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Ph.D. defensePh.D. defense
The measurement of the Lorentz angle The measurement of the Lorentz angle in the BTeV pixel detectors: in the BTeV pixel detectors:
the new PCI based DAQ, the setup and the resultsthe new PCI based DAQ, the setup and the results
Lorenzo UpleggerLorenzo Uplegger
Milano 26/01/2004
BTeV main goalsBTeV main goals
• Study of the beauty baryons
• Research of new phenomena beyond the Standard Model
• Definitively the measurement of the elements of the Cabibbo-Kobayashi-Maskawa matrix
The BTeV experiment will investigate one of the most fundamental problems of elementary particle physic, the CP violation. Some of the most important aspects of this kind of physics are:
• CP violation in b and c quark sector
• Measurement of the mixing phenomena of the B0s meson
BTeV main goalsBTeV detector layoutBTeV detector layout
•Pixel dimensions 50 m x 400 m •Plane dimensions 10 cm x 10 cm •Gap between stations 4.25 cm•Total number of stations 30•Total number of planes 60
Pixel vertex detectorPixel vertex detector
• The pixel detector will operate in a high magnetic field, so the position reconstructed by the pixels is shifted by the effect of the Lorentz force acting on the charge carriers in the silicon.
Pixel vertex detectorPixel vertex detector
• Thus I worked on the development of a setup which allowed me to measure the Lorentz angle.
• The effect on the carriers depends upon the different irradiation doses absorbed by the detector. Since the irradiation dose is greater close to the beam than in the outer regions of the pixel plane, different corrections must be applied in order to keep a good track resolution.
• I covered many aspects of the design and implementation of this new PCI based DAQ and I will first discuss all the features required by the test-beam needs.
DAQ for pixelsDAQ for pixels• Since the pixel detectors will be tested in a 120 GeV beam this year, the work was mainly directed to build a complete read-out system for test-beam studies but flexible enough to allow laboratory bench-test studies and in particular the Lorentz angle measurement.
• Furthermore the DAQ system has been designed with enough flexibility to accommodate the strip readout chip as well.
( I would recall that my group is responsible for the construction of the strip forward tracker )
1. Measure the spatial resolution of the sensors before and afterirradiation
2. Time-Walk studies
3. Test the read-out chip (ROC) in the real BTeV working conditions• data-driven mode
4. Build events using only the temporal information (time-stamp) associated to pixel cells without the aim of an external trigger
Test-beam main goalsTest-beam main goals
Experimental setupExperimental setup
Experimental setupExperimental setup
•FPIX0
•FPIX1
•preFPIX2Tb
telescope
detectors under test
DAQdedicated
PC
Detector Mezzanine-card PCI card
Detector Mezzanine-card PCI card
Detector Mezzanine-card PCI card
Readout &processes monitor PCI
extender
Read-out architectureRead-out architecture
Experimental setupExperimental setup
DATA
BCO CLOCK, READ CLOCK…
DATA
BCO CLOCK, READ CLOCK …
Experimental setupExperimental setup
DATA
DATA
Experimental setupExperimental setup
DATA
DATA
Experimental setupExperimental setupDATA
DATA
DATA
DATA
DATA
DATA
PCI BUS
Readout process
DAQ main featuresDAQ main features
Data1
Data2
Data3
Data5
Data4
Data6
Data7
Data8
Data11
Data10
Data9
Data12
EVENT
Readout process
DAQ main featuresDAQ main features
noise
noise
Readout process
DAQ main featuresDAQ main features
noise
noise
noise
Data1
Data4
Data6
Data11
Data10
Data7
Data8
Readout process
DAQ main featuresDAQ main features
noise
noise
noise
Data1
Data4
Data6
Data11
Data10
Data7
Data8
noise
Data1
Data4
Data10
Data7
Data1
Data2
Data3
Data5
Data4
Data7
Data10
Data9
Data12
?????
Data1Ts 2
Data2Ts 2
Data5Ts 2
Data6Ts 2
Data7Ts 2
Data8Ts 2
Data9Ts 2
Data3Ts 2
Data4Ts 2
BCO
time-stamp
DAQ main featuresDAQ main features132 ns
123456
Data1Ts 2
Data2Ts 2
Data6Ts 2
Data7Ts 2
Data8Ts 2
Data4Ts 2
Data1Ts 5
Data2Ts 5
Data6Ts 5
Data7Ts 5
Data8Ts 5
Data9Ts 5
Data4Ts 5
Data3Ts 5
Data5Ts 5
Data1Ts 5
Data2Ts 5
Data6Ts 5
Data7Ts 5
Data8Ts 5
Data9Ts 5
Data4Ts 5
Data3Ts 5
Data5Ts 5
Data1Ts 2
Data2Ts 2
Data6Ts 2
Data7Ts 2
Data8Ts 2
Data4Ts 2
Data9Ts 2
Data3Ts 2
Data5Ts 2
The read out system works in absence of an external trigger
The data collection from the different pixel detectors is therefore asynchronous
The DAQ must assemble the events in asynchronous mode
Events are built using the time-stamp information
DAQ main featuresDAQ main features
• It is important that the data flux from pixels to PCI is not hampered by the read-out system, which transfers data to the PC.
ALTERA FPGA FirmwareALTERA FPGA Firmware• This read-out is data driven: data are collected as soon as there is a hit above threshold in the detector
1. FPGA firmware2. PC Read-out software
• In order to balance the different acquisition rates between the detectors, PCI cards and the PC, we took particular care in the design of the
ALTERA FPGA FirmwareALTERA FPGA Firmware
Readout process
Bank0 Bank1
FPGA
Shared memory
Consumer
Disk writerTime
Interrupt handler
Reset interrupt
PCI card working mechanismPCI card working mechanism
Readout process
Bank0 Bank1
FPGA
Shared memory
Consumer
Disk writerTime
Interrupt handler
Reset interrupt
PCI card working mechanismPCI card working mechanism
Readout process
Bank0 Bank1
FPGA
Shared memory
Consumer
Disk writerTime
Interrupt handler
Reset interrupt
PCI card working mechanismPCI card working mechanism
Readout process
Bank0 Bank1
FPGA
Shared memory
Consumer
Disk writerTime
Interrupt handler
Reset interrupt
PCI card working mechanismPCI card working mechanism
Readout process
Bank0 Bank1
FPGA
Shared memory
Consumer
Disk writerTime
Interrupt handler
Reset interrupt
PCI card working mechanismPCI card working mechanism
Readout process
Bank0 Bank1
FPGA
Shared memory
Consumer
Disk writerTime
Interrupt handler
Reset interrupt
PCI card working mechanismPCI card working mechanism
Readout process
Bank0 Bank1
FPGA Interrupt handler
Reset interrupt
Shared memory
Consumer
Disk writerTime
PCI card working mechanismPCI card working mechanism
Readout process
Bank0 Bank1
FPGA
Shared memory
Consumer
Disk writerTime
Interrupt handler
Reset interrupt
PCI card working mechanismPCI card working mechanism
Readout process
Bank0 Bank1
FPGA
Shared memory
Consumer
Disk writerTime
Interrupt handler
Reset interrupt
PCI card working mechanismPCI card working mechanism
Readout process
Bank0 Bank1
FPGA
Shared memory
Consumer
Disk writerTime
Interrupt handler
Reset interrupt
PCI card working mechanismPCI card working mechanism
Readout process
Bank0 Bank1
FPGA
Shared memory
Consumer
Disk writerTime
Interrupt handler
Reset interrupt
PCI card working mechanismPCI card working mechanism
Readout process
Bank0 Bank1
FPGA
Shared memory
Consumer
Disk writerTime
Interrupt handler
Reset interrupt
PCI card working mechanismPCI card working mechanism
Readout process
Bank0 Bank1
FPGA
Shared memory
Consumer
Disk writerTime
Interrupt handler
Reset interrupt
PCI card working mechanismPCI card working mechanism
• This process of periodic memory swap and transfer to a shared memory continues indefinetely.
• We have several PCI cards playing this swap game in parallel: in order to be able to build events at a later stage, we needed a syncronization mechanism to keep the event builder as simple as possible.
• By synchronizing the swapping of all the memories we can build events in a very simple and immediate way.
PCI card working mechanismPCI card working mechanism
Interrupt handler A
Interrupt handler B
Interrupt handler C
Interrupt handler n
0 1Banks
1. The PCI card C redirect immediately the data flux to the other empty memory
Readout working mechanismReadout working mechanism
2. The interrupt handler of the PCI card C forces the other cards to swap and then starts flushing its content to the host PC
Each PCI card has its own interrupt-handler process listening for the memory-full signalLet’s suppose, for instance, that the PCI card C is the first being filled up.
3. The other cards start flushing their (partially) filled memory banks to the host PC.
0 1Banks Readout working mechanismReadout working mechanism
1. The PCI card C redirect immediately the data flux to the other empty memory
2. The interrupt handler of the PCI card C forces the other cards to swap and then starts flushing its content to the host PC
Each PCI card has its own interrupt-handler process listening for the memory-full signalLet’s suppose, for instance, that the PCI card C is the first being filled up.
Interrupt handler A
Interrupt handler B
Interrupt handler C
Interrupt handler n
0 1Banks Readout working mechanismReadout working mechanism
A0C0 … n0B0
• This architecture guarantees that events with contiguous time-stamps belong to buffers which are also contiguous in the read-out process.
Interrupt handler A
Interrupt handler B
Interrupt handler C
Interrupt handler n
0 1Banks
A0C0 … n0B0
Readout working mechanismReadout working mechanism
A1C1 … n1B1
• This architecture guarantees that events with contiguous time-stamps belong to buffers which are also contiguous in the read-out process.
Interrupt handler A
Interrupt handler B
Interrupt handler C
Interrupt handler n
0 1Banks
BUFi BUFi+1
A0C0 … n0B0 A1C1 … n1B1
Readout working mechanismReadout working mechanism• This architecture guarantees that events with contiguous time-stamps belong to buffers which are also contiguous in the read-out process.
• Events with the same time-stamp are contained within the boundaries of this overall buffer (BUFi), or at least in the next one, BUFi+1, but not in BUFi+2, making the event-builder an implementation of a sorting algorithm.
Interrupt handler A
Interrupt handler B
Interrupt handler C
Interrupt handler n
EventBuilder
Shared Memory
(unordered data)
Event buffer
(ordered data)
Timestamps:
Event
• Every hit with a new timestamp starts a new event (column) in a buffer• Other hits with the same time-stamp are appended to the right column in the buffer • When the analysis of the BUFFER i+1 is over, it is reasonable to assume that there are no more data related to an event that begun in BUFFER i.
Event builderEvent builder
DAQ conclusionDAQ conclusion• We built a DAQ system that will be used for the upcoming test-beam
• I covered many aspects in the design and implementation of this DAQ:1. I collaborated on the software development2. I personally took care of the FPGA programming
• I was then able to use this read-out system to measure the Lorentz angle in the silicon pixel detector
• I will show now the measurement and the results that I obtained…
E
L
X
Z280m
L effective = X/Z
Optical Fiber
Focusing Lens
Blue LED Light
~2m
B
X0 XL
Pixeldetector
Lorentz angleLorentz angle
Experimental setupExperimental setup
Optical FiberFocusing Lens
EBlue Light
B
Experimental setupExperimental setup
X
Y Bias E(V)B(KGauss)
Lorentz displacement400 m
50 m
• Pixel size in the Y direction= 400 m • Pixel size in the X direction = 50 m
• B parallel to the Y direction• Bias E along the Z direction
Lorentz displacement mainly in the X direction
Experimental setupExperimental setup
• The blue light illuminated several cells in two different columns.• With a threshold scan I was able to know the charge collected in each cell.• Knowing the charge, I could calculate the Center of Gravity of the cluster: a bidimensional point, X and Y.
YX
50 m 400 m
MeasurementsMeasurements
C.o.G. =
i
ii
PHxPH
B > 0B < 0
We expect displacements linearly proportional to the magnetic field and symmetric respect to the sign of the B field.Instead here is what I measured
X-measurementsX-measurements
B KGauss
x [
m]
Displacements are present even with B 0. Also in this case they are in the same direction reversing themagnetic field.
B > 0B < 0
Y-measurementsY-measurements
B KGauss
x [
m]
The reason for this effect can be due to:1. a movement of the apparatus caused by magnetic attraction 2. a residual hysteresis in the ferromagnetic parts of the
apparatus3. a combination of the previous two
• I excluded residual hysteresis effects by performing a full set of measurements along a complete hysteresis cycle and checking the reproducibility of the measurements. I didn’t observe any appreciable differences between measurements at the same B value at different points of the hysteresis cycle
• So I tried to investigate a possible movement due to the attraction of parts of the apparatus by the magnet
InvestigatingInvestigating
Since the Lorentz displacement is an odd function of the magnetic field, while any movement due to magnetic attraction is an even function of it, by taking the sum of the measured positions at opposite values of the magnetic field one can cancel the contribution of the Lorentz effect, and, vice-versa, by taking the difference one can cancel the mechanical effect.
( XB +X-B)/2 or ( YB +Y-B)/2 = Mechanical movement ( XB -X-B)/2 or ( YB - Y-B)/2 = Lorentz displacement
How to measure the mechanical movement?How to measure the mechanical movement?
This movement exists, as shown by the following histograms, and is proportional to the module of the magnetic field as expected,but it should not depend on the value of the BIAS voltage.I checked this by fitting all the movements to a common line (RED)
100 V
350 V300 V250 V
200 V150 V
400 V
Measured mechanical X-movementMeasured mechanical X-movement
Even in this view exists a unique movement that can fit all data witha good ²( ²/d.o.f = 1.14 ) even if it is not as good as in X
100 V
350 V300 V250 V
200 V150 V
400 V
Measured mechanical Y-movementMeasured mechanical Y-movement
• Taking the difference ( XB - X-B)/2 we then find the Lorentz displacement.• For consistency we have to check that the Lorentz displacement be strictly linear with the magnetic field.• We can easily see that now the movement is linearly proportional to the magnetic field and it decreases, as expected, increasing the Bias voltage.
Lorentz X-displacementLorentz X-displacement
100 V
350 V300 V250 V
200 V150 V
400 V
6m
7.8m 6.8m
m
5.5m 5m
9m
In the Y direction instead we can fit the data with a straight line, within the errors, but the movement doesn’t scale as expected with the Bias voltage. Nevertheless the residual Lorentz displacement in this view is very small (B is almost parallel to Y) and practically comparable with the errors.
ONLY1 m
Lorentz Y-displacementLorentz Y-displacement
100 V
350 V300 V250 V
200 V150 V
400 V
Position finding algorithmPosition finding algorithm• Since the position is deduced by computing the charge cluster center of gravity from binned pixel cells, an indetermination is introduced in the measurements by the finite size of the cells.• I investigated this effect by assuming a shape for the LED light given by a gaussian fit to the charge collected on the pixel cells
X-correctionX-correctionThe final correction curve is given by the following
plot• X = Measured Center of Gravity coordinate• Y = Corrected coordinate value
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3390
X-corrected mechanical movementX-corrected mechanical movementThe correction is really tiny for the mechanical movement.
100 V
350 V300 V250 V
200 V150 V
400 V
Corrected Lorentz X-displacementCorrected Lorentz X-displacement
100 V
350 V300 V250 V
200 V150 V
400 V
While for the Lorentz displacement the correction is ~1m at most
The same procedure has been applied to Y-coordinate where the effectis sizable due to the large discretization (400 m).I assumed the same LED shape as in X and I verified it rotating the optical fiber of 90 degrees
Y-correctionY-correction
Corrected Y-mechanical movement Corrected Y-mechanical movement
100 V
350 V300 V250 V
200 V150 V
400 V
Corrected Lorentz Y-displacementCorrected Lorentz Y-displacement
100 V
350 V300 V250 V
200 V150 V
400 V
1.6 m 2.6 m 1.5 m
1.5 m 0.8 m 1.8 m
1.2 m
Final Lorentz displacement at 2.8 KGaussFinal Lorentz displacement at 2.8 KGaussCombined X and Y displacement at different bias voltages
L
Lorentz displacement
Comparison with theoryComparison with theory
• The Lorentz angle is proportional to the magnetic fieldL = H Bwhere the proportionality factor, , is the Hall mobility which is related to the drift mobility viaH = rH ·
The Hall factor, rH, is a dimensionless value determined to berH = 1.15 for electrons, while the mobility can be well described by the empirical formula
=
)1 +( EEC)( 1/
• E is the electric field in the sensor• ,EC, are parameters determined empirically and are well measured at 300° K = 1450cm²/V·sEC = 7240 V/cm = 1.30
• I will compare the measured effective Lorentz angle as defined byL effective = X/Z with what expected from theory
Comparison with TheoryComparison with Theory
P+ implant
n+ n+ n+n+dVV db /)(
dVV db /)(
E
z
dVV
dz
dV
zE depbiasdep
1
2)(
• The electric field in the sensor can be well approximated by the following formula
L = dX/dZ = B dX = B dZ
…so it’s easy to calculate the trajectory of the carriers in the silicon
BdzrBdzX
d
H
d
HcEzE
0 10
/1)(
0
d n
Lorentz displacement at 2.8 KGauss
ResultsResults
² = 1.22
L
Effective Lorentz angle extrapolated at 1.6 Tesla
Bias: 100 Angle: 11.9 ±0.14 ± 0.3 Theory: 11.92Bias: 150 Angle: 10.6 ±0.16 ± 0.3 Theory: 10.4Bias: 200 Angle: 9.19 ±0.15 ± 0.3 Theory: 9.13Bias: 250 Angle: 8.02 ±0.15 ± 0.3 Theory: 8.1Bias: 300 Angle: 7.23 ±0.14 ± 0.3 Theory: 7.25Bias: 350 Angle: 6.83 ±0.16 ± 0.3 Theory: 6.56Bias: 400 Angle: 6.21 ±0.15 ± 0.3 Theory: 5.97
ResultsResults
L
I can even measure the three parameters, , 0 and Ec, by fitting my measurements with the theory model.The fit is reported in the plot and the returned parameters are0 = 1486 ± 123 = 1.19 ± 0.24 EC = 7706 ± 550
ResultsResults
( 0 = 1450 ) ( = 1.30 ) ( EC = 7240 )
ConclusionsConclusions• The measured Lorentz angle agrees very well with the previous measurements. -For instance, I obtain 9.3º ± 0.14º± 0.3º at 1.4 T and 150 V, to be compared with 9º ± 0.4º ± 0.5º measured by ATLAS
• I was able to measure the theory parameters which are in good agreement with those reported in literature.
• The scaling of the Lorentz angle with the bias voltage (i.e. mobility) follows the expectations
• The next step will be the measurement with irradiated detectors
• In the end, with a relatively simple apparatus, I was able to accurately measure the Lorentz angle.
BACK UP SLIDES
In order to improve the determination of the X displacement I add a correction in the X-direction performing a MonteCarlo simulation that takes into account this effect.
Data analysisData analysis
2. The mean value of these gaussians has been plotted in (a)
(a)
1. I generated a sample of 10,000 gaussians with fixed width and amplitude taken from the data sample shown in figure
3. For each chosen gaussian, the shape as been superimposed on a grid of pixels at fixed positions, and the fraction of gaussian area overlapping each bin has been computed. The mean values of these redistributed charges (C.o.G) have been computed and plotted in (b)
(b)
Data analysisData analysis
4. For each generated gaussian, the difference betweeen the input value (peak position of the gaussian) and the computed mean value after discretization is plotted in (c). The spread turns out to be of the order of 1m
5. Finally I obtained the correlation curve between continuous beam spot and the corresponding computed values from discretized quantities. See (d)
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3390
474 = 23,4% 1548 = 76,6%
To improve the determination of the center of gravity in Y I verified, rotating the optical fiber of 90 degrees, that the distribution of the light was almost the same in X and Y. So I could learn from the more precise determination of the X position also a correction to apply to the Y.
Data analysisData analysis
130 344 483 504 378 183
Beam Axis50 m
400 m
X Plane
Y Plane
50 m
400 m
Pixel vertex detectorPixel vertex detector