Scintillating double-beta-decay bolometers

We present the results obtained in the development of scintillating double-beta-decay bolometers. Several Mo and Cd based crystals were tested with the bolometric technique. The scintillation light was measured through a second independent bolometer. A 140-g CdWO₄ crystal was run in a 417-h live time measurement. Thanks to the scintillation light, the α background is easily discriminated, resulting in zero counts above the 2615-keV γ line of ²⁰⁸Tl. These results, combined with an extreme easy light detector operation, represent the first tangible proof demonstrating the feasibility of this kind of technique. The text was submitted by the authors in English.     

Design of quick-response bolometers

Design equations for a solid bolometer are derived. The expressions obtained take into account the temperature distribution in both the dielectric and heat removal layers. Criteria for selecting the proper materials in the design of quick-response bolometers and for computing the dielectric layer thickness are presented.The author is grateful to A. F. Mal'tsev and L. S. Kremenchugskii for their interest in the present study and to A. G. Momanchuk for his assistance in carrying out the numerical computations.     

The theory of metallic bolometers

In the paper all the important parameters of metallic bolometers are derived on the assumption that cooling only takes place by conduction through the gas surrounding the bolometer and these parameters are described by means of dimensionless relative quantities independent of the dimensions of the bolometer. Universally valid diagrams are given for the dimensionless parameters of metallic bolometers from which the properties of an arbitrary metal bolometer can be determined in a simple way. A general expression is derived for the relative sensitivity of the bolometer containing only the temperature coefficient and the temperature increase of the bolometer above its surroundings.     

Cosmic-ray backgrounds in infrared bolometers

Cosmic-ray events are produced in infrared detectors by energy impulses which result from the ionization trails of fast charged particles. Model calculations are compared to the pulse-amplitude spectrum observed in a balloon flight from Palestine, Texas, at an altitude of 38 km. The results are useful for understanding cosmic-ray backgrounds which occur in all applications of high sensitivity bolometers.     

Terahertz response of thin-film YBCO bolometers

The bolometric response of high-temperature thin-film YBCO superconducting detectors to an electromagnetic radiation with a frequency of 2.5 THz is measured for the first time. The minimum value of the noise-equivalent power of the detectors is 3.5 × 10^?9 W/ $ \sqrt {Hz} $ . The feasibility of further increasing the sensitivity of the detectors is discussed.     

Reflectance of broad band waveguide bolometers

We have calculated and measured the reflectance of a variety of absorbers for cryogenic waveguide bolometers. The best absorber has <–15 db reflectance across a band of width 25% of the K band (18–26 GHz) center frequency and <–6 db reflectance across the entire K band.     

Metallic bolometers in periodically interrupted radiation

A theory of metallic bolometers is given for the case that the measured radiation is periodically interrupted. The impedance of a bolometer through which a direct and an alternating current simultaneously flow, is derived generally. Separate solutions are given for bolometers cooled by heat conduction and bolometers with thermal impedance dependent on the temperature. Expressions are derived for the sensitivity and minimum measurable power and the conditions for the choice of a coupling transformer with optimum ratio. In conclusion the lowest value of the minimum power at all measurable by metallic bolometers working in interrupted radiation is determined.Delivered in an abbreviated form. at the congress of the Physics Society of the German Democratic Republic in Leipzig, April 11, 1957.     

A calibrator for low background infrared bolometers

A calibration system is described which is suitable for measuring, under laboratory conditions, the optical responsivity of infrared cryogenic bolometers developed for low background purposes, such as infrared astronomy from balloon or satellite platforms. The system consists of a series of diaphrams which reduce the optical throughput of the instrument by a known amount. In this way both a background simulating the inflight conditions and a calibration signal are generated using the ambient temperature blackbody radiation. The design, practical realization, and application of a calibrator of this kind are described.     

Metallic bolometers with temperature dependent thermal impedance

The paper gives the solution of a bolometer with thermal impedance, which is a general function of the temperature. The general solution is applied to a bolometer cooled by radiation for which the appropriate expressions fo the resistance, sensitivity and dynamic resistance are derived and the expression for the maximum sensitivity of this type of bolometers from nickel, platinum and electrolytical platinum is given. The case of a bolometer cooled by conduction through the air and radiation is solved as the case of a bolometer with several ways of cooling. The paper also gives a graphico-numerical method for determining all the important parameters of the bolometer from the measured dependence of the resistance on the electric power RI²; an example of the application of this method is given for a conduction and radiation cooled bolometer. Deviations from the theoretical course are explained by convection of heat caused by the flow of air along the bolometer due to the increase in temperature of the bolometer over that of its surroundings. The paper also gives the maximum attainable sensitivity of this type of bolometer and metallic bolometers in general.     

Seven matched bolometers for millimeter wave observations

We have fabricated a millimeter wave observation system using an array of bolometric detectors. The performance of the system depends largely on obtaining identical performance from each bolometer. We achieved a variance in the responsivity of less than 4% except for one of the seven elements. The bolometers had an electrical NEP of 1.3×10^–16 W/Hz under radiation background loading of 30pW. We used an AC bridge readout circuit to significantly improve the stability of the array and showed that observations can be done without a mechanical beam switch. The bolometer array is now in use on the Nobeyama 45-m telescope for 150GHz observations.H. Matsuo thanks those who helped him work on bolometers during his stay in Berkeley, especially to T. Wilbanks, M. Devlin and C. Inman. The authors are also grateful to S. Sato and T. Watabe at Nagoya University for their advice on the AC bridge circuit. This work is partly supported by the Shimazdu Science Foundation.     

On the theory of pulsed foil bolometers

We present simple formulas for estimating the temperatures attained during absorption of radiation in a foil. The method proposed for selecting materials for bolometers makes it possible to substantially improve the temporal resolution of the instrument.     

Large area bolometers for millimeter-wave power calibration

An accurate monolithic power meter has been developed for millimeter-wave applications. The detector is a large-area Bismuth bolometer, integrated on a fused-Quartz substrate. It simply measures the temperature change caused by the absorption of millimeter-wave radiation. The power meter is simple to fabricate, inexpensive, and can be easily calibrated using a low-frequency network. The measured responsivity for a 50 bolometer, with an area of 1×1cm, at a bias of 1V. and a video modulation of 100Hz, is 1mV/W. The noise spectrum exhibits a 1/f rolloff till 1KHz, and is limited by the Johnson noise for higher frequencies. The NEP of the detector is 3WHz^–1/2 at a video modulation of 1KHz. It is possible to decrease the current NEP by fabricating bolometers with higher responsivities. Possible application areas are absolute power calibration and localized power density measurements at millimeter and submillimeter wavelengths.     

An adiabatic demagnetization refrigerator for infrared bolometers

Adiabatic demagnetization refrigerators have been built and installed in small portable liquid helium cryostats to test the feasibility of this method of cooling infrared bolometric detectors to temperatures below 0.3K. Performance has been achieved which suggests that bolometer temperatures of 0.2K can be maintained for periods of 60 hours. Applications to sensitive infrared detection from ground based telescopes and space satellites are discussed. Design data are given which permit the evaluation of refrigerator performance for a variety of design parameters.     

Background study and Monte Carlo simulations for large-mass bolometers

Large-mass bolometers are today extensively used for dark matter and double beta decay searches, in both cases the ultimate experimental sensitivity is defined by the background level reached in such devices. The most common background sources and the techniques used for their identification and reduction are here reviewed, with a particular focus on double beta decay searches. The relevant role played by Monte Carlo simulations in this field is discussed. As a real case, the background optimization in the MiDBD experiment is described.     

Rejection of randomly coinciding events in ZnMoO_4 scintillating bolometers

Random coincidence of events (particularly from two neutrino double beta decay) could be one of the main sources of background in the search for neutrinoless double beta decay with cryogenic bolometers due to their poor time resolution. Pulse-shape discrimination by using front edge analysis, mean-time and \(\chi ^2\) methods were applied to discriminate randomly coinciding events in ZnMoO \(_4\) cryogenic scintillating bolometers. These events can be effectively rejected at the level of 99 % by the analysis of the heat signals with rise-time of about 14 ms and signal-to-noise ratio of 900, and at the level of 92 % by the analysis of the light signals with rise-time of about 3 ms and signal-to-noise ratio of 30, under the requirement to detect 95 % of single events. These rejection efficiencies are compatible with extremely low background levels in the region of interest of neutrinoless double beta decay of \(^{100}\) Mo for enriched ZnMoO \(_4\) detectors, of the order of \(10^{-4}\)  counts/(y keV kg). Pulse-shape parameters have been chosen on the basis of the performance of a real massive ZnMoO \(_4\) scintillating bolometer. Importance of the signal-to-noise ratio, correct finding of the signal start and choice of an appropriate sampling frequency are discussed.