CERN Axion Solar Telescope

The CAST experiment at the European Nuclear Research CERN. Shown is the superconducting CAST LHC magnet with tracking system and the helium cooling system for the operation of the magnet (left ) Template:. Infobox / Maintenance / image

CAST ( CERN Axion Solar Telescope ) is an experiment at the European Nuclear Research CERN, with about 60 scientists from 16 nations according to a new particle, the axion, are looking for. In July 2003, the experiment was first put into operation at CERN with the aim of searching for the end of 2010 according to solar axions with a mass of 0 eV to about 1.1 eV. The experiment was extended to 2013 [ deprecated] will be continued in the year.

• 3.1 X-ray telescope
• 3.2 Micromegas detectors
• 3.3 Barbe- detector
• 3.4 High-energy calorimeter
• 3.5 TPC detector

• 4.1 Solar axions

• 5.1 CAST Phase I
• CAST 5.2 Phase II

Detection principle

According to theoretical models are axions uncharged particles of very small mass that interact only very weakly with ordinary matter - a characteristic that can account for the experimental detection of axions to the challenge. Various experiments could restrict in the past 30 years the allowed Axionmassenbereich 10-6 eV to about 1 eV. Depending on their actual rest mass axions could explain part of the yet unknown dark matter. In addition, axions can be used in hot and dense plasmas, such as in the core of stars, are produced at a similar rate as neutrinos through the Primakoff effect as above.

Stellar and solar axions

Already in the nineties of the twentieth century has shown that hot and thermal stellar plasmas would be very efficient Axionquellen. The dominant process that contributes in nondegenerate plasmas for the production of axions, is the so -called Primakoff effect. In this case, a real photon interacts with the electric field of the charged particles in the plasma and converted to a Axion. The axions produced in this way would leave the stellar plasma because of their low interaction probability. From theoretical model calculations shows that the sun because of their small distance, for an observer on earth and because of the high Axionproduktionsrate in the solar plasma, the potentially strongest stellar source with an expected flux density of solar axions from

Represents. in which

On 10-10 normalized coupling constant ( interaction strength ) of axions to photons. The spectral energy distribution of solar axions is very similar to a thermal blackbody spectrum with a mean energy of 4.2 keV. Moreover Axione only be produced in a relatively small volume in the core of the sun with high efficiency. Said emission region is in the form of a spherical volume of radius, which corresponds approximately to 20% of the radius of the sun. In the outer layers of the Sun, the Primakoff conversion is strongly suppressed by the prevailing plasma conditions. In the event that axions are discovered, they would allow a direct view into the fusion regions of the sun.

Principle of an axion- Helioskops

1983 struck Pierre Sikivie from the University of Florida is a new revolutionary concept for detecting such lighter and low-mass solar axions before, the so-called axion- Helioskop principle: When aligned on the earth a transverse magnetic field on the sun, then can it theoretically from the sun emitted solar axions are converted into real photons. Analogous to the production of axions in the solar plasma here plays the Primakoff effect the decisive role. Happened a Axion a transverse magnetic field, so it can be converted by the time inverted Primakoff effect in a real photon. Since the propagation direction of the axions and the magnetic field direction must be at an angle of 90 ° to each other, the magnetic field of the sun must be tracked. This allows the longest possible observation time. The conversion of axions into a real photon is taking conservation of momentum and energy of the axions. The energy distribution of photons leaving the magnetic field corresponds, the energy distribution of the original solar Axione. These can be detected using suitable detection systems for X-ray at the end of the magnetic field.

Detection probability

The differential photon flux from Axionkonversion, leaving the magnetic field of a Helioskops results from the product of the conversion probability of axions in a photon, and the solar Axionfluss an observer would expect on the earth (for all equations was adopted ):

The solar Axionflussdichte can be calculated analytically and is determined by the relationship

Very well described. The probability that a consistent conversion of Axions takes place in a real photon in a vacuum in a uniform and transverse magnetic field, where:

Here describes the axion to photon coupling strength, the magnetic field strength, the length of the magnetic field and the momentum difference between the axion and the real photon, which depends on the mass of the axions as follows:

Thus, the related to one day differential flux density of the expected photon conversion results to

For large values ​​of the term suppressed

The conversion probability. This results in an upper mass limit of

Up to the theoretical maximum rate of conversion can be achieved with the CAST Helioskop for Axione having an average energy of approximately 4 keV. Above this mass limit assumes the conversion probability very quickly. From van Bibber et al. was proposed in 1989 that the sensitivity of a Helioskops can be extended over this limit mass addition, if the conversion volume is filled with a gas. Under these conditions, the photon has an effective mass

Which depends on the plasma frequency of the electron density and thus of the conversion volume. As a consequence, the momentum transfer from the Axion changes to the photon to

With the proviso that the material density in the conversion of volume, and thus the absorption coefficient of the medium is constant, the Axion to photon conversion probability then in its more general form

It is obvious that the term simplified in the limit to the original shape for the conversion probability in an evacuated volume conversion. The advantage of a gas volume conversion is to keep the maximum conversion probability for a very narrow mass range

Can be recovered. However, the conversion probability disappears almost completely outside this parameter range. Is the electron density in volume conversion systematically increased, this resonance migrates to higher Axionmassen. The sensitivity of the Helioskops can be adjusted by suitable choice of the electron density at various Axionmassen and tested stepwise by variation of the electron density, a wide mass range.

Based on this idea, it is possible to expand the area on a mass -sensitive Helioskop far beyond the limit for the mass conversion also in a vacuum. However, this experimental approach is limited. An upper mass limit is given by absorption and scattering of photons in the conversion volume conversion. Both effects increase with increasing gas density and suppress the number of expected from Axionkonversion photons. In addition, the saturated vapor pressure of the gas used is of a certain gas density is exceeded and the gas may condense in the conversion volume. In this case, no meaningful measurement is possible.

Application to CAST

Based on this Helioskopprinzips result for the CAST experiment, two basic experimental configurations:

• CAST Phase I: Operation of CAST with evacuated Helioskops conversion volume. In this configuration, the CAST Helioskop is sensitive to axions with a mass between 0 eV and 0.02 eV.
• CAST Phase II: Operation of CAST Helioskops with a gas-filled conversion volume of variable gas density. As the buffer gas gases with low atomic number, such as 4He and 3He are used. In this configuration, the CAST Helioskop is sensitive to axions with a mass of 0.02 to 1.12 eV.

To achieve a complete coverage of the mass range from 0.02 to 1.12 eV, while the phase II CAST, must be carried out with the CAST Helioskop measurements at approximately 1000 density steps. The resulting measurement time is nearly three years. Both configurations have been implemented in several measured sections with the CAST experiment.

Magnet, cryogenics and gas system

For the conversion of solar axions into observable photons in CAST experiment a superconducting dipole - similar to the magnets used in large hadron collider - used which generates a transversal to the propagation direction of the solar Axione and homogeneous magnetic field of up to 9.5 t. The magnet has within it two tubes having a length of 9.26 m and a diameter of 42 mm, which are used as conversion volumes. Both tubes are located within the cold mass of the magnet, in which a temperature of about 1.8 K there. The cooling system for the superconducting magnet was made ​​of components of the former LEP e e - rebuilt accelerator and the DELPHI experiment at CERN for CAST. The multistage He cooling system supplies the CAST magnet with liquid helium, whereby a maximum cooling power of about 300 W at a temperature of 4 K and 50 W is guaranteed at the operating temperature of 1.8 K.

The magnet is mounted on a moving and rotating frame with which it can be oriented to the sun or to other interstellar objects. The tilt angle of the magnet is limited to ± 8 ° elevation relative to the horizon is limited ( limitation of the cooling system ). In the azimuthal direction of the magnetic field at an angle of approximately 40 ° to 140 ° can be moved. This results in a maximum observation time of about 1.5 hours of sun during sunrise and sunset times throughout the year. The accuracy of the tracking system is about 0.01 ° and is checked at regular intervals by surveyors. In addition, can be checked twice a year with an optical telescope, the tracking accuracy of the CAST system. This telescope is aligned parallel to the optical axis of the magnets, and can be observed in the visible light, the sun.

To operate with a buffer gas in the conversion volume of the magnet is fitted with a hermetically sealed gas system. Core components of the gas system is a complex control and pumping system and specially developed for CAST and in X-rays transparent cold window. This only 15 microns thin polypropylene window separating the gas-filled and 1.8 K cold conversion volume of the detector systems, which are partly operated at room temperature. If the temperature rises in the magnet (for example, at a transition to a resistive line of the magnet ), the pressure in the volume conversion would increase proportionally to the magnet temperature. As a consequence, pressure differences could occur from about 1 bar to the cold windows. To protect in this case, the window before their destruction and the resulting loss of the buffer gas, the gas can be recovered from the conversion and volume are pumped into storage container. The gas or electron density in the conversion of volume can be stepwise or continuously, and reproduced at any time is adjusted and maintained constant for the time of the observations.

At the ends of both tubes are mounted four highly sensitive detectors, the energy of the X- radiation (0.5 keV to 20 keV) are sensitive. In addition, the sensitive energy range of CAST was expanded with a high-energy calorimeter for energies up to 100 MeV in 2003. Currently (as of summer 2009) is set up another detector system in the wavelength range of visible light. Since for given magnetic parameters ( maximum achievable field strength, length of the magnet ), the sensitivity of the CAST Helioskops is determined solely by the background of the detectors and their efficiency, the primary goal of CAST is to use efficient detectors with the lowest possible background.

Detector systems

X-ray telescope

The CAST - ray telescope occupies one of the four measuring stations of the CAST magnet and consists of an X-ray mirror optics Wolter type I with a focal length of 1600 mm. In the focal plane is an optimized for low background space-resolving silicon detector.

Existing at the time of 27 concentrically nested and gold-coated nickel shells CAST Wolter optics is a prototype that has been developed for the German X-ray mission ABRIXAS. The optics are mounted eccentrically at the end of the four magnetic openings so that photons would leave the magnet from Axionkonversion in a nearly parallel beam and enter the optics. The parabolic and hyperbolic shape of the mirror shells ensures that X-ray photons are focused at grazing incidence (total reflection ) to a focal spot with an area of ​​only 9.4 mm ². The thus achieved concentration of a potential signal at a small area leads to a reduction of the expected background by a factor of approximately 154 In addition, the X-ray telescope, the opportunity to observe a potential signal, and the detector background simultaneously, and thereby to minimize systematic effects. Due to the high spatial resolution of about 40 arc seconds, the X-ray telescope for the case could be that a signal is detected, measure a Axionbild the core of the sun and contribute significantly to the understanding of the structure of our neighboring star.

For the detection of the signal, back -lit, 280 micron thick and full verarmeter pn - CCD silicon detector is used, which was originally developed for the run by the ESA XMM-Newton X-ray emission. In addition to a very high quantum efficiency of about 95% for the relevant CAST energy range between 1 keV and 7 keV, provides the CCD with its 150 micron × 150 micron sized pixels necessary for the X-ray optics spatial resolution and allows the detection of single photons in the energy range of the X-ray radiation. A decisive advantage of the CCD detector with an integrated front-end electronics, the long-term stability of the detector. To minimize the influence of thermal noise of the CCD is cooled to a temperature of -130 ° C. The generated by the CCD images with a resolution of 12,800 pixels are read out after an integration time of about 70 ms in 6 ms. The detector is a multilayer passive shielding of deposited lead ( free of 210Pb ) surrounded and oxygen-free copper that shields the CCD against external gamma radiation. The thus achieved average differential detector background in the focal point is on average about 8 × 10-5 cm -2s - 1keV -1 ( in the energy range from 1 keV to 7 keV), which corresponds to about 0.24 events per 1.5 hours of observation time.

Micromegas detectors

At the three remaining test sites, one right next to the X-ray telescope and the two magnetic holes on the eastern side of the magnet, are equipped with detectors of Micromegas - type ( Micro Mesh Gaseous Structure). It is to gas detectors which are optimized for the efficient detection of photons with energies between 1 keV and 10 keV. The main advantages of these detectors are their low background, its very good spatial resolution, a high detection probability for X-ray photons and the low manufacturing cost. Technologically, the Micromegas concept is an evolution of Vieldrahtpropotionalzählers, wherein the wire mesh of the multiwire proportional counter has been replaced by a micro-structured copper foil with a hole diameter of about 25 microns. In the manufacture of the detectors has been taken in particular that only materials have been used with low intrinsic natural radioactivity. The housing of the detector is made for example from perspex. The current induced by external radiation background is suppressed by means of a multi-layer passive shielding. Since the beginning of the first measurement phase of the CAST Micromegas detectors have been continuously developed and replaced by newer, more efficient models. The detector background can be achieved on average is about 5 × 10-5 cm -2s - 1keV -1 ( in the energy range from 1 keV to 10 keV).

Barbe- detector

Unlike axions produced in the Sun's core, would axions or axionähnliche particles produced in electromagnetic fields of the solar corona, energies in the range of a few electron volts. If these Axione converted to photons in the CAST magnets these have a wavelength in the visible light region. To detect such low-energy photons, a new detector system ( Barbe- detector of Italian Basso rate Bassa Energia, dt, low rate, low energy ') is established and developed at the time of the CAST collaboration. In the final stage of the bar detector should be connected via a Galilean telescope to one of the magnet bore of the CAST magnet, that the system can be operated in parallel to one of the Micromegas detectors. The potentially emerging from the magnet bore photons from axion to photoconversion be coupled with a material transparent to X-ray film mirror out of the beam path of the magnet in the direction of cash telescope. As appropriate detectors and photomultiplier cooled avalanche photo diodes are studied. First successful test measurements have been carried out with the bar - telescope with two types of detectors and demonstrated by the achieved background of about 0.4 events per second a promising sensitivity. An increase in sensitivity is to be expected especially in the future by better shielded detectors. Other detector concepts are still in the development phase, so-called transition -edge sensors ( TES) or silicon DEPFET detectors.

High-energy calorimeter

Axions which are produced by nuclear processes rather than by Primakoff conversion in the solar plasma, would be monoenergetic, but possess kinetic energies ranging from a few tens of kilo-electron volts up to the range of gamma radiation with many mega-electron volts. In order to prove these axions, was in CAST, during the measurement phase in 2004, a high-energy calorimeter operated. The detector was on the side watching the sun during sunrise, next to the X-ray telescope and mounted behind one of the Micromegas detectors. The calorimeter consisted of a CdWO4/CWO scintillator crystal, which has a high probability of absorption of gamma radiation and a very low, caused by natural radioactivity background at a very good energy resolution. The scintillator was read out with an optically coupled photomultiplier. The detector was both active as well as passive shielding and the detector surrounding active plastic scintillator served as Muonveto. Passive components such as old seasoned lead served to reduce induced by gamma radiation background. The influence of radioactive decays of atmospheric radon has been minimized to the detector background by an additional N2 atmosphere to the detector. With the calorimeter the sun was observed through one of the Micromegas detectors for a total of 60 hours and dismantled after successful data acquisition.

TPC detector

During the measurement phases in 2003 and 2004, a time projection chamber ( TPC) was installed on the east side of the CAST magnet. The detector took two measuring stations on the eastern side of the CAST magnet and could therefore observe the sun during the sunset. The detector, with a drift length of 10 cm, was read out by a multi-wire proportional counter and reached a maximum sensitivity of about 60 % in the energy range between 1 keV and 10 keV. The main advantage of this detector system was in its very low background count rate of only about 4 × 10-5 cm -2s - 1 keV -1. After completion of CAST phase I the time projection chamber was replaced by two Micromegas detectors with improved sensitivity and better background suppression.

Results

Solar axions

With the 2003 carried out by the end of 2008 measurements so far could be detected with the CAST Helioskop no Axionsignatur. The increased by a factor of six sensitivity of CAST compared to previous experiments CAST can significantly restrict the interaction strength of the hypothetical axions with photons and make an important contribution to the understanding of the physics of axions and dark matter. With CAST, it is now possible, the sensitivity of an experiment for direct detection of axions and axionähnlichen particles over the previously best indirect astrophysical observations to improve also in a wide mass range. Only so-called microwave resonators (Microwave Cavity ) are in a narrow mass range higher sensitivity. The previously specified with the CAST Helioskop upper limits on the strength of interaction of axions with photons will be

• Gaγ ≤ v0, 88 × 10-10 GeV -1 for axions with a rest mass ma ≤ 0.02 eV and,
• On average gaγ ≤ 2.17 × 10-10 GeV -1 for axions with a rest mass of 0.02 eV ≤ ma ≤ 0.39 eV.

Measurements for Axionruhemassen in the range of 0.39 eV ≤ ma ≤ 1.12 eV are carried out at the time. First results are expected by the end of 2010. A summary of the previously achieved with the CAST Helioskop results is shown in the right figure. The results of various laboratory experiments and astrophysical studies are shown in comparison with the CAST result complementary.

History

CAST - phase I

On 9 August 1999, the CAST experiment has been proposed as part of an experiment proposal entitled " A solar axion search using a decommissioned LHC test magnet" CERN - SPSC Committee. Four years later, the experiment was first put into operation in May 2003 and the first measurement campaign will be successfully completed in November 2003. The sensitivity of CAST at the time was still limited because the optical alignment of the X-ray telescope was not monitored continuously. After a subsequent brief reconstruction phase of the operation of CAST was resumed in April 2004. The main component, which has been implemented in the conversion stage, an X-ray source was to monitor the orientation of the x-ray telescope. In order ≤ 1 × the sensitivity of the experiment was the first time clearly below gaγ 10-10 GeV -1. During the November 2004 ongoing Phase I of CAST, were all detector system with maximum possible sensitivity in operation.

CAST Phase II

In 2005, the CAST Helioskop was prepared in a longer construction phase to the operation with the gas 4He in the conversion volume. As a first step, the cold window and a simplified gas system without gas recovery have been implemented, which in November 2005 was put into operation for the first time. The phase II was heralded by CAST. There was a more than a year long first measuring section by December 2006. During this time it was the CAST collaboration has succeeded to investigate the Axionmassenbereich ma from 0.02 eV to 0.39 eV after a Axionsignatur. The final stage of the experiment was reached in late February 2008. A major technological challenge was the expansion of the gas system to operate with 3He dar. In contrast to the first stage of expansion of the gas system for operation with 4He, the gas with the extended system of the conversion volume to be recovered. Characterized the probability of loss of the expensive helium gas is minimized. After about a six -month hiatus in mid-2009 was added the data acquisition again and will continue until mid-2011. The scientific objective for this period of the investigation of the Axionmassenbereichs of 0.59 eV to 1.15 eV.

With 3He as a buffer gas and higher pressures, a better detection sensitivity than with 4He for a higher mass range of axions reach. In 252 density steps, each with a one-hour measurement eV searched for axions in the mass range from 0.39 to 0.64. Due to absence of the expected X-ray radiation, the upper limit for the coupling of axions to photons could gaγ ≲ 2.3 × 10-10 GeV -1 are set with a 95 -percent confidence interval.

For further planned measurements to search for axions is to be extended to the field up to 1.15 eV, which then largely overlaps with the boundaries of a hot dark mass in the universe. If then show no evidence of axions in the CAST experiment, a new detection device is required. However, the current experimental arrangement could be used to detect other WISPs use (english weakly interacting sub-eV particles, dt, weakly interacting particles in the sub -electron volt range '). .