ALPS II detector module including two double-stage SQUIDs and two TESs.

A PIXIS CCD camera was successfully used as a detector for ALPS I experiment. However the lowered photon energy of the ALPS II experiment (1.165 eV compared to 2.33 eV in ALPS I), which is very close to the silicone band gap of 1.14 eV, makes impossible to use the same CCD-based detector. The low photons energy as well as the expected low rate (1 photon every few hours) ask for a detection system with following characteristics:

  • High quantum efficiency (higher than 80 % for 1064 nm photons);
  • Low background rate (at least less than 10-3 s-1);
  • Long-term stability;
  • Good energy resolution;
  • Good time resolution.

Reaching such an extremely low dark count rate with a high quantum efficiency is the limiting factor for possible options for our detection system. ALPS collaboration made the choice to work with a tungsten Transition-Edge Sensor (W - TES), optimized for 1064 nm single photon detection and developed by the National Institute of Standards and Technology (NIST, USA).

Transition-Edge Sensor (TES)

Chip including two double-stage SQUIDS (view with a microscope).

TES sensor (20 um x 20 um) surrounded by silicon substrate (blue) (view with a microscope).

TESs are superconductive microcalorimeters measuring the temperature di fference dT induced by the absorption of a photon. They exploit the rapid change of the resistance at the superconducting phase transition (Tc = 140 mK).

The positioning of ALPS sensors within their superconductive transition (30% of their normal resistance) is induced through a thermal link to a bath (Tbath = 80 mK) and through a constant bias voltage. Thus, the TES chip needs to be cooled, for which we use an Adiabatic Demagnetization Refrigerator (ADR) designed by Entropy GmbH. The signal is fed to the TES chips using standard single-mode optical fibers. The read-out of the TESs is done using Superconducting QUantum Interference Devices (SQUIDs) developped by the Physikalisch-Technische Bundesanstalt (PTB, Germany).

ALPS II detector module mounted on its copper bench. A bare singlemode fiber is coupled to the lower sensor.

1064 nm single photon pulse.

A TES system is much more difficult to set up and operate than a CCD, but promises a large quantum efficiency (higher than 95% for a TES sensor)good time and energy resolutions as well as a very small intrinsic dark count rate (10-4 events/sec).

Characterization, optimization and background reduction of the sensors are currently under progress.

More details have been published in the Ph.D. thesis of Jan Dreyling-Eschweiler or can be found in the publications section.

ADR system in operation position (left). ADR system opened in maintenance position (right).

CCD camera

Quantum efficiency dependance on the wavelength of the Pixis CCD.

The PIXIS 1024 B CCD camera has been used for ALPS I. A low dark count rate of about 10-3 e-/(px*s) can be reached with the CCD chip being cooled.

The main disadvantage of the CCD for ALPS II is its relatively low quantum efficiency (about 1.2%) for the ALPS II infrared photons, which directly reduces the sensitivity.

Calibration results and statistical analysis methods can be found in the Ph.D. thesis of Jan Eike von Seggern.

For further information about the ALPS detector system please contact:

Klaus Zenker
Contact person for: Data analysis and detector
Institute: DESY
Tel.: +49-40-8998 4835
email: klaus.zenker(at)