Antiproton Decelerator

The Antiproton Decelerator

• Stochastic cooling
• Electron cooling

• ATHENA
• ATRAP
• ASACUSA
• ACE
• ALPHA
• AEGIS

The Antiproton Decelerator ( AD abbreviation; German: antiproton decelerator ) is a storage ring at CERN in Geneva. The aim of the AD is to decelerate the antiprotons produced by the proton synchrotron and to provide the various antimatter experiments are available.

• 3.1 ATHENA (AD -1) 3.1.1 Antiproton case
• 3.1.2 positron production and accumulation
• 3.1.3 The mixed case

• 3.2.1 positron production and accumulation
• 3.2.2 Current Development

• 3.6.1 Principle of measurement

History

At CERN synchrotron antiprotons have been produced since the late 1970s with the proton beam of the proton and for experiments with antimatter in the plants " Antiproton Accumulator " (AA), " Antiproton Collector" (AC) and " Low Energy Antiproton Ring" ( LEAR ) captured collected and braked. 1995 showed the PS210 experiment at LEAR storage ring at CERN, that it is in principle possible to produce antihydrogen. However, only nine antihydrogen atoms with a kinetic energy could be produced from about 1.2 GeV. This corresponds with antihydrogen at a speed of 0.9 c, or a temperature of 1.4 × 1013 ° C. Due to this high temperature, one also speaks of "hot " antihydrogen. As one with anti- atoms and theories such as can check the CPT theorem and various predictions about anti-gravity, it is of particular interest to perform experiments on antihydrogen. To perform high precision experiments, one needs much larger quantities and colder by several orders of magnitude antihydrogen atoms. This could not be achieved with the PS210 building. In 1996, the plants were shut down in favor of the LHC.

Due to the continued high level of interest on cooled antiprotons, it was decided, based on the components of the AC to construct the Antiproton Decelerator. The renovation plans were approved in February 1999. 1999, the Antiproton Decelerator was functional and is able to deliver 2 × 107 antiprotons with a kinetic energy of 5.3 MeV. After completion of AD antimatter different experiments were set up in the interior of the storage ring. Many are concerned with the production of cold antihydrogen (eg ATHENA, ATRAP ), others use the antiprotons for other purposes such as ASACUSA, which makes measurements of exotic atoms.

Operation

Antiproton production

Since antiprotons on earth are not naturally occurring, they must be produced artificially. This is usually done by pair production. You shoot a charged particle (eg, a proton p) with high kinetic energy on a target. Meets the blasting a nucleus, it interacts with a proton in the core and a particle Antiteilchenpaar is generated. Under certain circumstances produces a proton-antiproton pair.

The antiproton thus produced is separated by the mass spectrometer from the protons and the other generated particle-antiparticle pairs, so that one has only antiprotons in the beam. Due to the four-momentum conservation, the minimum kinetic energy

This corresponds to a momentum of 6.5 GeV / c. Since the protons are bound in the core of the target material, the actual energy is slightly lower and depends on the material used. Are usual here copper, iridium, and beryllium.

Since the antiproton formation with proton pulses greater than 6.5 GeV / c is much more likely (see cross section graph) using protons with a momentum of 26 GeV / c, corresponding to a kinetic energy of about 25 GeV. In order to provide these very high energy, a particle accelerator is required. In the case of AD, this is the Proton Synchrotron, which is also used as a pre-accelerator for the LHC.

Construction

The AD is a storage ring with a circumference of 188 m. It basically consists of the parts of the Antiproton Collector, a storage ring, which was previously used for collecting antiprotons and also used on the PS210 experiment. However, many parts were heavily modified. So all the power converters were better stabilized current and the vacuum compared to its predecessor by a factor of 20 ( in some AD 10-10 mbar) improved. To decelerate the antiprotons using acceleration cavities, but the " upside down " are operated so that the particles are slower after passing through the cavity. To reduce the particle beam emittance, AD has the ability to apply the both of the standard cooling methods stochastic cooling and electron cooling. The antiprotons will be redirected to the Abbremsprozedur with a kicker to the experiments. A kicker electromagnet that can be switched quickly, thereby changing the path of the particles. Skilful driving can thus be realized a kind of crossover for charged particles.

The AD can be filled with protons for calibration purposes. Since protons have the opposite charge of antiprotons, they are deflected by the deflection in the opposite direction. But that they can still be saved, they can be helped by a second beam pipe (in the picture the red loop ) inject in the opposite direction.

To take full advantage of the area of ​​the hall, one is based on the experiments inside the AD- ring.

Calibration

To calibrate and synchronize the components AD, it is taken into operation with protons. The advantage of protons over antiprotons offer is the fact that they are present in much higher amounts, as they can inject the proton synchrotron in the AD directly and does not need to produce in an intermediate step on the pair production. So typically there are 3 × 1010 protons are available, while there are only 5 × 107 in the surgical operation. In this way, the signals of the measuring instruments are stronger, and you get a better signal -to-noise ratio.

Operation

A Abbremszyklus begins with the antiprotons from the target with a momentum of 3.5 GeV / c are injected into the AD. Since the emittance is still very high, it is reduced by means of the stochastic cooling method (see graph cooling ). After the emittance has been sufficiently reduced, it begins with the actual braking process. In a few seconds the antiprotons with the help of the cavities are brought to a pulse of 2 GeV / c. In this case, however, the emittance is simultaneously increased again, so you have to re- apply the stochastic cooling. Note that cooling measures only serve to reduce the emittance and are not responsible for ensuring that the particle bunch is slower overall. If you were to bring the antiprotons directly to the desired pulse of 100 MeV / c, one would due to the increasing emittance lose too many antiprotons in the beam. After the second cooling can they slow down again and reduce the emittance by electron cooling. This is repeated one more time to achieve the desired pulse of 100 MeV / c. After this braking are about 2 × 107 slow antiprotons available. Comparing this with the 1013 protons impinging on the target, as is required on average 500,000 protons to produce a slow anti proton. With the help of a kicker the cooled antiprotons are directed to the experiments. After the slow antiprotons were forwarded to the experiments, one can fill the AD again with fast antiprotons, and the whole process starts again.

Experiments

After completion of AD antimatter different experiments were set up. Below is a selection of these is described.

ATHENA (AD -1)

Since the PS210 experiment, only 9 very hot antihydrogen atoms were produced, the ATHENA collaboration wanted to show that it is possible to produce large quantities of cold antihydrogen. To achieve this, an apparatus has been made ​​, which can be divided into three sections: first, the anti- proton trap in which the anti protons are trapped by the AD and further cooled, as the second the positron generating, collecting and cooling region, and finally the mixing region, in which the two constituent of antihydrogen are zusammgebracht and can recombine.

Antiproton case

Anti- proton from the AD ring have a pulse of 100 MeV / c, corresponding to a temperature of 1010 ° C × 6.2. It is therefore necessary to further cool. According to the Bethe formula charged particles lose kinetic energy when they pass through a solid state is likely to provide you the antiproton package a 130 micron thick film of aluminum in the way. As are present in the nuclei of aluminum atoms protons, one might think that the antiprotons annihilate immediately on contact, but the Annihilationsrate is highly dependent on the interaction time, which is very small. Therefore, only a very small percentage of the antiprotons by Annihilation is lost. Then enter the still -energy antiprotons into the prepared collection trap. The collection trap is a cylindrical Penning trap. In contrast to classical Penning trap the quadrupole electric field is not achieved by hyperbolic electrode, but by segmented cylindrical electrode, wherein each ring has a different potential. This makes it possible to form a potential well can be taken in the charged particles ( see picture right). Since the antiprotons only need to enter the potential well, it is open in the first 200 ns after the impact of the antiproton packet on the aluminum foil on one side while a voltage of 5 kV on the other side. Antiprotons which have less than 5 keV kinetic energy after passing through the foil, the potential barrier can not be overcome and are reflected. However, this is only less than 0.1 % of all antiprotons, leaving only about 10,000 left over from the original 2 × 107. Thus, the reflected antiprotons remain in the trap, a voltage of 5 kV must be applied after about 500 ns and on the other side of the trap. The antiprotons So now commute between the two potential walls of the case back and forth. To decelerate the 5 - keV antiprotons to a few meV, has summoned cold before the arrival of Bunchs ( about 15 K and 1.3 meV ) electrons in the trap. Since electrons as well as antiprotons are negatively charged, it is no problem to catch them in the same trap. Fly now the antiprotons by the cold electrons, so give this their temperature to the colder electrons and so lose energy. The heated electrons in turn give their energy by synchrotron radiation in the magnetic field of the trap. Antiprotons, which are about 1800 times heavier than positrons, although also send synchrotron radiation, but the radiation power strongly depends on the mass of the particles and increases rapidly with decreasing mass. After a few seconds, the antiprotons have fully discharged their thermal energy to the electrons, which have in turn reduces the temperature by synchrotron radiation. Finally, the captured particles are at about 15 K with the surrounding cooled superconducting magnets in thermal equilibrium and are now ready to be transferred into the mixing trap.

Positron production and accumulation

To produce the positrons for antihydrogen, one could proceed exactly as in the production of antiprotons, however, the nature here is an easier way ready. The radioactive isotope ² ² Na decays with a probability of 90 % by β decay in ² ² Ne, a positron, an electron neutrino and a high-energy photon.

The resulting fast positron is now also trapped and cooled in a cylindrical Penning trap. On the one hand is in the case of nitrogen gas at a very low pressure. Move the positrons through the gas, they encourage this. This is inelastic, so that the positrons lose kinetic energy and are slowed. Also here is another antimatter ( positrons) with normal matter ( shell electrons of nitrogen ) in contact and starts to annihilate. However, the pressure of the nitrogen gas is very low and the positron flux with 5 × 106 positrons / s is so high that losses are not significant. The other method used is the rotating wall technique, in which a rotating electric field is superimposed on the potential drop, which results in the magnetic field of the coil to a compression of the positron cloud. The time it takes the AD in order to brake the anti protons is used to accumulate the positrons in the Penning trap. So are finally about 3 × 108 positrons in the trap.

The mixed case

Now you have created the two components of an antihydrogen atom and must bring them into the same region of space, so they can recombine. This is done, as it did for the other two cases, a cylindrical Penning trap, which consists of many individual ring electrodes in order to realize the complex potential. First, the positrons are transported into the mixing trap. This is done by setting the potential on one side of the Positronfalle to zero; positron flow due to their small intrinsic speed of the positron trap out how gas from a gas cylinder. The potential of the mixed case is similar to that date which the antiproton case at t = 200 ns. Once the positrons have flooded into the empty mixing case, the potential is ramped up on the other side of the mixing trap, and the positrons are trapped in the mixing trap. During this process it loses about 50% of the positrons. Then the positron cloud is axially compressed, so they do not fill the whole mixed case volume. Now, one might add the antiprotons, however, one encounters the problem that the two particles are loaded differently ( antiproton negative, positive positron ) which means that they can not be stored together in a normal Penning trap. Intuitively, one can say that a potential well for positrons represents a potential barrier for antiprotons. To solve this problem, one puts the potential is seen in the picture below 1 ) to the case. The positrons as well as the antiprotons are each trapped in their potential well is "open" according to their charge in a different direction. To get now the antiprotons into the mixing region, we changed the potential so that it assumes the dashed curve in Figure 2). This allows the antiprotons to "slip " into the mixing trap. After the antiprotons have been transferred to the larger potential well, you put back the old potential of the trap (Figure 3) ). The resulting case is called a nested Penning trap, as they in a sense combines two Penning traps in itself. Although the picture makes it seem as if the two types of particles were separated from each other, but you have to remember that they reside in the same case volume and can recombine with each other. They are held by the potential only at the correct position.

When a positron and an antiproton have come together, formed of electrically neutral antihydrogen. This neutral antihydrogen is no longer supported by the trapping potential and the magnetic field, and so the anti-nuclear can move freely inside the case until it joins the ring electrodes of the Penning trap. There, the two particles annihilate with their respective partners matter of the electrode material. This characteristic annihilation radiation is emitted. This radiation is detected with a detector, and thus may be counted as many anti- hydrogen atoms were prepared.

2002 ATHENA could produce in this way a total of 500,000 cold antihydrogen atoms. The kinetic energy of 0.2 eV, which corresponds to a temperature of about 2000 ° C. While this is not "cold" in the sense of a few milli- Kelvin, the temperature, however, if one compares with the 1.4 × 1013 ° C at PS210, the expression is justified. In ATHENA, however, no high-precision experiments were carried out, it was only the production of larger quantities of cold antihydrogen demonstrated. Meanwhile, the project for successor experiments AEGIS and ACE has been set.

ATRAP (AD -2)

ATRAP is emerged at the same time as the ATHENA AD. Even with ATRAP the goal was the production of cold antihydrogen. The two experiments are very similar, except for the manner of positron accumulation which is described below for ATRAP.

Positron production and accumulation

There are two effective ways to slow down the fast positrons by inelastic processes. The ATRAP collaboration thereby chose a different path than ATHENA. The (as in ATHENA ) of ² ² Na emitted fast positrons were first slowed down by a 10 micron thick titanium foil and then met a 2 micron thick tungsten crystal. Then there is the possibility that a positively charged and a negatively charged electron positron assembles a Positroniumatom within the crystal. In this process, the positrons lose most of their energy so that it is no longer necessary as ATHENA she's here with nitrogen gas to decelerate further. Where the Positroniumatom now for Penning trap at the end of the apparatus, it is ionized there and caught the positron in the trap.

Since the positron accumulation was not particularly efficient in this way, even the ATRAP experiment has now switched to the method used in ATHENA.

Current Development

Unlike ATHENA ATRAP has not yet been set and has been continuously improved and expanded. So ATRAP now has a Penning - Ioffe trap that can store the electrically neutral antihydrogen using magnetic quadrupole fields. This is possible since the magnetic moment of anti- hydrogen non-zero.

ASACUSA (AD -3)

When ASACUSA experiment, has specialized in manufacturing of exotic atoms in the form of antiprotonic helium, ie a helium atom, in which a shell electron has been replaced by an antiproton. An examination of these atoms with spectroscopic methods, so you can test different aspects of the CPT theorem. This predicts, among other things, that the masses of the proton and antiproton are the same. The formula

Linked the measured wavelength λ of the emitted light with the atomic number Z, the Rydberg constant R ∞, the principal quantum numbers involved in the transition n1 and n2, the core mass M and the mass of the antiproton. This formula is indeed only a first approximation which relativistic and QED effects such as neglect the Lamb shift, for example, the idea behind the measurement illustrates, however, quite good.

Up to the wavelength λ and the antiproton mass all observables are known. One can determine the antiproton mass very precisely and compare with the mass of the proton that is, by high-precision measurement of the wavelength. If the values ​​within the measurement error from one another, so the CPT theorem is refuted.

ASACUSA has several radiative transitions measured with high precision, but could not detect deviations of the masses. The CPT theorem has therefore still applies.

ACE (AD -4)

The potential advantages of the use of antiprotons in radiotherapy of malignant tumors is being explored by the ACE collaboration. Due to the annihilation energy released, the dose is about doubled compared to protons in the Bragg peak at the same dose in the input channel. This could be spared in the vicinity of the tumor, the healthy tissue. Also hope for by detection of high-energy pions opportunities for online dose verification.

ALPHA ( AD-5 )

ALPHA is engaged in producing, trapping and measurement of anti-hydrogen molecules. These first positrons and antiprotons are stored in a Penning trap, and then merged in a magnetic octupole trap ( Ioffe trap). The anti hydrogens are indirectly detected by the annihilation particles in a Silicon Vertex Detector, photons in the case of positrons and pions for the antiproton.

ALPHA has been able to catch in 2010 as the first anti hydrogens. In 2011, it was possible to save 309 antihydrogen atoms for 1,000 seconds ( about 16 minutes). The first measurement of a transition in antihydrogen was published in 2012 by the same group.

AEGIS (AD -6)

As mentioned above, there are various theoretical descriptions of quantum gravity, which does not exclude the possibility that anti-matter in the gravitational field of the earth might know a different gravitational acceleration as ordinary matter. To check this, the AEGIS collaboration was established. Currently, the experiment is still in the planning and preparation phase, the basic structure is, however, already fixed.

The test specimen has been chosen for antihydrogen. The reason for this lies in the electrical neutrality and relatively simple production of antihydrogen. Other experiments that used as a specimen charged antiparticles (eg antiprotons ) failed due to the pressure acting on it electric and magnetic forces due to weak fields that are omnipresent and are generated by traps. This is understandable, considering the electric Coulomb force FC with the gravitational force FG of two electrons compared.

Gravity is in this case 4.2 × 1042 times weaker than the electric force.

Principle of measurement

First you shoot positrons with kinetic energies of 100 eV to a few keV to a target which consists of a nano- porous, non -conductive solids. Nano- porous as used herein means that the pore size in the range from 0.3 to 30 nm. The incident positron is slowed down in the material very quickly and can enter into a bound state with an orbital electron from the insulator under certain circumstances; In this way, positronium. Since the dielectric constant in the pores is smaller than in the solid state, thus increasing the binding energy of positronium, collects this preferred in these open spaces. There positronium collides repeatedly against the wall and loses more and more kinetic energy until it finally as large as the thermal energy of the target material. By cooling the insulator so very cold and thus very slow positronium can be accumulated. Has the thermalized positronium can diffuse out of the insulator out. In this entire process, a large proportion of positron annihilation with is lost. However, it can be ensured by appropriate dimensioning of the positron flux for a sufficiently large number of thermal positronium. If at this point the positronium with the previously accumulated and cooled in a Penning trap antiprotons together, the antihydrogen is formed. However, this reaction has a very low probability, as in positronium in the ground state, the positron is very strongly linked to the electron. In order to reduce the binding energy to the positronium using lasers to stimulate high principal quantum numbers ranging from n = 30 ... 40. Figuratively speaking, the two particles thereby remove from each other and feel the mutual attraction less. In the case of highly excited states ( also referred to as Rydberg states), the probability of antihydrogen formation increases approximately with the fourth power of the principal quantum number n Thus, the formation equation is as follows:

The star means that the atom is in a Rydberg state.

Antihydrogen is electrically neutral and can leave the trap in any direction, including the direction of the Stark acceleration electrodes ( see picture). As for the measurement of an antihydrogen beam is required, you have to speed up the slow antihydrogen specifically in one direction, however, this is not possible with a homogeneous electric field due to the electric neutrality. However antihydrogen has an electric dipole moment and can be accelerated in an electrical gradient. This situation is comparable to the everyday experience that a water jet ( which is actually electrically neutral) can be deflected with a charged comb. So water is accelerated in the inhomogeneous electric field of the comb to comb out. Since this technique is related to the antihydrogen with the Stark effect it is also called Stark acceleration. The speed v that is to be achieved thereby will be approximately 400 m / s. To measure the acceleration of gravity g, is allowed a certain distance L to fly the jet. "Fall" in the time T = L / v is the antihydrogen atoms in the gravitational field of the earth. Thus, the anti- atoms result from a horizontal throw. During the fall of the beam by the distance Ax is deflected from the horizontal. Since the velocity v is very small, one can apply classical Newtonian mechanics and receives

By the displacement measure ∂ x can be therefore determine the gravitational acceleration g for antimatter. This happens when the AEGIS experiment with a position-sensitive detector moire. As a first goal for measurement accuracy is a measurement error of 1% was targeted.

Related Projects

With the Fermilab Antiproton Accumulator, the U.S. has an anti- proton storage ring. In him was made ​​in 1997 with the E862 experiment in a similar manner as in the PS210 experiment 66 antihydrogen atoms.

With the FAIR accelerator center is from around 2020, a similar system in Germany are also available. For this purpose, the existing accelerator facility at GSI is greatly expanded. This plant will indeed be in Germany, but is similar to the CERN created as an international project.