\title{Justification for laser system}

\author{Sho Uemura for HENP Team}

\date{December 11, 2017}

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The laser pulser system will help the MVTX development in two
ways. It will aid ongoing studies of the ALPIDE parameter characterization, and it is
essential for necessary tuning of the in-pixel analog frontend of the ALPIDE.
The laser pulser will be faster and safer than the radioactive source that is currently in use.

\section{General ALPIDE studies}

We are still in the process of understanding operation of the ALPIDE sensor -
this effort has been steady for the last few months and will continue for
months more. So far the test stand work has mostly used a strontium-90 source, and
to some extent cosmic rays and the ALPIDE internal pulser circuit. 
Even with collimation, the Sr-90
source illuminates a large number of pixels, and it is difficult to align the source to illuminate specific parts of the ALPIDE (or hit the ALPIDE at all).
We have had significant difficulty in understanding which of
the effects we observe are due to the source setup, and which are due to the
ALPIDE.

The laser pulser would greatly simplify these studies. It has the advantage
of being completely repeatable: the ALPIDE should respond the same way every time. The pulser should completely replace the
Sr-90 source, and replace cosmics for everything except development of the
multi-chip or multi-stave telescope. This has benefits in terms of time and
safety: the laser pulser will generate test data faster than either the Sr-90
source or cosmics, and there is a safety benefit to eliminating the Sr-90
source that we believe outweighs the hazard of the laser pulser.

\section{Tuning the analog frontend}

MVTX will have a substantially different trigger latency from the ALICE ITS.
ALICE ITS will be triggered from a fast trigger with a latency ~1.2 us. The
proposed sPHENIX trigger provides only an L1 trigger with latency 4 us.

The sPHENIX trigger scheme is still under discussion, but it may be necessary
to operate MVTX with a larger latency than ALICE ITS: this would require the
shaping time of the analog frontend to be significantly increased. ALICE has
not investigated extending their shaping time.

The ALPIDE has an internal pulsing circuit, but the amplitude range of this pulser is limited and it cannot replicate the full range of charge depositions seen from real particles.
This means that some external source is needed for shaping time characterization.
This source must be able to deliver a known amount of charge in every event, so the laser pulser is essential (cosmics and the Sr-90 source give a broad distribution of charges).

%The basic shaping time studies
%can be done with the ALPIDE's internal pulser, but some sort of real signal
%(Sr-90, cosmics, or laser pulser) is needed for full understanding - as
%explained above, the laser pulser is the preferred tool.

%The ALPIDE analog frontend is highly nonlinear. Changing the shaping time
%will impact all other performance parameters; in particular it should be
%expected that the relation between threshold and cluster size will change,
%which in turn affects position resolution. CERN has typically performed these
%studies with test beams, which is not an option easily available to us.
%Cosmics could be used, but the laser pulser is a faster and simpler way to
%make this measurement.

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