Calibration System with Cryogenically-Cooled Loads

for QUIET-II Detector M. Hasegawa, O. Tajima, Y. Chinone, M. Hazumi, K. Ishidoshiro, M. NagaiHigh Energy Accelerator Research Organization, KEK

5. Principle

1. QUIET Phase-II

　 The Q/U Imaging Experiment (QUIET) is an experimental program to make very sensitive measurement of the Cosmic Microwave Background (CMB) polarization from the ground. The primary goal is to detect the degree-scale B-modes induced by primordial gravitational waves, which is a “smoking gun” signature of inflation. Having demonstrated our techniques in the first phase, we are preparing for the second phase of QUIET with ~500 detectors.

　 One of new challenges towards QUIET Phase-II is to (precisely) characterize the large number of detectors in the laboratory, and the calibration system reported here is newly developed for that purpose. 　

3. Calibration System

All-in-one polarization source - Blackbody emitters are cooled with a cryocooler as the cold load (~10K)

- Radiation is polarized by reflecting off a metal mirror

Advantages - Well-characterized polarization signal

Amplitude :

Direction : Parallel to the mirror surface

- Two temperature loads

Unpolarized response can be calibrated simultaneously

- Reproducing the observing condition (chilean sky load : ~10 K)

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P = 2 16πνρε 0 tan(β )(Tplate −Tload )

This system allows us to calibrate all the necessary quantities to characterize the QUIET-II detector simultaneously under a load condition similar to the actual observation.

Definitive test system for CMB polarization detector

4. System Components

6. Performance

This is the first demonstration “in the laboratory” for the modulation of the polarization signal with a load temperature, which is as low as those at the observing site.

Detector Response

10K

30K

Inherent polarization signal

Resistivity vs Signal - Polarization signal

- Inherent pol. is signal at 0Ωcm

No inherent polarization signal.

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∝ ρ

Total Power Two levels are seen as expected. - 10K (upper) and 30K (lower)

Polarizaton Clear sinusoidal curve - Amplitude is as expected.

6. Summary

Blackbody emitter array

We employ Eccosorb CR-112 an iron-loaded epoxy as the material for the blackbody emitter. To minimize the surface reflection, loads are casted with pyramid-shape grooves on the front surface. The measured power coefficient is typically -30dB.

Metal Mirror Cryostat and cryocooler

2. Detector and Characterization

3cm

OMT

HEMT

QUIET Detector 　 QUIET employs a HEMT-based pseudo-correlation receiver. A large focal-plane detector array is realized by a breakthrough in millimeter-wave circuit technology and packaging at JPL. Pseudo Correlation Detector - Measure Q/U polarization simultaneously - Stable against gain fluctuation Modulation (Phase switching) - 1/f noise is strongly suppressed. - Cancel out fake pol. signals. Characterization The following quantities should be characterized in order to convert detector time stream into polarization power spectra.

- Detector responsivities (mK/mV) - Detector polarization angle - Instrumental fake polarization

Well-characterized polarization signal is required.

Cylindrical cryostat (D 540mm,H 500mm) 2 stage GM cryocooler - enable two temperature loads - thermal loads is 18/7W on 1st/2nd stage in normal operation.

Use 3 different materials to vary the pol. Amplitude.

Rotate in the vacuum with rotary feed-through shaft.

All the calibration parameters are extracted from the response with respect to the mirror rotation angle.

Responsivity (pol.) : Amplitude/P

Responsivity (power) : DVI / DTload

Polarization Angle

: Phase of the sinusoidal curve

Instrumental fake polarization

: DVQ,U / (RK DTload )

Expected Response

We have developed a polarization calibrator for QUIET-II detectors, and successfully demonstrated its use with an actual QUIET detector.