Takahiro Umeno

Development of GM cryocooler separate type liquid-helium-free 3He-4He dilution refrigerator system

We developed the new liquid-helium-free dilution refrigerator system, in which the Gifford-McMahon (GM) cycle cryocooler and dilution refrigerator (DR) unit are separated. We obtained the base temperature below 50 mK in this DR system. In usual liquid-helium-free DR systems, the DR unit directly couples with GM-cryocooler in the same vacuum chamber. Therefore the mechanical vibration of GM-cryocooler is hardly removed from DR unit. In order to eliminate the vibration problem, the separated vacuum chamber contacting the GM-cryocooler is connected with the DR unit chamber by the flexible hose with length of about 1 meter. Thin flexible tubes used for circulation of the refrigerant gas and radiation shield are installed in the connection hose. The 4He gas, cooled in the GM-cryocooler unit, transfers to the DR unit throw the thin flexible tubes. After cooling the DR unit, the gas returns to GM-cryocooler unit with cooling of the radiation shield. We expect that our separate-type dilution refrigerator becomes a useful piece of apparatus for the low temperature experiments.

Development of Dielectric Microcalorimeter

A prototype dielectric microcalorimeter was fabricated from 0.99SrTiO3-0.01SrTa2O6, a dielectric material. To estimate the operation performance of the microcalorimeter, a simple evaluation model is introduced by employing the block diagram formalism. Pulse height and fall time constant of the detection signal are analyzed as a function of the thermal conductance of the heat link. The dielectric microcalorimeter maintained at 100mK was irradiated by 5.5 MeV α-particles emitting from an 241Am source. Responsivity was found to be 2.7x10−19C/eV.

Measurement of Output Signals from an Avalanche Photodiode by Irradiating with X-rays in the Temperature Range from 150 mK to 4.2 K

We measured leakage current and gain characteristics of a reverse-type avalanche photodiode (APD). The leakage current generated below the breakdown voltage was found to be lower than 1 pA at temperatures below 200 K. Avalanche multiplication of the APD was achieved in the temperature range from 150 mK to 300 K. To characterize charge carrier properties of the APD, output signal pulses from the APD were observed by irradiating the APD with X-rays in the temperature range from 150 mK to 4.2 K. The yield of signal charge was found to abruptly change in the temperature range from 1 to 2 K, where the yield of charge at 1 K is about 50% of that at 2 K.

Performance of compact liquid helium free 3He-4He dilution refrigerator directly coupled with GM cooler in TES microcalorimeter operation

A superconducting transition edge thermosensor (TES) microcalorimeter was cooled by a compact liquid-helium-free 3He-4He dilution refrigerator with loading a Gifford-McMahon (GM) cooler for detection of LX-ray photons emitted from an 241Am source. The first and second stages of the GM cooler are directly coupled with the first and the second precool heat exchangers of a stick shaped dilution unit through copper plates in the vacuum chamber, respectively. The circulating 3He-4He gas through the precooled heat exchangers is condensed into a liquid of condense mixture by the isoenthalpic expansion through the Joule-Thomson impedance. A cascade of two mixing chambers are employed for achieving sufficient cooling power. The helium-free dilution refrigerator performs the cooling power of 20 μW at 100 mK. The TES and SQUID chips suffered from mechanical vibrations induced by a reciprocating motion of the displacer of the GM cooler. Detection signals of LX-ray photons emitted from 241Am source were observed by operating the TES microcalorimeter in severe noise environment induced by mechanical vibrations.

Development of a microcalorimeter with transition edge sensor for detection of LX rays emitted by transuranium elements.

A transition edge sensor (TES) microcalorimeter has been developed for use as an energy dispersive X-ray spectrometer. The TES microcalorimeter is a thermal detector that enables one to determine the energy of an incident photon by measuring the resultant increase in temperature. In this work, a Ti/Au TES microcalorimeter was developed to measure LX rays emitted by transuranium elements. The phase transition temperature was set at ~200 mK by using a bilayer structure composed of a 110-nm-thick Au layer and a 40-nm-thick Ti layer. An Au of 5 µm thickness was deposited on the Ti/Au bilayer to achieve an absorption efficiency of 35-80 % for the energy range of LX rays (10-25 keV). The developed TES microcalorimeter was irradiated with LX rays emitted by an (241)Am source at an operating temperature of 140 mK. An energy resolution of ~80 eV (full width at the half maximum) was obtained for L(β1)X ray of 17.75 keV.

Detection of the scintillation light emitted from direct-bandgap compound semiconductors by a Si avalanche photodiode at 150 mK

In this work, the direct-bandgap compound semiconductor materials are irradiated by α particles emitted from 241Am for the detection of scintillation light at the temperature of 150 mK. For the irradiation experiment, two disk shaped samples were fabricated from an epoxy resin mixed with the powder of PbI2 and CuI, respectively. Each disk-samples was cooled down to 150 mK by a compact liquid helium-free dilution refrigerator. A Si avalanche photodiode (APD) was employed for detecting the scintillation light emitted from the disk-sample inside the refrigerator. The detection signal current of Si APD was converted into the voltage pulses by a charge sensitive preamplifier. The voltage pulses of the scintillation light emitted from the direct-bandgap semiconductors were observed at the temperature of 150 mK.

Development of TES microcalorimeter for high precision spectroscopy of LX-ray photons emitted from transuranium elements

A microcalorimeter with the superconducting transition edge sensor (TES) with a Au absorber of 5.0 μm thick was fabricated for the energy dispersive measurement of LX-ray photons emitted from transuranium elements. The TES microcalorimeter and a superconducting quantum interference device (SQUID) amplifier chips were cooled by operating a compact 3He-4He dilution refrigerator without consuming liquid helium by loading a Gifford-McMahon (GM) cooler. Since the cold stages of the GM cooler are tightly coupled to the heat exchangers of the dilution refrigerator, the TES and SQUID chips suffered from mechanical vibrations induced by a reciprocating motion of the displacer of the GM cooler. Detection signals of LX-ray photons emitted from an 241Am source were observed by operating the TES microcalorimeter in severe noise environment induced by mechanical vibrations.

Development of a Flexibly Separated 4K Head Refrigerator for a Squid System

A refrigerator with a 4K helium recondensing head located 1.5 m from the main refrigerator unit has been developed. The 4K head is separated from the main refrigerator unit ( a G-M/J-T 4K refrigerator) by a 1m flexible vacuum insulated line. When the 4K head is inserted in a SQUID cryostat, the flexible line cuts the mechanical vibration from the main refrigerator unit and allows the cryostat to be easily handled. The refrigeration power was obtained as a function of the cold head temperature and of the J-T loop mass flow in the system. We also examined the refrigeration power both in the case that there is no extension in the J-T line (without the flexible line) and in the case that the cold head is 1.5 m from a J-T valve (with the flexible line). The experimental results show that a degradation of 0.5 W is caused by the extension of the flexible line. And it is concluded that a degradation of 0.3 W is caused by heat leak and a degradation of 0.2 W is caused by the decrease of mass flow. An additional 4K cold head will be separated by 5 m , and in the future the field noise of the SQUID system will be measured. In that case, a high pressure helium gas flow from the 70K heat exchanger of the refrigerator will be lead along the 4K-flow line as a thermal shield. In regard to the mass flow decrease; the J-T valve must be opened to keep a refrigeration power of 3 W. This increases the refrigeration temperature. A refrigeration temperature with the 5 m flexible line is predicted to be 4.61 K.