97chapter3 - PiBeta - University of Virginia

solesudaneseΠολεοδομικά Έργα

25 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

69 εμφανίσεις


Chapter 3


Experimental Technique



This chapter will discuss the experimental techniques used to acquire data
presented in this work. The accelerator facility at the Paul Scherrer institute (PSI) is
discussed including descriptions of the main components

of the accelerator and the
process by which it creates a pion beam. The detector design for the pibeta is also
discussed with each of the components described in some detail. Finally, the data
acquisition system composed of the electronics and computer sy
stem is discussed.

The goals of the 1996 and 1997 pibeta beam times were:




Collect data which would measure the detector response of the CsI crystals



Collect data which would measure the detector response of the plastic veto



Test the electronics and
the logic which they implemented



Mount the CsI crystals and plastic veto half
-
staves on one of the final pibeta
endcaps to test mechanical compatibility under in
-
beam conditions.



Make a measurement of the Panofsky ratio for pion decays.


37

For part of the

data taken in 1996 and 1997, the trigger electronics were set up to measure
the



e
+


decay rate. The analysis of those data is presented in Chapter 5. The
measurement of the Panofsky ratio is to be presented in the thesis of University of
Virginia st
udent Penny Slocum.


3.1 The Paul Scherrer Institute accelerator


The accelerator at the Paul Scherrer Institute (PSI) is a vital component of the
pibeta experiment. The PSI facility was chosen because of it the time structure of its
primary production bea
m. "Buckets" of beam particles are delivered every 20 ns at PSI as
opposed to 5 ns at the Los Alamos Meson Physics Facility (LAMPF) and 12 ns at the
Tri
-
Universities Meson Facility (TRIUMF). A longer period between beam buckets
makes it easier to distingu
ish beam pions from muon and positron contaminants.
Additionally, it is easier to reject events in which two adjacent beam buckets both contain
particles. This section will provide an overview of the accelerator and its components and
a description of how
the pion beam used by the pibeta experiment is produced.


3.1.1 Accelerator components


Figure 3.1 shows an overview of the accelerator facility at PSI. The accelerator
complex at PSI consists of four main parts: the pre
-
injector, the Injector II cyclotron
, the
ring cyclotron, and the Injector I cyclotron. Only the first three are of interest here. The
"Injector I" cyclotron was used from 1974 to 1984 but is now only used when a polarized
beam is required and is therefore unnecessary to the pion beta decay
experiment.


38


Figure 3.1: Overview of the PSI accelerator facility taken from the 1994 user's manual.




39




Figure 3.2: Schematic of a Cockroft
-
Walton type DC accelerator like that used at PSI as
a pre
-
injector. An alternating current with a peak to peak p
otential of 2V (where V is
some voltage) charges each of the capacitors (C1,C2,C3,C4,…) such that a large (870
kV) potential is achieved between the top and bottom of the diode
-
capacitor ladder.


3.1.1.1 Pre
-
Injector


The pre
-
injector initially accelerates

protons to an energy suitable for introduction
to the “Injector II” The pre
-
injector is a Cockroft
-
Walton DC accelerator which uses an
ion source to supply the "Injector II" cyclotron with 870 keV protons. The schematic
diagram shown in Figure 3.2 helps t
o illustrate the principles of how this accelerator
works. An alternating current with a peak
-
to
-
peak potential of 2
V

is supplied as indicated
in the diagram (where
V

represents some voltage). On the first half of the cycle, the first

40

diode is forward bias
ed and capacitor C1 is charged to +2
V
. On the second half of the
cycle, the bottom of C1 is at +
V

and the top is at +3
V
. The bottom of capacitor C2 is now
at
-
V

while the top is at +3
V

charging C2 to +4
V
. This continues up the diode
-
capacitor
ladder chargi
ng all other capacitors to +4
V

producing a very large, positive voltage at the
top of the last capacitor. This voltage is applied to a large, hollow conductor in which the
ion source resides. The field experienced by the proton as it emerges from the condu
ctor
accelerates it to 870 keV.


3.1.1.2 Injector II


Built in the early 1980's at PSI, Injector II accelerates protons from the pre
-
injector to a fixed energy of 72 MeV before sending them to the ring accelerator
described below. Injector II is a separate
d sector cyclotron. It consists of four large
magnets and two RF
-
cavities. Figure 3.3 shows a photograph of the Injector II cyclotron.
The vacuum pipe which carries the proton beam from the pre
-
injector (not shown) to the
center of the cyclotron can be see
n in Figure 3.3.

As the protons gain energy, the radius of the orbit increases for the protons until
reaching the extraction point after about 100 revolutions. By using the injector cyclotron
to feed the ring accelerator, much greater extraction efficien
cy is achieved. Prior to the
construction of the PSI accelerator in the early 1970's (then called SIN), typical extraction
efficiencies were on the order of 50
-
70%. This was due to overlapping particle orbits at
the outer edge of the cyclotron where the pa
rticles are extracted. By using this method of
stages, greater separation of orbits is achieved at the extraction radius of the ring
accelerator, increasing the extraction efficiency to greater than 90%.


41




Figure 3.3: Injector II. The 870 keV proton beam

enters from the top of the
picture and exits to the right after being accelerated to 72 MeV.


3.1.1.3 Ring accelerator

The ring accelerator at PSI, shown in Figure 3.4, accelerates the protons in the beam
from 72 MeV to 590 MeV. Built at PSI, the ring acc
elerator was first commissioned in
early 1974. The ring

accelerator is an azimuthally varying field (AVF) cyclotron
producing a fixed energy 590 MeV proton beam. The ring consists of eight magnets
pitched at 33 degrees with respect to a radial. The ring ac
celerator is capable of producing
a 1.5 mA proton beam. Acceleration is achieved via four RF cavities. The RF cavities

42

operate at a frequency of 50 MHz and each exposes the protons to a 600 kV field across a
20 cm gap.

43



Figure 3.4: Ring accelerator. The
fields of the eight large magnets keep the protons
orbiting through the cyclotron while the four RF cavities increase the energy of the
protons by a total of 2.4 MeV per rotation.


With an energy gain of 2.4 MeV per revolution, the protons make around 220
revolutions
in the ring before extraction. The protons are injected at a radius of 2.1 m where they

44

experience a magnetic field of 1.5 T. They

are extracted at a radius of 4.5 m where the
field is 2.1 T.


3.2 The Paul Scherrer Institute beam lines


The bea
m lines at PSI are responsible for carrying the proton beam from the ring
accelerator to the production targets and experimental areas. Figure 3.5 shows the beam
lines in the PSI experimental hall. The proton beam is split into three beams two of which
lea
d to a production targets in different sections of the experimental hall. Users are given
the responsibility of controlling the magnets in the beamline after the production target in
order to achieve the beam tune they desire.


3.2.1 Production targets


Th
e pion beta decay experiment will be carried out in the same

E
1 experimental
area as the 1997 test runs. The

E
1 area is fed by production target E. Target E consists of
a thick graphite wheel which is rotated on an axis parallel to the incident proton beam.
Rotation is necessary so as not to overheat the target by

continually exposing a single area
to the proton beam. Using a rotating target also reduces the frequency with which the
targets must be replaced. Targets E and M are called production targets because they
produce a wide variety of subatomic particles as
a result of being struck by the proton
beam. Figure 3.6 is a photograph of one of the PSI production targets. Among the
particles produced are large fluxes of pions. This capacity for producing large current

45

pion beams has earned PSI (along with accelerato
rs such as LAMPF and TRIUMF) the
name



Figure 3.5: Schematic of the PSI Beamlines. Target M produces
particles for the

M1 and

M3 experimental areas while target E
produces particles for the

E1,

E3,

E5,

E1, and

E4
experimental
areas.



46



Figure
3.6: Production target at PSI. Muons and electrons
produced at the production target along with muons and
electrons produced from in
-
flight pion decays contaminate
the pion beam and must be dealt with in any experimental
setup.


"meson factory". Muons, ele
ctrons, and their anti
-
particles are also produced by the
production target.


3.2.2 The

E
1
beam line


The

E
1 beam line consists of nine quadrupole and two dipole magnets used to
select the momentum of the particles transported to the experimental area. In addition,
there exist vertical and horizontal beam collimators as well as two c
arbon degraders, all
of which can be controlled remotely. For the summer 1996 beam time, there were three
additional quadrupoles and a large dipole in the

E
1 area. It was quickly determined that

47

the dipole magnet spread the beam spatially and extended the

focal point of the last
quadrupole enough to significantly spread the beam momentum. The decrease in beam
quality during data runs due to the effects of the field of the dipole magnet in 1996 caused
the omission of this magnet during data runs in 1997.

Th
e facility is capable of producing up to 2.4


10
8

pions per second and
momenta of up to 200 MeV/c. For the 1996 and 1997 beam times, a pion momentum of
116 MeV/c was used. A large pion momentum means fewer pions will decay in
-
flight due
to kinematics and relativity. The beam line collimators were us
ed to reduce the pion stop
rate such that the detector saw good events at a rate slightly higher than the data
acquisition rate (~100 Hz). This minimized the number of accidental coincidences
without compromising our rate of collection.


The beam line mag
nets at PSI are under user control. A computer program called
TRANSPORT
, written at PSI, allows users to determine optimal magnet settings for a
given momentum tune and particle type. This program requires some experience to use
properly. Pibeta beam times

typically start out using beam tunes from past experiments
then incorporate the use of another computer program called
OPTIMA
. This program uses
a configuration file to change the setting for each magnet in the beam line one at a time.
OPTIMUM
checks the
rate of a user
-
supplied signal to determine the optimal magnet
settings for maximum beam rate. The user
-
supplied signal can be as simple as a single
scintillator/phototube detector placed in the beam, or a more complicated logic made
from fast electronics.

Pibeta beam times usually used a 2


2 mm
2

"pill" counter
positioned at the target center to determine a beam tune focused at that point.


48


3.3 Experimental Apparatus


The experimental detectors, electronics, and various support structures to be used
in th
e pibeta experiment are discussed here. Most of these components are already on site
at PSI and many have been used during in
-
beam tests of the equipment including the
1997 measurement of the

+



e
+


(

) branching ratio.


3.3.1 The pion beta decay project

detector


The design of the pion beta decay detector involves several components illustrated
in Figure 3.7. Many of these components were used in the 1996 and 1997 beam times at
PSI. The detector, when completed, will cover nearly 80% of

sr as seen b
y the center
of the target. For the summer of 1996, an array of 40 CsI crystals covered ~ 13% of

sr
however, the presence of 4 “dummy” crystals (plastic pieces shaped like the crystals used
for mechanical support) reduced the usable area of this array s
ignificantly. By 1997, the
“dummy” crystals had been replaced by CsI crystals giving a coverage of > 14% of

sr.


3.3.2 Targets


Two different targets will be used during the acquisition of data for the pion beta
decay experiment. The first is depicted i
n Figure 3.8 and is intended for measuring
stopping distributions of the pions (i.e. how stopped pions are distributed throughout the
target) as well as providing some tracking ability for decay particles leaving the target.
There are 69 3

3 mm
2

scintill
ating fibers each mated with a fiber optic line leading to a

49

phototube outside the detector on the downstream side. These 69 pieces are enclosed by
eight additional larger scintillator pieces formed so as to give the entire target package a
cylindrical sha
pe.

The target constructed and used in the 1994 beam time will not be used for pion
beta decay data runs because the granularity makes accurate energy reconstruction



Figure 3.7: Diagram of the pion beta decay detector [Brö 96]. 1) Central beam
trajecto
ry 2) vacuum pipe 3) active degrader 4) active target 5) fiber optic light guides

50

6) cylindrical MWPC's 7) segmented plastic veto detector 8) 1" PMTs 9)CsI shower
vetos 10) CsI shower calorimeter 11) 3'' PMTs





Figure 3.8: The granular target
constructed in 1994 and used in the 1994 beam time.


difficult for the escaping positron and therefore reduces the overall resolution of the
detector.

The second target is shaped identically to the first except that it is a solid piece of
plastic scintilla
tor. This target was used in the 1996 and 1997 beam times and is
discussed in more detail below. An schematic view of the array showing the location of
that target is shown in Figure 3.9.


3.3.3 Multi
-
wire proportional chambers


51


The pibeta detector will in
corporate two cylindrically shaped multi
-
wire
proportional chambers (MWPCs) to obtain accurate position information for particles
passing between the target and the CsI calorimeter. This will be most useful in identifying


Figure 3.9: CsI crystals, half
-
length plastic veto, target, degrader, target veto from 1996
GEANT simulation


accidental coincidences occurring when two muons decay simultaneously in the target.
These coincidences might otherwise be misinterpreted as



e


events which would
compromis
e the accuracy of the normalization used in the pion beta decay measurement.
The wires are located at a radius of 60.15mm (120.1mm) for the inner (outer) chamber
from the beam axis. They both have wire spacings of 1.96 mm and have a resolution of
0.2 mm.


52


3.3.4 Detector components used in the 1996 and 1997 beam times


The 1996 and 1997 pibeta beam times incorporated similar detector and beamline
components. The two most significant changes from 1996 were:



The replacement of four "dummy" crystals with rea
l ones



The removal of a large dipole magnet from the

E area (discussed in section 3.2.2)



The following is a list of detector components used in the 1997 beam time. Each
is discussed below.



Platform



Beam counter and collimator



Multi
-
wire proport
ional chambers



Thermal house



Degrader and target



Plastic veto array



CsI crystal array


Figure 3.10 shows a diagram of how most of these components were positioned during
the 1997 beam time.


3.3.4.1 Platform


Most of the apparatus was mounted on a
base structure made of steel
girders. Figure 3.11 shows an overhead view of this structure indicating the placement of

53

the attached elements. Mounting much of the experimental apparatus rigidly on this
structure made it possible to connect many of the wire
s and to position many of the
detector components prior to gaining access to the

E1 area. The structure contained the


54


Figure 3.10: Arrangement of the detector elements used in the 1997 beam time.



55

H
i
g
h

V
o
l
t
a
g
e

S
u
p
p
l
i
e
s
A
i
r

C
o
n
d
i
t
i
o
n
e
r
D
e
l
a
y

C
a
b
l
e
s
T
h
e
r
m
a
l
h
o
u
s
e
E
l
e
c
t
r
o
n
i
c
s

R
a
c
k
s
E
l
e
c
t
r
o
n
i
c
s

R
a
c
k
s
E
l
e
c
t
r
o
n
i
c
s

R
a
c
k
s
E
l
e
c
t
r
o
n
i
c
s

R
a
c
k
s
C
o
n
c
r
e
t
e

B
l
o
c
k
s


Figure 3.11: The 1996 and 1997 experimental apparatu
s. Elements are supported by a
structure of steel girders from underneath. Cables were run from the detectors inside the
thermal house under the concrete blocks and between girders to the electronics racks.


electronics racks, thermal house and swing, dela
y cables, air conditioner, and high voltage
power supplies. It weighed over 5 metric tons, and was moved into the area using a large
crane which spanned the experimental hall. Large concrete blocks were then placed on
the girders between the electronics ra
cks and the detectors so as to provide shielded
access to the electronics while the beam was on.


3.3.4.2 Beam counter and collimator


Just after entering the

E1 area, the particle beam was collimated using a lead
brick with a 1.5 cm diameter hole in it i
n the beam direction. Just on the downstream side

56

of the collimator was a 1mm thick plastic scintillator detector. This was called
B0
and

was used for detecting beam particles as they entered the

E1 area
.



3.3.4.3 Wire chambers


Two multi
-
wire proportio
nal chambers were positioned between a triplet of
quadrupole magnets (QSK51,QSL55, and QSK52) and the thermal house. Unlike the
MWPCs built for the pion beta decay detector, the chambers used in 1996/1997 were flat
and square. Each of the MWPCs contained
both an X and a Y plane. All planes had a
wire spacing of 1mm. The MWPCs were read out using a LeCroy Research Systems
(LRS) PCOS III system, the same system that will be used to read out the cylindrical
MWPCs to be used in the full pion beta decay detecto
r. The two MWPCs were placed
with their centers 507 mm apart and normal to the beam. These were useful only for beam
profiling.


3.3.4.2 Thermal house


The thermal house (TH) was constructed out of wood and dense, thick Styrofoam.
The shelter completely en
closed the swing, CsI crystals, plastic veto, and target/degrader
package. The TH served to provide a temperature and humidity regulated environment for
the CsI crystals improving their behavior and stability. Using heaters inside the TH, a
control system
was able to keep the temperature there stable (±1° C) at a few degrees
above 0° C. The temperature regulator was designed and built at the University of
Virginia and incorporated circuits that would switch on and off the heaters as the 50 Hz
AC passed thr
ough zero volts. This helped to reduce the amount of noise introduced into

57

the signal electronics by the heaters switching on and off. The TH sat on a large, rotating
platform such that the focal point of the CsI crystals was on the axis of rotation. The T
H
and all its contents could thus be rotated >30 degrees to either beam left or beam right.
Markings on the rotation mechanism allowed the angle at which the TH was positioned
(relative to the beam) to be read easily and accurately.


3.3.4.5 Target and deg
rader


Two cylindrical plastic scintillator detectors were used to stop pions in the beam
near the focal point of the CsI array allowing them to decay in the rest frame of the lab. A
diagram of these pieces is shown in Figure 3.12. The upstream detector (c
alled
D0

)was
35 mm long with a radius of 20 mm and was used to slow the pions such that they would
stop at the center of the target. The downstream detector, called T0, was 50 mm long with
a radius of 20 mm and was the target in which the pions were stop
ped. The
D0

and
T0

detectors were individually optically coupled to small, surface
-
mounted Hamamatsu
photosensors and sealed from light leaks. They were mounted on a stand at the focal point
of the CsI array.





58

Figure 3.12: Target and degrader. The pio
ns, entering from the right, are slowed by the
degrader so as to stop near the center of the target.

3.3.4.6 Plastic veto array


The main uses of the plastic veto are (1) to distinguish between the lighter,
minimum ionizing particles (positrons and electro
ns) and the heavier charged particles
(pions and muons) going from the target to the CsI calorimeter, and (2) to distinguish
between different particle types as illustrated in Figure 3.13. For the 1996/1997 runs, the
plastic veto array consisted of eight l
ong, thin scintillator staves positioned between the
target and CsI crystals. The pieces were arranged to form a partial cylinder whose center
coincided with the beam line. The preparation and properties of the PV detector is
discussed in detail in chapter

4. For the 1997 beam time, half
-
sized staves were used.


3.3.4.7 CsI crystals


An array of 44 pure CsI crystals was used in the 1997 beam time. For the final
pion beta decay experiment, an array of 240 crystals will be used. The crystals fit together
so a
s to form a spherical shape which will cover nearly eighty percent of
4


as seen by a
particle at the sphere's center. The sphere will be open on two opposing sides for the
beam entrance upstream and downstream for access and wiring for the inner detectors
.

Pure CsI was chosen because of its fast recovery time (the fast component of the
CsI pulse is on the order of 30 ns wide so the crystal can accept a high rate of events
without overlapping signals). This is needed to handle the high rates necessary to c
ollect
the necessary statistics in a reasonable amount of time (1 year of beam time). There are
crystals of six different shapes: (a) Pentagon, (b) HexA, (c) HexB, (d) HexC (e) HexD,

59

(f) HexD2 and (g) Veto . The pentagons are the only regularly shaped cry
stals. Due to the
precision desired for the pion beta decay experiment and the exceptionally small
branching



Figure 3.13: Plot made from one of the plastic veto half
-
staves during the 1997 beam
time. The narrow peak to the left is made from the minimu
m ionizing positrons. The
broad peak to the right is from heavier, slower moving particles (e.g. pions being
scattered elastically off a nucleus).


ratio for pion beta decay, a large degree of modularity was required for the calorimeter.
The final design w
as achieved by a geodesic breakdown of an isocahedron [Ass 95].


3.4 Data acquisition system


60


The data acquisition system for the pibeta experiment has been designed using
fast electronics and custom acquisition software. A high data rate is critical to m
ake the
experiment a success using the allotted beam time. Sophisticated logic has been
hardwired into the electronics as described in section 3.4.1. Section 3.4.2 discusses the
data acquisiton system designed specifically for the pibeta experiment.


3.4.
1 Trigger electronics


The triggering scheme for the pion beta decay experiment has been developed, as
with most experiments, to maximize acceptance while minimizing background. It is
particularly important here because of the low uncertainty desired (<0.5
%). Previous test
runs have shown areas where improvement could be made on the original design. In
particular, the summer 1996 beam time revealed timing problems with the LeCroy
Research Systems (LRS) 2373 Memory Lookup Unit (MLU) modules used in the final

stages of the trigger electronics. In fact, the somewhat poor quality data taken in 1996 can
be attributed to the identification of this problem late in the beam time. The 1996 beam
time can be considered a success for this discovery alone for it led to m
uch improved
quality data in 1997.


3.4.1.1 Overview


The pion beta decay experiment requires a trigger logic capable of distinguishing

+




0

e
+



events from the

+



e
+



events which will be used for normalization. The
signature of a

+




0

e
+



event consists of observing the back to back ~70 MeV

61

photons from the

0

decay. This will be done using a three stage process: (1) formation

of
"clusters", (2) formation of "superclusters", and (3) logically combining the
"superclusters" and other signals from beam line detectors to create the trigger.


Clusters

are combinations of several adjacent CsI crystals. GEANT simulations
have shown c
lusters of about 7 crystals to be optimal in efficiency for determining
showers above 55 MeV [Brö 96]. In the final detector, 60 overlapping clusters will be
used. Clusters are formed using the UVa 125 summer/discriminator NIM style modules
designed specif
ically for this task. The UVa 125 adds the voltages from up to 9 inputs.
Since the modules do no integration of signals, it becomes imperative to have the input
signals timed with one another as accurately as possible. The UVa 125 discriminates on
the summ
ed signal using two externally supplied thresholds.


Ten different
superclusters

are defined, each having two logic signals
corresponding to the higher and lower discriminator thresholds in the UVa 125. The two
discriminator outputs from a single cluster
are input into two different LRS 4564 OR
logic units. Signals from six different clusters are OR'ed together to create a supercluster
signal. Two diametrically opposed supercluster signals must occur in a small time range
(~20 ns) in order for a pion beta
decay trigger to fire.


Each of the CsI crystals (except the few odd
-
shaped half
-
D and veto crystals
surrounding the endcaps) is viewed by a 3" PhotoMultiplier Tube (PMT) manufactured
by EMI Technologies (UK). The tubes are powered by bases designed and bu
ilt at the
University of Virginia. Each PMT signal is split at the base into two separate lines. One is
delayed through about 75 m (385 ns) of PK
-
50
-
2
-
16

cable and will be referred to as the
"analog leg". The PK
-
50
-
2
-
16 cable was manufactured in Siberia an
d was used because it

62

was considerably less expensive than RG
-
58. The second line from the base feeds what
will be referred to as the "trigger leg".


3.4.1.2 Analog leg


After the delay cable, the PMT signal is split again via a University of Virginia
UVa
122 splitter. One half of the signal goes to a discriminator whose output provides the
stop pulses for the LRS 1877 FASTBUS time to digital converters (TDC) as shown in
Figure 3.14. The other half goes to an LRS 1882F FASTBUS analog to digital converter
(A
DC). The TDC start and ADC gate pulses come from the trigger leg and are created
during the time the analog leg signals spend in the 385ns delay cables.


3.4.1.3 Trigger leg


Figure 3.15 shows a schematic of the trigger leg of the pibeta electronics. The
t
rigger leg PMT signals are split via a UVa 126 1:3 splitter. The outputs of the splitter are
sent to various UVa 125 summer/discriminator modules which form "clusters". Each
crystal can contribute to the formation of up to three different clusters. Any unu
sed splits
are terminated to ensure all the crystals of a given cluster will contribute equally. The
clusters are formed in the UVa 125 modules as described above. The two externally
supplied discriminator threshold voltages for the UVa 125 modules are dri
ven by two DA
500 CAMAC modules. The DA 500 is a digital to analog converter whose DC output
voltage can be set via CAMAC. The programming of the UVa 125 discriminator
thresholds is integrated into the online data acquisition and control systems.


63

Once the
cluster signals are formed, they are sent to an LRS 4564 OR logic
CAMAC module where the superclusters are formed and logic is performed to output
four signals.These four signals are

HI
,

LO,

e

HI

and

e

LO
. Since a partial array of
only 44

64


Figure 3.14: Analog leg of the data acquisition electronics. Signals from all the detectors
are split so that one part is digitized while the other part is used to create logic signals
used
by time to digital converters and scalers.



65

cluster07:
C00 C01 C02
C09 C10 C11
C19 C20
cluster13:
C05 C06 C14
C15 C16 C24
C25
cluster12:
C04 C12 C13
C14 C15 C22
C23 C24 C31
cluster08:
C19 C20 C21
C28 C29 C35
cluster17:
C03 C04 C05
C13 C14
cluster16:
C02 C03 C04
C11 C12 C13
C20 C21 C22
cluster15:
C21 C22 C23
C29 C30 C31
C32 C36 C37
cluster14:
C23 C24 C25
C26 C31 C32
C33 C37 C38

EMPTY

EMPTY
cluster22:
C06 C07 C08
C16 C17 C18
C25 C26 C27
cluster21:
C26 C27 C33
C34 C39
low
high
low
high
low
high
LRS 2365
LRS 4564
LRS 4518
LRS 4516
SIN I O 506
LRS 4448
LRS 4564
SCHI
SCLO
tr igger
These two modules
allow maski ng of
tr igger types coming
fr om the LRS 2365.
This modul e reads
out the bit patter n of
val id tr igger types
for each event.
UVa 126 1 to 3 splitters
C00
C01
From
beamli ne
detectors
UVa 126 summer/di scr. modules
SIN DA 500
SIN DA 500
UT
LT
Upper and lower discrimint aor t hresholds
daisy chained t o all UVa 126 modules.


Figure 3.15: Trigger leg of the data acquisition electronics. One of the outputs from each
phototube is split into four signals. Clusters are formed from analog additions of these
signals while superclusters a
re formed from the logical ORing of cluster signals.


66

CsI crystals were used during the 1997 beam time, only one supercluster was available.
Two supercluster signals
SC0
HI

and SC0
LO

were used in place of

HI

and

LO

while
the

e

HI

and

e

LO

inputs were left unconnected.

The
SC0
HI

and SC0
LO

signals were sent through an LRS 4518 programmable
delay unit along with several signals generated by beam line detectors that are described
below. The LRS 4518 can
be used to adjust the timing of each of its inputs before the
signals are passed on to an LRS 2365 Octal Logic Matrix module. The LRS 2365 is a
programmable logic module

that will generate up to sixteen outputs, each made from a
different logical combinati
on of the sixteen inputs. A diagram showing the inputs and
outputs of the LRS2365 used by the pibeta experiment can be seen in Figure 3.16. For the
1997 beam time, only eight of the outputs were used, each representing a trigger for a
different event type.


The LRS 2365 output is sent to the input of an LRS 4516 logic unit where they are
logically ANDed with eight signals from an IO 506 input/output CAMAC module. By
writing a logical "false" to certain lines of the IO 506, certain trigger types can be "turn
ed
off" via the computer. A copy of the outputs of the LRS 4516 are eventually sent to an
LRS 4448 coincidence register.

The LRS 4448 coincidence register will latch the input lines during an externally
supplied gate. This allows the bit pattern indicating

which trigger types were valid for a
specific event to be read out and recorded with the event. This feature has several very
useful benefits. Besides making the experiment easier to maintain, this feature allows the
accumulation of data of different type
s evenly over time. This is particularly beneficial

67


Figure 3.16: Inputs and outputs of the LRS 2365 module at the heart of the trigger
electronics.


when using events of one trigger type to calibrate events of another trigger type. In the
case of
gain m
atching, the use of alternate "gain matching" and "pion decay" runs would
mean the appropriate CsI gain matching factors would be accurately known only during
times when no pion decay data were being taken. This can mean significant
improvements in overall

detector resolution if the gains tend to drift over time. Gain drifts
in the 1997 data are discussed in the section on data analysis.


68

Throughout the design of the trigger logic for the pion beta decay experiment,
emphasis has been made on making the syste
m configurable via computer. This means
few, if any , changes should ever need to be made to the wiring once the initial setup is
complete. Additionally, the trigger conditions will be stored automatically with each run
reducing the chance for human error
.



3.4.1.4 Beam line detectors

Several detectors in the beam line were used to generate inputs to the LRS 2365 logic
module discussed above. Specifically, these were the upstream detector B0, the

active
degrader D0 and the active target T0. The signals fr
om these detectors were combined in
several ways in order to create triggers. The most important of these were:




stop

(
B
D
T
HI
0
0
0


). This signal is created when a pion is stopped in the target
(T0). Created from the logical AND of B0, D0, and T0
HI
. The T0 detector
determined
the timing and was cut on a high threshold so as not to trigger on decay products in
the target



D

G
The delayed pion gate is generated by the

stop signal but delayed by 10 ns to
allow prompt events, such as pion scattering, to occur before loo
king for signals in the
calorimeter. This pulse was 75 ns wide. A supercluster signal in coincidence with this
signal will trigger the digitization electronics to begin recording values for all of the
detectors



D

G'

The delayed pion gate prime is generate
d by the D

G signal but delayed to
start ~100ns after D

G starts. The D

G’ captures events in which the calorimeter is

69

hit long after most pions have decayed and so provides a good measurement of the
Michel (i.e.



+



background. A supercluster signa
l in coincidence with this
signal will trigger the digitization electronics to begin recording values for all of the
detectors.


A timing diagram showing the relation of these signals can be seen in Figure 3.17.


3.4.2 Data acquisition software


The data
acquisition software used by the pibeta changed in 1997 from HIX
(Heterogeneous Information Exchange) to MIDAS (Maximum Integrated Data
Acquisition System). Both of these packages were written by Stefan Ritt, a collaborator in
the pion beta decay experimen
t. The MIDAS system is based, primarily, on Intel x86
class CPUs communicating via Ethernet
-
TCP/IP. MIDAS has been successfully used in
several experiments at PSI and TRIUMF. The MIDAS system has been designed with
multiple layers to allow for easy compila
tion on most any platform. The system has a
Remote Procedure Call system and database structures designed specifically for the pibeta

experiment optimizing on speed. During the 1997 beam time, the setup consisted of three
PCs running the Windows NT operati
ng system. Two of these were "frontends" while the
third was the "backend".


One of the most useful features of the MIDAS system is the incorporation of its
own analyzer software. The analyzer allows users to define their own software "modules"
to manipul
ate or create new data banks. The analyzer program can be run online or offline
making it particularly simple to reproduce results identical to those seen online. It also

70

10ns
0ns
108ns
85ns
SC
0
HI
SC
0
LO

stop
D

G
D


G

Figure 3.17: Trigger timing for several signals created and
used in the electronic

trigger logic.


allows easy implementation of offline successes online during the next beam time.
Another very useful feature of the analyzer was its ability to keep the last 10,000 events in
a global section of memory while being run online. This memory
was accessible by the
CERN software package PAW (Physics Analysis Workstation) making many aspects of
the data observable online. The backend computer was stationed upstairs in the counting
house.


3.4.2.1 Trigger frontend computer


This computer was respo
nsible for reading events from the FASTBUS crate and
sending them over the network to the backend computer. This computer was stationed in
the electronics racks just outside of the

E1 area and ran in "DOS" mode, eliminating any
overhead in maintaining a g
raphical environment.



71


3.4.2.2 Slow controls frontend computer


This computer was responsible for : (1) communicating with the two LRS 1440
HV power supplies, (2) communicating changes in the trigger configuration to the
appropriate CAMAC modules, (3) mon
itoring temperatures from probes in and around
the detector apparatus, and (4) monitoring the gas flow of the MWPCs. The slow controls
computer was also stationed in the electronics racks just outside of the

E1 area.


3.4.2.3 Backend computer


This computer was a 133Mhz Pentium and was the most powerful of the three
MIDAS computers. The backend computer was responsible for:



Holding the online database in shared memory



Writing data to hard disk and two DLT

tape drives



Running the analyzer program to allow online analysis of data as it came in.

A total rate of >75 events per second was achieved for pion decay data.