FPCCD Neutron Damage test at Tohoku CYRIC

From afar the ILC detectors are going to have a very similar look to the LHC detectors ATLAS and CMS. They are cylindrical objects, with different sub-detectors for measuring different properties of the high-energy particles that will be produced by the collisions of bunches of electrons and positrons. Closest to the interaction point in the center of the detector will be the vertex detector for the precise measurement of the decay point of long-lived particles. Next, a tracking detector measures the momenta of charged particles from their trajectories in a magnetic field. The calorimeters outside of the tracking detectors measure the energy of the particles.
Yet, at second glance, because of much cleaner environment of the ILC detectors, particularly the inner detectors look very different from their LHC counterparts. While for the LHC detectors the requirement of radiation hardness puts a serious constraint on the achievable precision, ILC vertex detectors hope to achieve about an order of magnitude better precision on momentum and impact parameter measurements. Nevertheless, radiation damage is also a concern for ILC detectors. The beams after the collision cannot be re-used for collisions. (But we can hopefully recover at least some of their energy.) The current plan is to safely dispose of them in a beam dump. Some of the neutrons produced there can travel back up the beam pipe and enter the detector. Their energy has been studied and roughly looks like this.

 

Expected Neutron energies from the ILC beam dump.
The first layer of a vertex detector at the ILC has to deal with
up to 10¹⁰ neutrons / cm² / year

Not a big problem compared to the factor of about 1,000 that the LHC detectors have to cope with, but nevertheless something we need to study. We have a facility at Tohoku University where we can get a neutron beam. The energy is close enough to what we would expect at the ILC.

Distribution of Neutron energies in the
Tohoku CYRIC facility. We still have to
evaluate our own measurements of
 neutron flux and energy.

After some intense preparation and frantic manufacture of beam profile monitor from scintillator bars, we now have pixel detectors with the equivalent of about 2.5 years equivalent of ILC neutron dose. Analysis of the data is underway, so I can't show anything yet. We are still not even done with the complete analysis of last year's data. Here are two plots to give you just a rough idea of what we are studying. I will try to go into more detail in some future blog posts.

Active area of the FPCCD chip. The red dot is a "hot pixel".
Similarly to a dead pixel in your LCD screen, this is bad.
In this case, we define a "hot pixel" as one that has a signal
significantly above the (blue) background.

We are measuring the number of pixels that was affected
by the neutron radiation. As you can see, measurements
at low temperatures are significantly less affected by the
damage, but the difference before and after irradiation is
obvious.