Series for Educators
A detector's chambers are crammed with intricate layers
of materials and electronics to detect and record the results
of billions of high-speed collisions between protons and antiprotons
at Fermilab's Tevatron particle accelerator
Detectors such as CDF and DZero at Fermilab
are complex systems, layers of detectors. This article explains
a few of the interior detectors and the need for upgrading to
keep up with the advancements of the accelerators.
As Fermilab prepares for the future in high-energy physics, the
Laboratory must maintain a certain synergy. If technology and
engineering advance in one field, other areas must progress in
parallel, as with the DZero and CDF experiments and their future
benefactor - the Main Injector.
The Main Injector, which will begin operating by 1999, will greatly
increase the luminosity of the Tevatron, resulting in many more
collisions per second at the detectors. Without upgrades, the
detectors at the two experiments would not be able to keep up
with the higher luminosity, and all those extra collisions would
be for naught.
"Detectors have to keep up with the accelerator to achieve
more precision in the physics measurements that we are making,"
said Ron Lipton, a DZero experimenter and head of his experiment's
silicon detector project.
These studies include "better measurements of the mass of
the top quark, [and] more accurate measurements of the intermediate
bosons, which are carriers of the electroweak force. All of those
things benefit from having higher luminosity. . ."
Because upgrading the detectors is a laborious and expensive task,
requiring great expertise in multiple fields, CDF and DZero are
cooperating on the project. When scientists complete the upgrades,
components in the two detectors will be different in detail but
similar in concept and development. Both experiments are using
a common facility for silicon detector construction, a common
engineering staff to design them, and a common electronics development
effort as well, according to Lipton.
"It is really important to understand that the two experiments
have worked together to build the infrastructure of the detectors,"
"The Laboratory just can't afford to build many independent,
similar devices. It is just too expensive in terms of dollars,
but mostly in terms of manpower, expertise and technical abilities.
And so I think the cooperation has worked out [fairly] well."
Jim Hylen, a CDF physicist and head of his experiment's scintillating
fiber project, agreed and said the sharing of knowledge has greatly
Of the many facets to the upgrades of the two experiments, some
of the most ambitious involve the two inner layers of detectors­p;the
silicon vertex tracker and scintillating fiber technology, according
CDF developed much of the technology for the silicon vertex tracking,
and DZero pioneered the development of the scintillating fiber
Silicon Vertex Tracker
A silicon vertex tracker, made possible by technology that grew
out of the microelectronics industry, consists of a piece of silicon
with microscopic detector strips imprinted directly on it. The
strips are sensitive to particles passing through them. Technicians
connect sets of 128 strips to an SVX chip with a wire thinner
than a human hair. Scientists and engineers from both Fermilab
and the Lawrence Berkeley Laboratory in California developed the
SVX computer chip.
As a particle passes through the detector, it knocks out electrons
that the microstrip collect; this is the 'signal' of the particle.
The signal then goes through the wire to the chip. The chip amplifies
the signal, then delays the charge. A "delay" stores
the event information for 32 collisions; this infinitesimal amount
of time gives the trigger hardware a chance to analyze the event
and decide if it should be kept. The trigger will generally keep
one interesting event out of approximately 1,000 total events.
If the event is worth keeping, the SVX chip turns that charge
into a digital number, and a computer converts the digitized information
to a readout that physicists can study.
When silicon vertex trackers were first used for experiments at
the Large Electron Positron collider at CERN, each separate silicon
detector had 40,000 - 50,000 "channels" for detection.
The detectors now being developed for CDF will have about 400,000
channels per detector, and the DZero version will have about 800,000
channels, allowing much more precise measurements than any seen
in the past.
"In the end, when you display a track coordinate, a single
piece of information about that track coordinate is contained
in that one channel of electronics. So, the more channels you
have, the more detail [you] can measure about the tracks,"
Scintilating Fiber Technology
The next layer of detectors after the silicon vertex trackers
is the scintillating fiber. Technicians weave fibers together
in every precise array of ribbons and then place the ribbons and
their mountings onto cylinders. When a particle passes through,
a brief flash of light occurs. That light travels down an optical
fiber, the scintillating fiber, similar to the kind used in telecommunications.
The light then hits a Visible Light Photon Counter. The VLPC changes
the flash of light into an electrical signal for a better reading
and then converts the signal to digitized information. A computer
translates that data into information physicists can study. Scientists
keep the VLPC very cold because it is sensitive to thermal "noise"
generated by heat.
"The scintillating fiber is not as fine grained as the silicon,
but more fine grained than the next layer of detectors in CDF,
the wire drift chambers," said Hylen.
Rockwell International Corp. developed VLPC technology for the
military, and Fermilab staff helped adapt it for the Laboratory.
A goal of the new detector technology will be to pinpoint the
"displaced vertex" of a particle. This technique enables
scientists to track a particle back to where if first decayed.
A particle often decays before it gets out of the vacuum pipe
and hits the first detectors. Physicists use the displaced vertex
to find out where the particle first decayed by "retracing"
its steps. CDF has used this technique in the past, but Dzero
did not previously have the technology to exploit it. With the
upgrades, both experiments will be able to use the displaced vertex
Along with the inner layers of the detectors, other components
of CDF and DZero will be getting upgrades. For example, scientists
in DZero are developing a new muon system to measure muons that
don't pass through the entire steel system. DZero and CDF are
also revamping the calorimeter electronics system and other electronics
components to keep up with the higher luminosity generated by
the Main Injector.
The next article, Reading
the Data at Fermilab
,will discuss the computer interfaces
to translate the information received by the detectors.