius M., Pickett, G.R. 1970, Phys. Rev. Bi, 2888.
16. Nishioka, N.S., Richards, P.L., Woody, D.P. 1978, AppL Opt. 17, 1562.
17. Specific Heat - Metallic Elements and Alloys, 4 (Y. S. Touloukian, ed.,
Plenum Press, 1970).
52
Table 1
Bolometer Thermal Data at 0.1 K
Material Volume
Bismuth Film 1.1 x 10 -3 mm 3
Sapphire Substrate 2.2 mm 3
Ge:Ga Thermistor
Epoxy
Brass Wires
In Solder
1.0 mm 3
0.01 mm3
3.0 x 10 -3 mm 3
4.2 x 10 -3 mm 3
Est. Heat Capacity, C
1.3 x 10 -13 J/K
8.4 x 10 -13 J/K
3.5 x 10 -12 J/K
7.0 x 10 12 J/K
1/3 x 3.4 x 10 -11 J/K
5.5 x 10 -13 J/K
Re__jf.
[15]
[16]
[16]
[16]
[17]
[16]
Total heat capacity
Thermal conductance of brass wire
Time constant
Theoretical
2.3 x 10 11 J/K
2.0 x 10 .8 W/K
1.1 ms
Experimental
6 x 10 -12 J/K
2 x 10 -9 W/K
3 ms
53
Table 2
Measured bolometer electrical parameters
(Using notation of reference [1])
Z, dynamic impedance
R, resistance
I,current
Vn, voltage noise
Ts, bolometer sink temperature
RL, load resistance
TL, load resistor temperature
S, responsivity
NEPelec
36 M_
42 M.Q
9.8 x 10-11A
43 nV Hz-1/2at 20 Hz
0.0725 K
250 M_
2K
6.4 x 108V/W
6.7 x 10-17W Hz-1/2
Table 3
Theoretical Estimates of Bolometer NEPeleccontributions [1]
NEPthermal
NEPJohnson,R
NEPJohnson,load
NEPamplifier
2.3 x 10-17W Hz-1/2
1.6 x 10-17W Hz-1/2
3.3 x 10-17W Hz-1/2
2.3 x 10-17W Hz-1/2
NEPelec 4.9 x 10-17W Hz-1/2
54
Brass Wires
25 pm Diameter
Ge:Ga Thermistor
1 mmxlmmxl mm
Sapphire Substrate
3 mm x 3 mm x 0.25 mm
Indium Solder
120 nm Bi
Absorbing Film
Stycast 2850FT
Epoxy
Figure 1
Bolometer Materials and Dimensions
55
20
16 ................................................[...............
14 ...............................................i........................
>-
.......................................
6
4
2
0
0 100 200 300 400 500 600 700
I, pA
Figure 2
Bolometer Load Curve
56
_o
¢-
o
o_
09
o_
rr
1.2
0.8
0.6
0.4
0.2
Figure 3
Bolometer Response to Modulated Radiation
5"1



A progress report on using bolometers cooled by adiabatic demagnetization refrigeration

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