Triplet polarimeter - Arizona State Universitydugger/polToGo.pdf · Comparison of GEANT4 study of triplet polarimeter to previous study • The SAL detector ... Detector upstream

1

Triplet polarimeter

M. Dugger, March 2015

M. Dugger, February 2012 2

Triplet production

• Pair production off a nucleon: γ nucleon → nucleon e+ e-.

• For polarized photons σ = σ0[1 + PΣ cos(2φ)], where σ0 is the

unpolarized cross section, P is the photon beam polarization

and Σ is the beam asymmetry

• Triplet production off an electron: γ e- → eR- e+ e-, where eR

represents the recoil electron

• Any residual momentum in the azimuthal direction of the e- e+

pair is compensated for by the slow moving recoil electron.

This means that the recoil electron is moving perpendicular to

the plane containing the produced pair and can attain large polar

angles.

3

Event generator

• Diagrams used

• Screening correction

• Most important diagrams

4

Triplet production (pair like)

• Here we have two electrons in final state and must include

diagrams that have 4 ↔ 5 interchange

1

2

3 4 5

q

Q 1

2

4 3 5

q

Q

1

2

3 5 4

q

Q 1

2

5 3 4

q

Q

time

5

Triplet production (Compton like)

• Includes 4 ↔ 5 exchange

1 2

4 Q

q

5 3

1 2

4 Q

5 3

1 2

5 Q

q

4 3

1 2

5 Q

4 3

time


Screening correction

7

Screening correction

• Leonard Maximon informed me that the screening correction for

triplet production should not be important for the beam asymmetry

but was very important for the cross section

• Used the screening correction provided in the paper by Maximon

and Gimm [1] to compare cross section results of the event

generator to values from the NIST

[1] L. C. Maximon, H. A. Gimm Phys. Rev. A. 23, 1, (1981).

8

Comparison plot of total cross section for

triplet production • Black line: Total cross

section from NIST

• Blue points: Total cross

section from event

generator without

screening correction

• Red points: Total cross

section from event

generator with screening

corrections included

• Very nice agreement once screening is included

9

Most important diagrams

• The Mork paper [2] says that the Compton-like diagrams and the

switched electron leg diagrams should be negligible at high

photon energy

[2] K. J. Mork Phys. Rev. 160, 5, (1967).

10

Calculations

• Ran 100,000 events at Eγ = 9.0 GeV using:

• Full calculation: σ = 10.856 mb

• Reduced calculation: σ = 10.855 mb

• Reduced calculation neglects Compton-like and crossed electron-

line diagrams

• Reduced calculations are well within 0.1% of full calculations

1

2

3 4 5

q

Q 1

2

4 3 5

q

Q

time


Comparison of GEANT4 study of triplet

polarimeter to previous study

• The SAL detector

• δ-rays

• Pair to triplet ratio

• ASU simulation of SAL

• Could SAL have been modified to work in the Hall-D environment?


The GW SAL detector • Eγ = 220 to 330 MeV

• 2 mm scintillator converter

• Polarimeter located ~39 cm downstream of converter

• Recoil θ = 15 to 35 degrees

• Recoil φ = 0, 90, 180, 270 degrees with Δφ = 44 degrees

• Analyzing power at the event generator level = 12%

• Analyzing power from simulation 3-4% (post experiment)

• Measured analyzing power = 2.7%

• Quick check - Can ASU reproduce the GW results?


δ-ray comparison with Iwata 1993 simulation

• Eδ is δ-ray kinetic energy

after traveling through 1 mm of

scintillator using Iwata’s polar

geometry and scintillator

widths

• BLUE: Current ASU

GEANT4 results

• RED: Iwata GEANT3 (scaled

to GEANT4 results by ratio of

signal integration)

• Shapes of the distributions

look similar

Eδ (MeV) • ASU simulation Eγ = 300 MeV

• Iwata simulation Eγ = 250, 365, 450 MeV

Note: ASU simulation did not wrap scintillators


NIST cross sections for triplet and pair

production off carbon

• σpair/σtriplet:

5.75 @ 300 MeV

5.16 @ 9.0 GeV

• Ratio does not vary much over large energy range


Comparison of ASU MC of SAL detector to GW results Note: ASU results are for Eγ = 300 MeV and GW is of Eγ = 220 to 330 MeV

Analyzing power:

• At event generator level: 12.6 ± 0.1 % ASU; ~12% GW (no error reported)

• ASU simulation: 2.65 ± 0.05%

• ASU simulation (30 μm Al wrapped scintillators): 2.8 ± 0.1 %

• GW experiment: 2.7% (no error reported)

• GW simulation: 3-4% (range given with no error reported)

• ASU results are in agreement with the GW results for the SAL detector ☺


How could the SAL detector be modified to

work in Hall-D environment?

Air → Vacuum Converter rad length → 10-4

E → 9 GeV ΔE pair < 1.5 GeV Analyzing power

2.8(1) %

X 6.9(1) %

X X 12.5(2) %

X X X 10.6(2) %

X X X X 17.5(3) %


Dependence of analyzing power on

ΔEpair for 16 sector detector

• Same parameters as previous best configuration but now with 16

sectors instead of 4 paddle SAL design

• Analyzing power fairly constant

for ΔEpair < 1750 MeV

• The 16 sector design increases

the analyzing power to 19.1 ± 0.7 %

from 17.5 ± 0.3 % of the 4 paddle

design (ΔEpair < 1500 MeV) ΔEpair (MeV)

An

alyzi

ng

po

wer

(%

)


Recap of modifications that would make

SAL type detector work in Hall-D

Air → Vacuum Converter rad length → 10-4

E → 9 GeV ΔE pair < 1.5 GeV 16 Sector Analyzing power

2.8(1) %

X 6.9(1) %

X X 12.5(2) %

X X X 10.6(2) %

X X X X 17.5(3) %

X X X X X 19.1(7) %

19

Detector • Decided to use

Micron S3 design

instead of the S2

• The S3 has 32

azimuthal sectors

instead of 16 for the

S2

22 mm

70

mm

1 mm thick

20

Preliminary design (slide 1)

• Micron S3

• Converter

• 200 events thrown

21

• Micron S3

• Converter

• Mounting plate and brackets

• Having a removable plate will allow for

easy modification of how the detector is

mounted without having to modify the

chamber


22

• Micron S3

• Converter

• Mounting plate and

brackets

• Chamber with

electrical feed-

through flange, and

blank flange

• Inner dimensions:

11in by 9in by 9in


Actual design: 12 in by 12 in by 12 in

23

• Includes ribbon cable from

detector to electrical feed-

through


24

• With front door in

see through mode

• 200 events thrown


25

Simulation

φ (degrees)

En

erg

y d

epo

site

d (

MeV

)

• Generated pairs and

triplets with Eγ= 9 GeV

• Required energy of e+

e- pair to be within 1.75

GeV of each other

• Required energy

deposition in detector

to be greater than 200

keV

• Converter: 35 μm beryllium

• Used full calculation (all diagrams included)

26

Simulation results

cro

ss s

ecti

on

wei

gh

ted

co

un

ts

φ (degrees)

• Assumed collimated photon

rate in coherent peak : 99 MHz

• Δt = 4 hours

• Analyzing power: 0.186

• Polarization uncertainty: 0.01

Rates:

• Total rate on device = 955 Hz

• Rate surviving software and trigger cuts = 64 Hz

121

22

PNP

P

where N ≡ Rate surviving cuts * 4 hour,

P ≡ Polarization = 0.4, and

α ≡ analyzing power

Assumed

Polarimeter location (red box)

27

Future home

• Sources of magnetic fields in red circles

28

B-field study

• Study performed in April 2012

• Applied magnetic field in vertical direction

M. Dugger, April 2012 29

Effect of B-field on δ-rays

x (cm)

x (cm)

y (

cm)

y (

cm)

No field →

350 gauss field →

30

Azimuthal distribution with applied B-field

φ

• 350 gauss field applied A[1+Bcos(2φ)]

A[1+Bcos(2φ)+Ccos(φ)]

31

Analyzing power vs. B-field

Anal

yzi

ng p

ow

er (

B)

Field strength (gauss)

Small field → Small systematic effect

32

Polarimeter stand in collimator cave

33

Upstream of polarimeter stand

• Secondary

collimator

• Secondary

sweep magnet

• Polarimeter

stand

34

Further upstream of polarimeter stand

• Primary sweep

magnet

• Secondary

collimator

35

Construction

36

Vacuum system

• When the polarimeter is installed in the collimator cave, the

vacuum will come from the beam line

• In the test bench, the vacuum has to be provided by a

temporary system

• Using a rotary vane pump for the vacuum system of the test

bench

• Rotary vane pumps will back-stream oil and this issue must be

addressed

37

VisiTrap

• VisiTrap will

catch any back-

streaming before

it hits the

vacuum hose

38

Molecular Sieve and Stinger

• Molecular sieve

will catch stray

contaminates

• Loaded sieve

with zeolite and

heated for two

hours

39

Vacuum system attached to chamber (view 1)

• Cleaned chamber

with:

• Acetone

• Methonal

• DI water

• Attached the

vacuum system

40

Leakage and outgassing tests

Procedure:

• Pump down chamber

• Close butterfly valve between vacuum system and chamber

• Record the pressure as a function of time

cycle 1

Slope = 0.0266 mTorr/min

41

Vacuum leakage test results (empty chamber)

• For area and volume calculations of the chamber I included all of

the flanges

• Volume ~ 29.5 liters. Surface area ~ 6500 cm2

• Outgassing rate = (Volume/Area)*dP/dt

• Tim Whitlatch said steel will outgas at a rate of

2E-09 Torr*l/(cc*s)

• It looks like the test is consistent with the dP/dt of the chamber

being from the outgassing of steel: No big obvious leaks

Cycle dP/dt (mTorr/min) dP/dt (Torr/s) V/A*dP/dt (Torr*l/cc*s)

1 0.0266 4.4 * 10^-7 2.00E-09

2 0.0216 3.6 * 10^-7 1.60E-09

3 0.0169 2.8 * 10^-7 1.30E-09

4 0.0091 1.5 * 10^-7 6.90E-10

42

• Five hour pump down to ~20 mTorr

• Ql = V*dp/dt found to be 1.80x10-4 Torr*liter/s

• A turbo pump on the chamber with a flow rate of 100 liters/s with

a working pressure of 2x10-5 Torr will have a Qw = 2x10-3

Torr*liter/s

• Pfeiffer vacuum says that a system is adequately tight if Qw >

10*Ql

• Putting a turbo pump with flow rate 100 liters/s on the base of the

chamber should be sufficient to maintain a vacuum of 2x10-5 Torr

Vacuum test results (detector in chamber)

43

Detector upstream view with source stand and

Po210 source

• Teflon fasteners

connect detector

to supports

44

Detector downstream view

45

Preamps

• Decided to have a parallel development of the preamps:

• Glasgow is building a pre-amplification system based off

of the Rutherford Appleton Laboratory RAL-108 pramps

and custom motherboards

• ASU is using a pre-amplification system from Swan

research (“Box16 preamps”)

46

Swan preamps The STARS detector uses Micron S2

with swan research preamps

Input view Output view

16 channel box

Preamp hybrid Stars detector

47

Preamp to feedthrough cable assembly

• The Micron S3 detector

uses special ribbon

cable connectors

• Could not find suitable

ribbon cables. Instead

used Kapton wires that

were individually

placed in the cable

connector

• 40 Wires attached to

connector in picture

shown

48

Detector, cable and source

49

Ring side cable

• Only enough

preamps to

instrument the

sectors but

made the ring

side cables

first

50

Preamps wired up and ground connections

• Sector side

cables

• Ring side set

to ground

51

Distribution box connected to preamp

enclosure

• Original

distribution

box

52

New distribution box (view 1)

• While Kei was at

ASU getting trained

to be a polarimeter

expert he was able to

help assemble to new

distribution box

Signal plate

53

New distribution box (view 2)

Power plate

54

Copper preamp-box grounding (slide 1)

• Preamp supports made

out of anodized

aluminum

• Decided to help ground

the preamp boxes by

using copper foil on the

preamp supports

55


• Lined three sides of the

preamp enclosure with

the copper foil

• View: looking into the

preamp enclosure

through the opening for

the vacuum chamber

feedthrough flange

• Ground connector to ring side of detector

56


• View: looking into the

preamp enclosure from

the top

• Can see the EM-

shielding copper mesh

for the fan inlet/outlet

grounded to the copper

foil

57

Signal plate grounding

• Cutting copper foil

for the signal plate

grounding

• Also grounded to

the input voltages

(power plate)

58

Signal plate and power plate grounding

59

Fan leads

• Routed the fan leads

through the preamp

enclosure towards the

distribution box

60

View of polarimeter with original distribution

box completely removed

61

• Wrapping signal wire around

toroidal core reduces noise

• Putting AC Power Entry Module

(with inline filter and earth-line

choke) into LV supply also helped

with the noise

Noise reduction

62

The silicon detector

• The detector is very much like a diode operated in reverse bias

mode

• As the voltage is increased across the detector, the depletion

region gets larger

• The larger the depletion region, the smaller the capacitance of

the detector

• For each 3.6 eV of energy deposited in the depletion region there

is one electron-hole pair that is created and then swept out of the

detector

63

Noise of preamps versus input capacitance

• Preamp noise has a linear

relationship with the

detector capacitance

Typical noise versus

detector capacitance plot

64

High voltage

Tennelec TC 952 • For the test bench we are using a

temporary power supply that is

rather old

• The permanent power supply will

be provided by JLab and will be of

higher quality

• The temporary power supply has a

ripple of about +/- 5 mV at 60 Hz

65

Ripple and other noise as function of HV (slide 1)

HV = 0V HV = 20V

HV = 40V HV = 60V

10 mV/div

10 ms/div

Using

Po210

source

66


HV = 80V HV = 100V

HV = 120V HV = 140V

10 mV/div

10 ms/div

67


HV = 160V HV = 180V

HV = 200V HV = 200V

200 mV/div & 1 µs/div

10 mV/div

10 ms/div

α

68

Alpha o-scope picture

• Polonium 210

source

• Alpha energy =

5.3 MeV

• Signal about 500

mV

69

Electron o-scope picture

• Cesium 137

source

• Signal about 25

mV for this shot

• Finer time scale

for this shot (50

ns/div)

70

Data acquisition system at ASU

• Using a Tektronix logging

oscilloscope as a slow ADC

• Acquisition rate ~ 1 Hz

LabView signal express GUI

71

Fit to signal

• Assume voltage has same form

V = [Γr Vm/(Γr - Γf)][exp(Γrt) – exp(Γrt)]

Volt

age

time (μs)

Po210 signal

72

Calibration (sector 3)

Counts

mV

Center = 505.5 mV

• Po210 alpha source

• Ek = 5304.33 keV

• One hour of data

• Calculated sensitivity = 95mV/MeV

73

Alpha-test widths

• Looked at all 32 sectors

• Looks good except for a

single channel (sector 32,

lower preamp-box channel

10)

74

Typical fit to signal for Ba133 source

Volt

age

time (μs)

75

MC Data

• MC

• Generated photon energies: 223, 276, 302, 356, 383 keV

• Smeared energy deposited by standard deviation of 12 keV

• Fit:

• Centers locked to same photon energies that were generated in MC

• Standard deviation was the same for each Gaussian and allowed to vary

• Standard deviation from fit found to be 11+/- 1 keV

• Therefore, resolution of detector plus electronics is about 12 keV for this sector

Ba133 MC compared to data (sector 3)

76

Pile-up study

2.1% of total

• Threw 10 million photon events

• Only 469 events seen on detector

• Assume

• 108 Hz in photon range

between 8.4 and 9 GeV

• Timing window of preamp

pulse to be 18 μs

• For a single sector we expect 0.7%

of events to have more than one

signal in the timing window

• Pile-up should not be much of an

issue

77

Work still to be done at ASU

• Need to complete the positioning system (should be able to finish

this week)

78

Convertor tray

Bottom of converter tray

Top of converter tray

79

Positioning system

• Still need to clean

parts and install

limit switches

• I expect the

positioning system

to be ready to ship

by the end of this

week

80

Chamber crated up

81

Initial work to be done at JLab

• Set up the chamber with vacuum system attached and do leakage

and outgassing tests

• Attach the preamps and make sure that the signals look as they did

at ASU

• Send the signals through the fADC and take data

• Attach the position control system and test

New stuff

82

Chamber

Chamber at JLab

• JLab started

receiving the

polarimeter parts

last week

• Initial test bench

in the

Experimental

Equipment

Laboratory (EEL)

at Jefferson Lab

Documents

Triplet polarimeter - Arizona State Universitydugger/polToGo.pdf · Comparison of GEANT4 study of triplet polarimeter to previous study • The SAL detector ... Detector upstream