Nonlinearity of the transduction of a laser pulse by a fine-wire bolometer

The normalized efficiencies of transduction of the instantaneous power of optical radiation by a bolometer as a function of the average incident energy per unit length and the nonuniformity of its distribution are determined on the basis of the solution of the heat balance equation averaged over the length of the bolometer when the absorption efficiency factor, the temperature coefficient of resistance, and the specific heat of the material depend linearly on the temperature. Calculations are carried out for platinum bolometers and a radiation wavelength of 10.6 m. The amplitude distortions of the recorded signal do not exceed +20%.Translated from Izmeritel'naya Tekhnika, No. 10, pp. 33–35, October, 1993.     

Experimental Determination of the Kapitza Conductance Across Thin Film Bolometer Interfaces

Thin film bolometers are widely used in low temperature experiments. They are always connected in an electrical measuring circuit which involves a current, so an unavoidable Joule heating effect is generated. As a result the detector temperature can be appreciably higher than that of the cell, especially in the low temperature range. It is therefore important to be able to determine the bolometer net heat loss in order to evaluate the temperature gradient which could take place. We propose a simple analysis that provides an experimental determination of the values of the heat conductance across the interfaces of a thin film bolometer deposited onto a substrate. In this way, both temperatures of the bolometer and of the part of the helium film which covers it, as well as the respective heat flows through the film and the substrate are easily obtained. This method could prove useful in the future for a better understanding of surface exchanges and surface characterization.     

Amplitude Saturation and Non-Linearity of Pulsed Third Sound on a Glass Substrate

The amplitude of third sound has been measured as a function of the amplitude and width of a pulsed thermal drive for ⁴He film thickness values in the range 3–20 layers at T=1.11, 1.35 and 1.50 K. Saturation of the third sound amplitude and non-linearity were measured for large drive powers. The critical velocity of film flow is estimated from the saturated amplitude.     

Phonon Mediated Helium Atom Transmission through Superfluid Helium Four

We report results of experiments in which pulses of helium vapor with translational energies of 3 K are directed at a thin film of superfluid helium at a temperature of about 0.2 K suspended over a cesium covered orifice in a platinum film. The response of the superfluid film was detected by a super-conducting titanium bolometer placed on the side of the film opposite to that of the source. For films of approximately 1 mm in thickness we find no response of the bolometer within the limits of our detector. However, for films of less than 100μ thickness, we find a response which is of the same temporal shape, but smaller in amplitude, than that of the orifice when it is not covered with superfluid helium. We interpret these results to mean that we are seeing phonon mediated transmission in the thin films. Roton and condensate mediated transmission amplitudes for these conditions are apparently too small for us to detect in any of the films. This result is consistent with the theoretical results of Sobnack and Inkson [M. B. Sobnack, J. C. Inkson, and J. C. H. Fung, Phys. Rev. B 60, 3465 (1999)] concerning the amplitude of roton to atom and photon to atom conversion as a function of the atomic energy.     

Effect of heat link capacity on the energy pulse response of bolometers

A number of applications have been suggested for low temperature bolometers, for which their response to energy pulses should be optimized. When the heat capacity of the thermal link is not negligible, the bolometer response to energy pulses can be calculated by using programs written for analysis of electrical networks. The resulting response is not a single exponential, in agreement with experiments by McCammon et al.³ A satisfactory estimate of the signal-to-noise ratio is obtained by adding a third of the heat capacity of the link to that of the bolometer.     

Fabrication of ultra-low radioactivity detector holders for Edelweiss-II

In its first stage, the EDELWEISS-II dark matter experiment will use 21×320 g ionization-heat bolometers with NTD thermal sensors. To improve the present sensitivity of EDELWEISS to WIMP interactions by a further factor 100, ultra-low radioactivity detector holders exclusively made of copper and Teflon have been designed. The new design is using the relative expansion coefficients of copper, Teflon and germanium to hopefully ensure a dissipation-free detector holding.

In order to validate this new holder, we need to compare the vibration behavior of the old holder and the new one. The amplitude of the bolometer vibrations is measured using the variation of capacitance between electrodes sputtered on the Ge crystal and reference electrodes fixed above the detector holders. We present noise measurements and compare them to the equivalent measurements using the detector holders of the Edelweiss-I experiment, which gave very satisfying results.     

Superconducting Transition-Edge Sensor Bolometers with Integrated Electron-tunneling Refrigerators

We discuss the development of an array of superconducting Transition-Edge Sensor (TES) bolometers intended for use at millimeter wavelengths. Each bolometer in the array uses a proximity-effect TES sensing element and each has integrated Normal-Insulator-Superconductor (NIS) refrigerators to cool the bolometer below the reference bath temperature. The NIS refrigerators and bolometers are fabricated on a single Silicon substrate. Four short, thin and narrow legs suspend the millimeter wavelength radiation-absorbing element. The leg geometry is used to control and optimize the thermal conductance of the bolometer. Using the technology developed at NIST, we fabricated NIS refrigerators at the base of each of the suspension legs. These NIS refrigerators remove hot electrons by quantum-mechanical tunneling and should be capable of reducing the temperature of the biased (∼10 pW) bolometers by more than 100 mK when operating from a local thermal reservoir provided by a ³He system at ∼300 mK. This lower temperature at the bolometer will allow the detectors to approach background-limited performance despite the simple cryogenic system. In this paper we describe the progress toward the development of these detectors designed for use in observations at the 100 m Green Bank Telescope at 3 mm wavelength and the results of laboratory tests of a prototype detector.      

Development of a Bolometer Detector System for the NIST High Accuracy Infrared Spectrophotometer

A bolometer detector system was developed for the high accuracy infrared spectrophotometer at the National Institute of Standards and Technology to provide maximum sensitivity, spatial uniformity, and linearity of response covering the entire infrared spectral range. The spatial response variation was measured to be within 0.1 %. The linearity of the detector output was measured over three decades of input power. After applying a simple correction procedure, the detector output was found to deviate less than 0.2 % from linear behavior over this range. The noise equivalent power (NEP) of the bolometer system was 6 × 10⁻¹² W/Hz at the frequency of 80 Hz. The detector output 3 dB roll-off frequency was 200 Hz. The detector output was stable to within ± 0.05 % over a 15 min period. These results demonstrate that the bolometer detector system will serve as an excellent detector for the high accuracy infrared spectrophotometer.     

Analysis of Transient Heat Conduction in a Hollow Sphere Using Duhamel Theorem

In this article, analytical modeling of two-dimensional heat conduction in a hollow sphere is presented. The hollow sphere is subjected to time-dependent periodic boundary conditions at the inner and outer surfaces. The Duhamel theorem is employed to solve the problem where the periodic and time-dependent terms in the boundary conditions are considered. In the analysis, the thermophysical properties of the material are assumed to be isotropic and homogenous. Moreover, the effects of the temperature oscillation frequency, the thickness variation of the hollow sphere, and thermophysical properties of the sphere are studied. The temperature distribution obtained here contains two characteristics, the dimensionless amplitude (A) and the dimensionless phase difference (j{\varphi}). Moreover, the obtained results are shown with respect to Biot and Fourier numbers. Comparison between the present results and the findings from a previous study for a hollow sphere subjected to the reference harmonic state show good agreement.     

Nonlinearity in the response of a thin-wire bolometer to periodic laser pulses

The normalized response coefficient for detection of the instantaneous power of a periodic train laser pulses by a thin-wire bolometer has been obtained. The duration of the pulses is significantly less than the pulse interval period which is comparable to the thermal time constant which, in turn, is dependent on the incident thermal energy and its non-uniform distribution. A linear temperature dependence has been noted for the coefficient of resistance, the absorption efficiency factor the coefficient of heat transfer with external media,and the specific heat of the bolometer. Calculations show that the systematic error in the recording of instantaneous pulsed light power with platinum bolometers for a wavelength of 10.6 μm does not exceed 19%.Translated from Izmeritel'naya Tekhnika, No. 2, pp. 30–32, February, 1995.     

Voltage-Biased Superconducting Transition-Edge Bolometer with Strong Electrothermal Feedback Operated at 370 mK

We present an experimental study of a composite voltage-biased superconducting bolometer (VSB). The tested VSB consists of a Ti-film superconducting thermometer (T(c) ~375 mK) on a Si substrate suspended by NbTi superconducting leads. A resistor attached to the substrate provides calibrated heat input into the bolometer. The current through the bolometer is measured with a superconducting quantum interference device ammeter. Strong negative electrothermal feedback fixes the bolometer temperature at T(c) and reduces the measured response time from 2.6 s to 13 ms. As predicted, the measured current responsivity of the bolometer is equal to the inverse of the bias voltage. A noise equivalent power of 5 x 10(-17) W/ radicalHz was measured for a thermal conductance G ~ 4.7 x 10(-10) W/K, which is consistent with the expected thermal noise. Excess noise was observed for bias conditions for which the electrothermal feedback strength was close to maximum.     

Excitation and Relaxation of Film Flow Induced by Third Sound

High amplitude third sound waves are observed to create and to destroy persistent flow states within a circular resonator. These changes are necessarily a result modifying the underlying distribution of pinned vortices responsible for the flow. At low temperatures, large oscillatory flows associated with the third sound wave agitation are required to both increase and decrease the flow. At higher temperatures, thermally assisted de-pinning enhances the destructive aspects of lower amplitude third sound agitation. The constructive, or swirling tendency of the third sound agitation is also enhanced, but only in the presence of the wave excitation. At the higher temperatures, the ability of the induced flow to be trapped as a persistent current is diminished.     

Measurement of the distribution of intensity in a beam of thermal radiation

An experiment is described which involves measurements of the distribution of intensity in a beam of optical radiation using a grid of metal wires (bolometers). The resistance of each bolometer is determined, which depends on the intensity of radiation at the location of the bolometer. The detector rotates around the axis passing through the center of the grid normally to its plane. The resistance of the bolometers is measured for several positions of the grid. The resultant data are processed in a computer by an algorithm similar to that of tomography.     

Dynamical Theory of a High-Temperature Superconductor Photo-Thermoelectrical Bolometer

In this paper, we develop a dynamic theory starting from the basic heat balance equation for a high-temperature superconductor (HTSC) photothermoelectrical bolometer. We include dynamics via a small infrared sinusoidal signal incident on the sensitive area of the bolometer. The sensitive area of the bolometer is considered to be approximately mm × mm, with a film thickness in the micron range. This area is a thermoelectrical junction between an HTSC—for example, YBCO—and a high thermoelectic figure of merit material—for example, BiSb—grown on a substrate—for example, Yttria stabilized zirconia (YSZ). The two legs of the thermocouple are connected in a parallel geometry. Heat is transferred by direct conductance to the substrate underneath as well as toward the cold end of the thermojunction—that is, Peltier heat in the plane of the substrate—is also taken into account. It is found that if the bolometer is operated under a voltage bias condition, there is substantial improvement in the response time and the responsivity of the bolometer. This appears to be due to the photothermoelectrical operation superimposed on the normally used photoelectrical operation.     

Predicting the response of a submillimeter bolometer to cosmic rays

Bolometers designed to detect submillimeter radiation also respond to cosmic, gamma, and x rays. Because detectors cannot be fully shielded from such energy sources, it is necessary to understand the effect of a photon or cosmic-ray particle being absorbed. The resulting signal (known as a glitch) can then be removed from raw data. We present measurements using an Americium-241 gamma radiation source to irradiate a prototype bolometer for the High Frequency Instrument in the Planck Surveyor satellite. Our measurements showed no variation in response depending on where the radiation was absorbed, demonstrating that the bolometer absorber and thermistor thermalize quickly. The bolometer has previously been fully characterized both electrically and optically. We find that using optically measured time constants underestimates the time taken for the detector to recover from a radiation absorption event. However, a full thermal model for the bolometer, with parameters taken from electrical and optical measurements, provides accurate time constants. Slight deviations from the model were seen at high energies; these can be accounted for by use of an extended model.     

Characterization of a SrF2 Scintillating Bolometer

We present the analysis of the data obtained with a 53?g SrF₂ scintillating bolometer operated at 20?mK. We have analyzed its heat and light response (time constants, linearity and energy resolution) and measured its scintillation relative efficiency factor for different particles (alpha, beta/gamma and neutrons). We have studied the spatial uniformity of the light output profiting from its internal contamination. The light amplitude of alphas from the delayed coincidence ²²⁴Ra→²²⁰Rn→²¹⁶Po (emitted from the same crystal position) shows a positive correlation, evidence of a non-uniformity that worsens the light signal energy resolution by more than?50%.     

Vibration of a single microcapsule with a hard plastic shell in an acoustic standing wave field

Observation techniques for measuring the small vibration of a single microcapsule of tens of nanometers in an acoustic standing wave field are discussed. First, simultaneous optical observation of a microbubble vibration by two methods is investigated, using a high-speed video camera, which permits two-dimensional observation of the bubble vibration, and a laser Doppler vibrometer (LDV), which can observe small bubble vibration amplitudes at high frequency. Bubbles of tens of micrometers size were trapped at the antinode of an acoustic standing wave generated in an observational cell. Bubble vibration at 27 kHz could be observed and the experimental results for the two methods showed good agreement. The radial vibration of microcapsules with a hard plastic shell was observed using the LDV and the measurement of the capsule vibration with radial oscillation amplitude of tens of nanometers was successful. The acoustic radiation force acting on microcapsules in the acoustic standing wave was measured from the trapped position of the standing wave and the radial oscillation amplitude of the capsules was estimated from the theoretical equation of the acoustic radiation force, giving results in good agreement with the LDV measurements. The radial oscillation amplitude of a capsule was found to be proportional to the amplitude of the driving sound pressure. A larger expansion ratio was observed for capsules closer to the resonance condition under the same driving sound pressure and frequency.     

Detection of single X-ray quanta with a magnetic calorimeter

We give a status report on the development of a particular low temperature calorimeter with new experimental results. On absorption of an X-ray photon the increase of temperature changes the magnetization of a diluted magnetic sample, and this quantity is measured with a SQUID-magnetometer. It is a special feature of this experimental method that the magnetic sample has a very high heat capacity and an additional absorber for a compound detector does not change the sensitivity essentially. Besides a short summary on earlier measurements we present new results with metallic magnetic samples, which give shorter signal rise times (below 100 μs). On a compound detector with 0.1 g of LaB₆:Er and an absorber of 12 g sapphire, the energy resolution for 5.9 keV and 60 keV X-ray sis 1.6 keV and 2.6 keV (FWHM), respectively. With a silicon absorber an energy resolution of 1.4 keV at 5.9 keV has been found. The energy resolution is in any case limited by two effects. On the one hand the signal height is strongly reduced due to an additional heat capacity of the magnetic sample and on the other had we have an additional noise from the conduction electrons of the metallic sample. Possible improvements with respect to both effects are discussed.     

Factors determining the stability, resolution, and precision of a conventional Raman spectrometer

We verified the performance of a conventional Raman spectrometer, which is composed of a 30 cm single polychromator, a Si based charge-coupled device (CCD) camera, and a holographic supernotch filter. For that purpose, the time change of the peak positions of Raman spectra of naphthalene and fluorescence spectra of ruby (Cr-doped Al(2)O(3)) were monitored continually. A time-dependent deviation composed of two components was observed: a monotonous drift up to 0.4 cm(-1) and a periodic oscillation with a range of 0.15 cm(-1). The former component was stabilized at approximately 2000 s after the CCD detector was cooled, indicating that incomplete refrigeration of the CCD detector induced the drift. The latter component synchronized with the periodic oscillation of the room temperature, indicating that thermal expansion or contraction of the whole apparatus induced this oscillation. The implemental deviation is reduced when measurements are conducted using a sufficiently cooled CCD detector at a constant room temperature. Moreover, the effect of the room temperature oscillation is lowered in a spectrum acquired over a duration that is longer than one cycle of this oscillation. Applying the least squares fitting method to carefully measured spectra enhanced the precision of the determination of the peak position to 0.05 cm(-1) using the spectrometer with pixel resolution of 1.5 cm(-1).     

Photothermal radiometric detection and imaging of surface cracks

This paper discusses surface crack detection by photothermal radiometric imaging (PRI). In PRI, also called dynamic infrared scanning, a surface is scanned with a spot of heat. Imperfections are detected by radiometrically sensing changes in the surface temperature of a small area in the vicinity of this spot. In the work described, cracks narrower than 25 µm (0.001 in.) in a lightly rusted steel surface have been detected. Indiscrete scanning an amplitude modulated heating beam is moved in steps, remaining at each location long enough to measure amplitude and phase of the AC temperature. Incontinuous scanning a constant intensity heating beam is moved continuously while the temperature deviations are measured. This paper presents methods of calculating amplitude and phase of surface temperature for discrete scanning and instantaneous temperature for continuous scanning across a surface crack. For a steel surface scanned by a watt-level laser beam, predicted surface temperature deviations when crossing the crack are several degrees Celsius, with expected radiometrically detected power several orders of magnitude above the detector noise. In experiments performed, both techniques easily detected narrow cracks in a smooth, clean surface. Discolorations and pits, on the other hand, generate a disturbing type of surface noise. This noise was minimized by differential detection. Based on results obtained, continuously scanned PRI with a fan-type heating beam and array detection could become a viable way of mapping surface cracks at practical scanning speeds.