Solar axion search experiment with a superconducting magnet

The axion is a light neutral pseudoscalar particle yet to be discovered. It was introduced to solve the so-called strong CP problem. The expected property of the axion is characterized mostly by its mass, ma. If 10-5<ma<10-3eV, the axion can be copiously produced in the early universe so that they can close the universe as dark matter.

Another astrophysically interesting mass region is at around one to a few eV. Such axions can be allowed for some hadronic axion models by other astrophysical or cosmological constraints. In this region, the axion would be produced in the solar core through the Primakoff effect. It has an energy spectrum very similar to that of black body radiation photons with an average energy of about 4 keV. It can be converted back to an x-ray in a strong magnetic field in the laboratory by the inverse process. The principle of the detection is illustrated in the following figure.






We constructed a long superconducting magnet called the axion helioscope to search for the solar axions. A schematic view of the helioscope is shown in the next figure.

It consists of three parts, a tracking system, superconducting coils and x-ray detectors. The magnetic field strength in the aperture is 4 T. The tracking system is an altazimuth mount and drives a 3-m long vacuum cylinder of the helioscope to track the sun. Its trackable altitude ranges from -28 degrees to 28 degrees and almost any azimuthal direction is trackable. This view range corresponds to about 50% of duty cycle for the sun measurement in Tokyo. The other half of a day we measure the background. This helioscope mount is driven by two AC servo motors controlled by a personal computer (PC) through CAMAC bus. The PC regularly monitors two precision rotary encoders through CAMAC bus and forms a feedback control loop.

The superconducting coils and the x-ray detectors are mounted in the cylinder. The coils are kept at 5-6K during operation. They are directly cooled with two Gifford-McMahon refrigerators and no cryogen is needed.

我々は、太陽アクシオンを検出するため、下図のような全長約3mの超伝導電磁石で中心磁場4Tを発生するAxion Helioscopeを作った。



Sixteen PIN photodiodes, Hamamatsu Photonics S3590-06, are used as the x-ray detectors. The chip size is 11x11x0.5mm3. The x-ray detectors are mounted in a thermal as well as radioactive radiation shielding box made of oxygen-free high conductivity copper (OFHC Cu) which is operated at about 60K. The following picture shows a unit of four PIN photodiodes with their front-end amplifiers. X線検出器としては、本来光センサーである浜松ホトニクスのPINダイオードS3590-06を16枚用いている。1枚の大きさは11x11x0.5mm3である。この検出器は熱兼放射線遮蔽用の無酸素銅の箱に収められ、温度約60Kで作動している。次の写真はフロントエンドアンプとPINダイオードを4組のユニットにしたものである。

We put limits on axion-photon coupling constant by the search for the solar axions with the Axion Helioscope. The limits are illustrated in the following figure. Our limits are more stringent than any existing experimental limits in this mass region, and also than the limit inferred from the solar age consideration and also more stringent than the recent helioseismological bound. These results are described in the latest publication.

In the next phase of the experiment, we try to go to still higher mass toward the region upto ma=2eV.




As published in our resent report, the AXION HELIOSCOPE has also been used to search for possible axions emitted by other celestial objects. We scanned about 10% of the celestial sphere as shown in the following figure and searched for point axion sources. We also searched for axions from four compact objects, the galactic center, Sco X-1, Vela X-1, and Crab nebula. The AXION HELIOSCOPE is further directed toward the soft gamma ray repeater SGR 1900+14 to search for axions produced in it with its very strong magnetic field. No positive signal is found so far, and we put limits on the axion flux coming from them for the first time. Axion Helioscopeは太陽以外の天体でも、axionを放出する可能性のあるものを同様に観測することができる。太陽の場合と同様な議論で、いろんな天体から飛来しているaxionのフラックスも計算されている。われわれは、これまでに下図に示すような全天球の10%をスキャンして、axionの点源を探した。また、銀河中心、Scorpius X-1、Vela X-1、Crab nebulaをそれぞれ追尾観測した。また、大変強い磁場を持つと考えられているsoft gamma ray repeater SGR 1900+14についても追尾観測を行っている。いずれも、negative resultであるが、axionフラックスの上限値を与えている。結果の一部は、最近のわれわれの論文に報告されている。


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on 2010-08-19