An optical imaging system for milliKelvin temperatures has been developed based on a regular B/W surveillance camera (25 frames/s), with its CCD sensor inside the 4‐K vacuum can of our nuclear demagnetization cryostat. The heat leak to the nuclear stage, caused by the operation of the video camera, was reduced below 1 nW by careful rf shielding. The construction of the system and its limits of operation are discussed.Peer reviewe

Alles, H.

Babkin, A. V.

Hakonen, Pertti J.

Manninen, A. J.

Pekola, Jukka P.

Ruutu, J. P.

H. Alles

J. P. Ruutu

A. V. Babkin

P. J. Hakonen

A. J. Manninen

J. P. Pekola

Crossref

Cooled video camera for optical investigations below 1 mK

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This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Alles, H. & Ruutu, J. P. & Babkin, A. V. & Hakonen, P. J. & Manninen,A. J. & Pekola, JukkaTitle: Cooled video camera for optical investigations below 1 mKYear: 1994Version: Final published versionPlease cite the original version:Alles, H. & Ruutu, J. P. & Babkin, A. V. & Hakonen, P. J. & Manninen, A. J. & Pekola,Jukka. 1994. Cooled video camera for optical investigations below 1 mK. Review ofScientific Instruments. Volume 65, Issue 5. 1784-1785. ISSN 0034-6748 (printed). DOI:10.1063/1.1144827.Rights: © 1994 AIP Publishing. This is the accepted version of the following article: Alles, H. & Ruutu, J. P. & Babkin,A. V. & Hakonen, P. J. & Manninen, A. J. & Pekola, Jukka. 1994. Cooled video camera for opticalinvestigations below 1 mK. Review of Scientific Instruments. Volume 65, Issue 5. 1784-1785. ISSN0034-6748 (printed). DOI: 10.1063/1.1144827, which has been published in final form athttp://scitation.aip.org/content/aip/journal/rsi/65/5/10.1063/1.1144827All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Cooled video camera for optical investigations below 1 mKH. Alles, J. P. Ruutu, A. V. Babkin, P. J. Hakonen, A. J. Manninen, and J. P. Pekola  Citation: Review of Scientific Instruments 65, 1784 (1994); doi: 10.1063/1.1144827 View online: http://dx.doi.org/10.1063/1.1144827 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/65/5?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in A Continuous Adiabatic Demagnetization Refrigerator for Cooling to 10 mK and Below AIP Conf. Proc. 850, 1573 (2006); 10.1063/1.2355306  Characterization of ZYX exfoliated graphite for studies of monolayer 3 He below 1 mK Rev. Sci. Instrum. 74, 4448 (2003); 10.1063/1.1606540  Spin structure of solid 3He below 1 mK neutron diffraction AIP Conf. Proc. 103, 82 (1983); 10.1063/1.34214  Superconducting switch for large heat flow below 50 mK Rev. Sci. Instrum. 49, 396 (1978); 10.1063/1.1135416  Hyperfine enhanced nuclear cooling to 1.3 mK in PrNi5 AIP Conf. Proc. 24, 238 (1975); 10.1063/1.30069    This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Fri, 29 May 2015 06:05:17Cooled video camera for optical investigations below 1 rnK H. Alles, J. P. Ruutu, A. V. Babkin, P. J. Hakonen, A. J. Manninen,a) and J. P. Pekolab) Low Temperature Laboratory, Helsinki University of Technology, 02150 Espoo, Finland (Received 8 November 1993; accepted for publication 2.5 January 1994) An optical imaging system for milliKelvin temperatures has been developed based on a regular S/W surveillance camera (25 frames/s), with its CCD sensor inside the 4-K vacuum can of our nuclear demagnetization cryostat. The heat leak to the nuclear stage, caused by the operation of the video camera, was reduced below 1 nW by careful rf shielding. The construction of the system and its limits of operation are discussed. The desire to obtain images of the 3He superfluid phases below 3 mK has made it necessary to develop new optical measurement techniques. So far, cryostats with optical access from room temperature, through a set of 3 to 5 windows, have reached temperatures down to about 25 mK.“2 Re- cently, temperatures below 1 mK have been obtained in a setup where laser illumination was brought to 4 K by a single mode optical fiber and the image was transmitted out of the cryostat along a coherent bundle of 30 000 fibers.3 In such an imaging system, the resolution is limited by the coherent fiber bundle since the amount of pixels in a CCD (charged coupled device)-type of camera is typically ten times larger than the number of fibers in the bundle. Another scheme for milliKelvin imaging, pioneered by Wagner et al.,4 is to employ a cooled CCD sensor inside the vacuum can immersed in liquid helium at 4 K. Since the dark current decreases exponentially with temperature, this allows the use of the so-called slow-scan mode which facilitates the observation of very weak light signals. The cooled slow-scan cameras have sweep frequencies around 100 kHz which re- sults in rather slow read-out speeds, on the order of 1 frame/s. In this Paper, we describe a milliKelvin temperature im- aging system, based on a cooled CCD which operates a real- time video speed. We use in our setup a regular B/W surveil- lance camera (JVC, model TK-S 200) whose sensor (Matsushita, MN 3745, 518HX582V pixels) is.capable of operating down to about T=60 K. One of the main prob- lems in the remote operation of this CCD sensor is the phase shift of the clock signals, caused by the delay in the extended 1.5 m long electrical wires between. the sensor chip and its room temperature control electronics. This distorts the gray scale of the picture severely. We have compensated for the phase shift by inserting two advanced CMOS gates, with a total delay of about 10 ns, into the clock signal line of the CCD read-out control (see Fig. 1). The leads from the control unit to the sensor were in- stalled into the pumping line of the vacuum can. Coaxial cables (type Cl, 105 pF/m, Lake Shore, Westerville, Qhio) are used for the read-out signal and for the horizontal (10 ‘)Present address: Department of Physics, University of Manchester, Manchester, Ml3 9PL, UK. h)Present Address: Department of Physics, University of Jyv;iskyIii, P.0. Box 35, 40351 Jyvaskyli, Finland. MHz) and vertical (15 kHz) drives. The rest of the leads were made of 0.3 mm diam manganin wire, insulated by Teflon tubing. The six coaxial cables for the drives were slipped inside a greased copper braid that was employed for heat sinking at 4 K. To provide a separate electrical shield, the braid was grounded only from one point at its low tem- perature end. The same procedure was applied to the manga- nin wires. The CCD read-out line was shielded and grounded in a similar way but kept separate from the other coaxial cables. The sensor itself was mounted on a circuit board located inside a tight cylindrical copper box ($=35 mm, L-35 mm), as illustrated in Fig. 2. The upper end of the box has a hole of 7X10 mm2 capped with a 2 mm thick fused silica thermal filter. A copper cold finger ($=5 mm) connects the body of the box to the vacuum can flange immersed in the liquid He bath. In order to keep the CCD sensor at its lowest working temperature, around 70 K, the CCD chip was heat sunk, using grease secured with GE 7031 varnish, to a 0.6 mm thick temperature-controlled copper plate. An 80 mm long copper wire of 0.5 mm diameter provides the weak thermal contact between the plate and the cold finger. The power generated by the CCD chip itself was measured to be about 170 mW which is not enough to raise the temperature sufficiently; extra heating on the order of 150 mW is needed for proper operation of the CCD sensor. Our optical setup is a modification of the scheme em- ployed in Ref. 3. The He-Ne laser illumination is guided into the cryostat via a single-mode optical fiber. The beam is expanded to 10 mm diameter and directed by a beam splitter and by a set of mirrors through the cell. Only about 0.05% of the light is reflected to the camera from the sample, i.e., from the reference plate (reflection coefficient R=3 X 10s4) and from the free surface of liquid 3He (R-2 X 10A4), while the rest of the illumination traverses the cell and is absorbed by the still radiation shield. A single bi-convex lens (f= 125 mm) forms the interference image, created by the two re- flected beams, to the CCD sensor. In order to block the infrared radiation, emerging from the CCD sensor at 70 K, we installed two additional thermal filters, made of sapphire and CaF, (with thicknesses 2 and 8 mm, respectively), to the optical path leading to the sensor (see Fig. 2). These filters reduce the thermal radiation from the sensor to its surroundings below a few nanowatts.’ The filters and windows used in our optical setup were antireflec- 1784 Rev. Sci. Instrum. 65 (5), May 1994 0034-6748/94/65(5)/1784/2/$6.00 Q 1994 American Institute of Physics  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Fri, 29 May 2015 06:05:17CONTROL UNJT (T= 300 K) r----------------------------. I 8 1 I I TIMrNci DELAY VIDEO t UNIT SIGNAL : / I GENERATOR I VIDEO I 1 OUT L________---______--___----___I READ-OUT PROM SENSOR FIG. 1. Principal scheme of the remote-controlled CCD imager. tion coated to avoid problems with secondary reflections. Initially, heat leaks in excess of 1 ,uW were observed to the nuclear demagnetization stage when the camera was op- erated. This was assigned to rf heating caused by the 10 MHz digital signals entering the cryostat. Therefore, extreme care was taken to shield the milliKelvin parts against the r-f radia- tion from the 10 MHz drive signals. We tried to make the inner radiation shield around the nuclear stage a true “Fara- day cage.” Ground loops due to the 3He till lines were bro- ken inductively by six-turn spiral (+lO mm) sections, grounded from the middle when penetrating the shield. The only electrical wires entering the inner radiation shield are those needed to operate the heat switch and the pulsed plati- num NMR thermometer. The leads of the heat switch magnet and of the Pt static coil (wound from CuNi-clad multifila- mentary NbTi wire) have 47 nF ceramic capacitors to ground and across the lines at the entry point to shunt high frequency disturbances. The twisted pair for the rf coil is shielded by a Cu-Ni tube covered by tin solder. Because of limitations imposed by the tuning of the Pt tank circuit, only a 220 pF mica capacitor is connected across the leads of the rf coil. To eliminate possible electrical potential differences, we employ superconducting wire along the dilution refrigerator between the still and the mixing chamber. Owing to our experimental needs we have two 12 mm diam holes in our “Faraday cage;” they do not seem to be TOP FLANGE OF k\\\\\\\\\?+h\\\\\\\x “Am&j CAN INTERFERENCE IMAGE PROM OPTICAL CELL FIG. 2. Mounting of the CCD sensor inside the 4 K vacuum can. Cables connecting the circuit board to the control unit are not shown. F=thermal filter, M=mirror. too detrimental. One of them is along the axis of the cryostat and allows the laser illumination to enter the experimental space whereas the other lets the nonreflected light to escape from the coldest parts of the cryostat to the still radiation shield. The basic heat leak of our nuclear demagnetization cry- ostat is 10 nW. Operation of the camera does not change this within an accuracy of lo%, i.e., the heat leak from the CCD is less than 1 nW. An additional source of heating is the laser light used for illumination. The sensitivity of our CCD is such that a light power of 40 PW is required for successful imaging. About 0.5% of the light is absorbed to the fused silica reference plate inside the experimental cell, which means a heat leak of 100 nW to the 3He sample under inves- tigation. We find that heat absorbed by the fused silica is transmitted to liquid 3He with an effective time constant of 100 ms. Typically, we employ 20 ms light pulses. A repeti- tion rate of 1 Hz results then in an average heat leak of 2 nW. For a bulk sample, this is not critical but with thin films a strong fountain effect is observed. Thus real-time imaging can be performed on bulk specimens over relatively short time intervals (a few seconds). This is, nevertheless, suffi- cient to study the surface dynamics of superfluid 3He (Ref. 6). To decrease the heat leak, laser light of lower power level would be preferable. One possibility is to enlarge the reflection coefficient of the reference plate but this leads to a degraded contrast in the interference image. The most straightforward solution would be to use a slow-scan camera which takes advantage of the small dark current at low tem- peratures. Thereby an increase by a factor of 100 in the light sensitivity could be obtained but the real-time imaging capa- bility is lost. However, the light sensitivity of a regular CCD can also be improved by installing a separate, low-noise pre- amplifier to increase the S/N ratio at small illumination lev- els. We are indebted to G. Frossati, R. Wagner, and 0. V Lounasmaa for fruitful discussions. One of us (A. Babkin) thanks for hospitality during short visits to Leiden. Scholar- ships from the Vaisala Foundation (H. A.) and the Wihuri Foundation (J.R.) are gratefully acknowledged. This work was financially supported by the Academy of Finland. ‘K.-P. Miiller and P Haarer, Phys. Rev. Lett. 66, 2344 (1991). 2E. Rolley, E. Chevalier, C. Guthmann, and S. Balibar, Phys. Rev. Lett. 72, 872 (1994). “A. J. Manninen, J. P. Pekola, G. M. Kira, J. P. Ruutu, A. V. Babkin, H. Alles, and 0. V. Lounasmaa, Phys. Rev. L&t. 69, 2392 (1992). 4R. Wagner, P. J. Ras, P. Remeijer, S. C. Steel, and G. Frossati, JLTP (to be published). ‘A. J. Manninen, H. AIles, A. V. Babkin, and J. P. Pekola, Physica B 178, 3.52 (1992). “H. Alles, J. P. Ruutu, A. V. Babkin, and P. J. Hakonen, JLTP (to be pub: lished). Rev. Sci. Instrum., Vol. 66, No. 5, May 1994 Notes i765  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Fri, 29 May 2015 06:05:17

An optical imaging system for milliKelvin temperatures has been developed based on a regular B/W surveillance camera (25 frames/s), with its CCD sensor inside the 4-K vacuum can of our nuclear demagnetization cryostat. The heat leak to the nuclear stage, caused by the operation of the video camera, was reduced below 1 nW by careful rf shielding. The construction of the system and its limits of operation are discussed.</p

Hakonen, P. J.

Pekola, J.P.

VTT Research System

Aaltodoc

This is an electronic reprint of the original article.
This reprint may differ from the original in pagination and typographic detail.
Author(s): Alles, H. & Ruutu, J. P. & Babkin, A. V. & Hakonen, P. J. & Manninen,
A. J. & Pekola, Jukka
Title: Cooled video camera for optical investigations below 1 mK
Year: 1994
Version: Final published version
Please cite the original version:
Alles, H. & Ruutu, J. P. & Babkin, A. V. & Hakonen, P. J. & Manninen, A. J. & Pekola,
Jukka. 1994. Cooled video camera for optical investigations below 1 mK. Review of
Scientific Instruments. Volume 65, Issue 5. 1784-1785. ISSN 0034-6748 (printed). DOI:
10.1063/1.1144827.
Rights: © 1994 AIP Publishing. This is the accepted version of the following article: Alles, H. & Ruutu, J. P. & Babkin,
A. V. & Hakonen, P. J. & Manninen, A. J. & Pekola, Jukka. 1994. Cooled video camera for optical
investigations below 1 mK. Review of Scientific Instruments. Volume 65, Issue 5. 1784-1785. ISSN
0034-6748 (printed). DOI: 10.1063/1.1144827, which has been published in final form at
http://scitation.aip.org/content/aip/journal/rsi/65/5/10.1063/1.1144827
All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, and
duplication or sale of all or part of any of the repository collections is not permitted, except that material may
be duplicated by you for your research use or educational purposes in electronic or print form. You must
obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or
otherwise to anyone who is not an authorised user.
Powered by TCPDF (www.tcpdf.org)
Cooled video camera for optical investigations below 1 mK
H. Alles, J. P. Ruutu, A. V. Babkin, P. J. Hakonen, A. J. Manninen, and J. P. Pekola 
 
Citation: Review of Scientific Instruments 65, 1784 (1994); doi: 10.1063/1.1144827 
View online: http://dx.doi.org/10.1063/1.1144827 
View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/65/5?ver=pdfcov 
Published by the AIP Publishing 
 
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Rev. Sci. Instrum. 74, 4448 (2003); 10.1063/1.1606540 
 
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 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:
130.233.216.27 On: Fri, 29 May 2015 06:05:17
Cooled video camera for optical investigations below 1 rnK 
H. Alles, J. P. Ruutu, A. V. Babkin, P. J. Hakonen, A. J. Manninen,a) 
and J. P. Pekolab) 
Low Temperature Laboratory, Helsinki University of Technology, 02150 Espoo, Finland 
(Received 8 November 1993; accepted for publication 2.5 January 1994) 
An optical imaging system for milliKelvin temperatures has been developed based on a regular S/W 
surveillance camera (25 frames/s), with its CCD sensor inside the 4-K vacuum can of our nuclear 
demagnetization cryostat. The heat leak to the nuclear stage, caused by the operation of the video 
camera, was reduced below 1 nW by careful rf shielding. The construction of the system and its 
limits of operation are discussed. 
The desire to obtain images of the 3He superfluid phases 
below 3 mK has made it necessary to develop new optical 
measurement techniques. So far, cryostats with optical access 
from room temperature, through a set of 3 to 5 windows, 
have reached temperatures down to about 25 mK.“2 Re- 
cently, temperatures below 1 mK have been obtained in a 
setup where laser illumination was brought to 4 K by a single 
mode optical fiber and the image was transmitted out of the 
cryostat along a coherent bundle of 30 000 fibers.3 In such an 
imaging system, the resolution is limited by the coherent 
fiber bundle since the amount of pixels in a CCD (charged 
coupled device)-type of camera is typically ten times larger 
than the number of fibers in the bundle. 
Another scheme for milliKelvin imaging, pioneered by 
Wagner et al.,4 is to employ a cooled CCD sensor inside the 
vacuum can immersed in liquid helium at 4 K. Since the dark 
current decreases exponentially with temperature, this allows 
the use of the so-called slow-scan mode which facilitates the 
observation of very weak light signals. The cooled slow-scan 
cameras have sweep frequencies around 100 kHz which re- 
sults in rather slow read-out speeds, on the order of 1 
frame/s. 
In this Paper, we describe a milliKelvin temperature im- 
aging system, based on a cooled CCD which operates a real- 
time video speed. We use in our setup a regular B/W surveil- 
lance camera (JVC, model TK-S 200) whose sensor 
(Matsushita, MN 3745, 518HX582V pixels) is.capable of 
operating down to about T=60 K. One of the main prob- 
lems in the remote operation of this CCD sensor is the phase 
shift of the clock signals, caused by the delay in the extended 
1.5 m long electrical wires between. the sensor chip and its 
room temperature control electronics. This distorts the gray 
scale of the picture severely. We have compensated for the 
phase shift by inserting two advanced CMOS gates, with a 
total delay of about 10 ns, into the clock signal line of the 
CCD read-out control (see Fig. 1). 
The leads from the control unit to the sensor were in- 
stalled into the pumping line of the vacuum can. Coaxial 
cables (type Cl, 105 pF/m, Lake Shore, Westerville, Qhio) 
are used for the read-out signal and for the horizontal (10 
‘)Present address: Department of Physics, University of Manchester, 
Manchester, Ml3 9PL, UK. 
h)Present Address: Department of Physics, University of Jyv;iskyIii, P.0. Box 
35, 40351 Jyvaskyli, Finland. 
MHz) and vertical (15 kHz) drives. The rest of the leads 
were made of 0.3 mm diam manganin wire, insulated by 
Teflon tubing. The six coaxial cables for the drives were 
slipped inside a greased copper braid that was employed for 
heat sinking at 4 K. To provide a separate electrical shield, 
the braid was grounded only from one point at its low tem- 
perature end. The same procedure was applied to the manga- 
nin wires. The CCD read-out line was shielded and grounded 
in a similar way but kept separate from the other coaxial 
cables. 
The sensor itself was mounted on a circuit board located 
inside a tight cylindrical copper box ($=35 mm, L-35 
mm), as illustrated in Fig. 2. The upper end of the box has a 
hole of 7X10 mm2 capped with a 2 mm thick fused silica 
thermal filter. A copper cold finger ($=5 mm) connects the 
body of the box to the vacuum can flange immersed in the 
liquid He bath. In order to keep the CCD sensor at its lowest 
working temperature, around 70 K, the CCD chip was heat 
sunk, using grease secured with GE 7031 varnish, to a 0.6 
mm thick temperature-controlled copper plate. An 80 mm 
long copper wire of 0.5 mm diameter provides the weak 
thermal contact between the plate and the cold finger. The 
power generated by the CCD chip itself was measured to be 
about 170 mW which is not enough to raise the temperature 
sufficiently; extra heating on the order of 150 mW is needed 
for proper operation of the CCD sensor. 
Our optical setup is a modification of the scheme em- 
ployed in Ref. 3. The He-Ne laser illumination is guided 
into the cryostat via a single-mode optical fiber. The beam is 
expanded to 10 mm diameter and directed by a beam splitter 
and by a set of mirrors through the cell. Only about 0.05% of 
the light is reflected to the camera from the sample, i.e., from 
the reference plate (reflection coefficient R=3 X 10s4) and 
from the free surface of liquid 3He (R-2 X 10A4), while the 
rest of the illumination traverses the cell and is absorbed by 
the still radiation shield. A single bi-convex lens (f= 125 
mm) forms the interference image, created by the two re- 
flected beams, to the CCD sensor. 
In order to block the infrared radiation, emerging from 
the CCD sensor at 70 K, we installed two additional thermal 
filters, made of sapphire and CaF, (with thicknesses 2 and 8 
mm, respectively), to the optical path leading to the sensor 
(see Fig. 2). These filters reduce the thermal radiation from 
the sensor to its surroundings below a few nanowatts.’ The 
filters and windows used in our optical setup were antireflec- 
1784 Rev. Sci. Instrum. 65 (5), May 1994 0034-6748/94/65(5)/1784/2/$6.00 Q 1994 American Institute of Physics 
 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:
130.233.216.27 On: Fri, 29 May 2015 06:05:17
CONTROL UNJT (T= 300 K) r----------------------------. 
I 8 
1 I 
I TIMrNci 
DELAY VIDEO 
t UNIT SIGNAL : / 
I GENERATOR I VIDEO 
I 1 OUT L________---______--___----___I 
READ-OUT 
PROM SENSOR 
FIG. 1. Principal scheme of the remote-controlled CCD imager. 
tion coated to avoid problems with secondary reflections. 
Initially, heat leaks in excess of 1 ,uW were observed to 
the nuclear demagnetization stage when the camera was op- 
erated. This was assigned to rf heating caused by the 10 MHz 
digital signals entering the cryostat. Therefore, extreme care 
was taken to shield the milliKelvin parts against the r-f radia- 
tion from the 10 MHz drive signals. We tried to make the 
inner radiation shield around the nuclear stage a true “Fara- 
day cage.” Ground loops due to the 3He till lines were bro- 
ken inductively by six-turn spiral (+lO mm) sections, 
grounded from the middle when penetrating the shield. The 
only electrical wires entering the inner radiation shield are 
those needed to operate the heat switch and the pulsed plati- 
num NMR thermometer. The leads of the heat switch magnet 
and of the Pt static coil (wound from CuNi-clad multifila- 
mentary NbTi wire) have 47 nF ceramic capacitors to ground 
and across the lines at the entry point to shunt high frequency 
disturbances. The twisted pair for the rf coil is shielded by a 
Cu-Ni tube covered by tin solder. Because of limitations 
imposed by the tuning of the Pt tank circuit, only a 220 pF 
mica capacitor is connected across the leads of the rf coil. To 
eliminate possible electrical potential differences, we employ 
superconducting wire along the dilution refrigerator between 
the still and the mixing chamber. 
Owing to our experimental needs we have two 12 mm 
diam holes in our “Faraday cage;” they do not seem to be 
TOP FLANGE OF 
k\\\\\\\\\?+h\\\\\\\x “Am&j CAN 
INTERFERENCE 
IMAGE PROM 
OPTICAL CELL 
FIG. 2. Mounting of the CCD sensor inside the 4 K vacuum can. Cables 
connecting the circuit board to the control unit are not shown. F=thermal 
filter, M=mirror. 
too detrimental. One of them is along the axis of the cryostat 
and allows the laser illumination to enter the experimental 
space whereas the other lets the nonreflected light to escape 
from the coldest parts of the cryostat to the still radiation 
shield. 
The basic heat leak of our nuclear demagnetization cry- 
ostat is 10 nW. Operation of the camera does not change this 
within an accuracy of lo%, i.e., the heat leak from the CCD 
is less than 1 nW. An additional source of heating is the laser 
light used for illumination. The sensitivity of our CCD is 
such that a light power of 40 PW is required for successful 
imaging. About 0.5% of the light is absorbed to the fused 
silica reference plate inside the experimental cell, which 
means a heat leak of 100 nW to the 3He sample under inves- 
tigation. We find that heat absorbed by the fused silica is 
transmitted to liquid 3He with an effective time constant of 
100 ms. Typically, we employ 20 ms light pulses. A repeti- 
tion rate of 1 Hz results then in an average heat leak of 2 nW. 
For a bulk sample, this is not critical but with thin films a 
strong fountain effect is observed. Thus real-time imaging 
can be performed on bulk specimens over relatively short 
time intervals (a few seconds). This is, nevertheless, suffi- 
cient to study the surface dynamics of superfluid 3He (Ref. 
6). 
To decrease the heat leak, laser light of lower power 
level would be preferable. One possibility is to enlarge the 
reflection coefficient of the reference plate but this leads to a 
degraded contrast in the interference image. The most 
straightforward solution would be to use a slow-scan camera 
which takes advantage of the small dark current at low tem- 
peratures. Thereby an increase by a factor of 100 in the light 
sensitivity could be obtained but the real-time imaging capa- 
bility is lost. However, the light sensitivity of a regular CCD 
can also be improved by installing a separate, low-noise pre- 
amplifier to increase the S/N ratio at small illumination lev- 
els. 
We are indebted to G. Frossati, R. Wagner, and 0. V 
Lounasmaa for fruitful discussions. One of us (A. Babkin) 
thanks for hospitality during short visits to Leiden. Scholar- 
ships from the Vaisala Foundation (H. A.) and the Wihuri 
Foundation (J.R.) are gratefully acknowledged. This work 
was financially supported by the Academy of Finland. 
‘K.-P. Miiller and P Haarer, Phys. Rev. Lett. 66, 2344 (1991). 
2E. Rolley, E. Chevalier, C. Guthmann, and S. Balibar, Phys. Rev. Lett. 72, 
872 (1994). 
“A. J. Manninen, J. P. Pekola, G. M. Kira, J. P. Ruutu, A. V. Babkin, H. 
Alles, and 0. V. Lounasmaa, Phys. Rev. L&t. 69, 2392 (1992). 
4R. Wagner, P. J. Ras, P. Remeijer, S. C. Steel, and G. Frossati, JLTP (to be 
published). 
‘A. J. Manninen, H. AIles, A. V. Babkin, and J. P. Pekola, Physica B 178, 
3.52 (1992). 
“H. Alles, J. P. Ruutu, A. V. Babkin, and P. J. Hakonen, JLTP (to be pub: 
lished). 
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Cooled video camera for optical investigations below 1 mK

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