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HELEN Program
Final report
Grant-holder: Daniel de Miranda Silveira (UFRJ – Brazil)
Grant: Advanced Training (AT) fellowship
Experiment: ALPHA/AD-5 (Antihydrogen Laser Physics Apparatus)
Laboratory: CERN
Period: 20/03/2006 – 20/09/2006 (6 months)
Brief introduction:
The ALPHA experiment aims to perform a high precision test of CPT symmetry, by doing
a spectroscopic comparison between Hydrogen (H) and antiHydrogen (Hbar). This will be
accomplished through high resolution laser spectroscopy of cold (T < 1 K), magnetically trapped
Hbar atoms. The collaboration includes many former members of ATHENA, the experiment
which pioneered the formation of cold Hbar atoms, and retains most of the expertise needed for
Hbar production and detection, as well as some of the key parts of the apparatus, such as the
positron accumulator and the laser system.
In 2006, ALPHA is having its first experimental run, after the AD (Antiproton Decelerator)
- the machine that provides the low energy antiprotons for the antihydrogen experiments experienced a two-year shutdown. For this run, the goal is to produce a good amount of Hbar
atoms, cold enough to allow them to be confined by a magnetic trap. My proposed activities
program comprised a 3-month period to be spent in the comissioning of the experimental
apparatus and the last 3 months of my stay at CERN would be devoted to actually running the
experiment
Activity report:
As stated in the working program submitted to the HELEN program, I was initially
involved in the assembly and commissioning of the apparatus. I soon became responsible for
the vacuum systems for the experiment: chambers, gauges, valves, pumps, residual gas
analysers, leak detectors, feedthroughs and cables.
Our apparatus has 3 independent vacuum systems, namely the AD vacuum, the trap
vacuum and the isolation vacuum. AD vacuum is the vacuum space adjacent to the AD
machine which, upon opening of a vacuum valve (AD valve), delivers the 5 MeV antiproton
(pbar) beam which is trapped in the catching trap. Inside this vacuum system, there is a
rotatable degrader (to reduce the energy of the pbar beam) and the beam counter (a positionsensitive silicon detector used to measure the pbar beam intensity and position). It is a high
vacuum system, but its vacuum requirements aren´t very strict: we just have to keep its
pressure below a threshold value (5 x 10-6 mbar), above which the AD valve closes to preserve
the AD performance. The pump down is provided by a 1000 l/s turbo pump, while the high
vacuum operation is ensured by a 300 l/s ion pump. The catching trap cables reach the external
space through the AD vacuum, so the chamber with the catching trap feedthroughs has its own
vacuum system (an 8 l/s ion pump), because it is connected to the pumps through a lowconductance line.
The second (and most important) vacuum system is the trap vacuum. It is separated from
the AD vacuum by a Titanium vacuum window, which has the additional function of performing
the final degrading of the pbar beam. In this vacuum space, we store, cool and manipulate the
cold pbar and positron plasmas from which we produce Hbar. Trapped particle lifetimes depend
critically on the pressure in this part of the apparatus: a vacuum lower than 10 -10 mbar is
desirable. To reach this value, we performed a very judicious choice of materials, together with
good cleaning and handling procedures. After a several day bakeout we reached the 10-9 mbar
range and, upon cooling the cryostat which surrounds the trap space to 4 K, we reached the 10 10
mbar range. The pumping system consists of a 360 l/s turbo pump and a 150 l/s ion pump.
The last vacuum system (which was the last one to be assembled) is the isolation
vacuum. The purpose of this vacuum space is to thermally isolate the helium vessel (at 4 K)
inside the cryostat from the 300 K outer walls. There is no need for extremely low pressures,
since below a certain pressure the thermal conductivity of all gases is independent of the
pressure: a vacuum between 10-4 and 10-6 mbar is desirable. The cryostat is being pumped by a
mechanical pump with a base pressure of 5 x 10-4 mbar with good results, but when the cryostat
is cold, as soon as the pumping is interrupted the pressure rises and the cryostat boil-off
increases. This indicates that there is a leak in the isolation vacuum, but so far we were not able
to find and fix it.
Beside the vacuum systems, I was also involved in the trap and magnets assembly, as
well as the cryostat comissioning, testing and operation.
The start of the data taking period was delayed by a serious problem with the PS, the
accelerator which supplies the pbar beam for the AD. The AD Physics program, which was
scheduled to start on June 6th. was transfered to July 17th.. After the PS was back in operation,
the AD experienced some technical problems, being unable to deliver the pbar beam to our
apparatus. This was fixed by the AD team, and the 2006 run officially started on August 28 th..
Initially some time was spent in steering and focusing the beam into our apparatus, with special
attention to beam focusing in the presence of our new 3 T magnet. We proceeded by doing
some degrading studies, tuning the rotatable degrader to maximize the number of pbars. After
the trap was at 4 K (and the vacuum was in the 10 -10 mbar range) we moved forward quickly:
we trapped pbars in the catching trap, injected electrons into the catching trap and observed the
electron cooling of trapped antiprotons. The day I left CERN, we were optimizing the pbar
stacking, measuring which number of AD shots could be taken without saturating the trapped
pbar number. The collaboration now proceeds with the aim of producing and trapping
antihydrogen during this year´s run, which was extended until November 20th..
Commentaries:
This was a very important period for my scientific training. I was involved in all aspects of
a large international scientific collaboration: planning the experiment, designing the apparatus,
integrating all the systems (vacuum, cryogenics, electronics, data acquisition), running the
experiment and analyzing the data. Unfortunately, due to some technical problems external to
the collaboration (the PS and AD problems), the run was delayed and I couldn´t be there for the
whole run. Despite that, I am really sure that I learned a lot and I also contributed to the
collaboration with my experience.
I continue to be a member of the ALPHA collaboration and I plan to return to CERN for
the next runs of the experiment.