Compton Storage Rings

Transcription

1 Compton Storage Rings Wolfgang Hillert ELectron Stretcher Accelerator Physics Institute of Bonn University

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3 Møller-Polarimeter Compton-Polarimeter Mott-Polarimeter

4 Compton Scattering Differential cross section: dσ dω ( SP, ) = Σ 0 + Σ1( S1) + Σ2 ( S3 P), Polarised electrons: 2Z Compton kinematics: i i ( ϑ ) Σ 2( S3, P) = SP 3 z CK f sinϑ ( 1 cosϑ ) sinϕ S3Ps CK f ( 1 cosϑ )( K f + Ki ) cosϑ Σ Σ 2 r K e f C = 2 K i 2 spatial asymmetry counting rate asymmetry K f K = 1 + K 1 cos 2S

5 Measuring Principle shift of the centre of the photon spatial distribution integral up-down counting rate asymmetry N u N d silicon strip detector and pair conversion required: rather complicated at least 2 crystal detectors and no conversion required: rather simple

6 Design Criteria Basic Parameters: crossing angle: δ 3mrad electron beam width: σ 1mm interaction region: l 0.7m Background due to Beam-Gas Radiation: straight section should be considerably short! 0.7 m dipole quadrupole 5.9 m dipole

7 Numerical Simulations 1. Ideal beams (σ x =σ z =0) and infinite detector (simple): integral asymmetry: A int π + + N sin un d d d NuN ϕ ϑ ϑ d 2Z = = P + + N N + N N dϕ sinϑ dϑ u d u d π 0 0 π π γ 0 0 Σ Σ 0 P e shift of center of spatial distribution: 2π π dϕ sin ϑ dϑ z( ϑ, ϕ ) 2Z z = z+ z = P dϕ sinϑ dϑ 0 0 2π π γ 0 0 Σ 0 Σ P e ( ) ( ) D sinϑ β cosϑ ( with z( ϑ, ϕ ) = cosϕ ) 2 γ 1 βcosϑ 1-dim integration:

8 Numerical Simulations 2. Ideal beams (σ x =σ z =0) and finite detector (moderate): integral asymmetry: A int X det X /2 Z /2 shift of center of spatial distribution: ( ϑ ( x, z), ϕ ( x, z) ) det = X X det X det X / 2 0 det /2 Z /2 /2 0 (, ) det ϑ ϕ dx dz Σ2Z ( xz, ) det (, ) d ϑ ϕ Σ0 ( x, z) xz x /2 Z /2 /2 Z /2 ( ϑ (, ), ϕ (, z) ) ( ϑ ( xz, ), ϕ ( xz, )) det det = z X det X det /2 Z /2 (, ) det ϑ ϕ dx dz z ( xz, ) det ( ϑ, ϕ ) dx dz /2 Zdet / 2 ( x, z) Σ Σ 2Z 0 ( ϑ ( xz, ), ϕ ( xz, )) P γ P P γ e P e ( ) ( xz) ( ) 3 2 ϑ, ϕ γ 1 βcosϑ (with: = ), D β cosϑ 2-dim integration:

9 Numerical Simulations 3. Real beams (σ x,σ z, σ x,σ z 0) and finite detector (!!!): a) 2-D intensity profile of backscattered photons: S ' ' max max max max Z X Z X 1 N (, ( x ), z ) ds dz dx dz dx ϑ ϕ d d e(,,,, ) (,, ) ( x, ) sx s dσ = ρ zx z ργ x z z dω ' ' 0 min min min min S Z X Z X 5-dim integration:

10 Numerical Simulations 3. Real beams (σ x,σ z, σ x,σ z 0) and finite detector (!!!): b) Asymmetry and shift from mean values: integral asymmetry: z j 0 N = N ( x, z ) +, +, u i j + + i, j N un d N un d A z 0 int = j + + +, +, N un d + N un d d = ( i, j) i, j N N x z shift of center of spatial distribution: + j i j j i, j i, j z = z N ( x, z ) z N ( x, z ) # dim integrations: i j 5-dim integration for each point: detector size = 4.0 x 3.84 cm 2 detector pitch = 100µm in total 41 x 384 points

11 Counting Rate Asymmetry

12 Shift of Spatial Distribution

13 Numerical Simulations 4. Pair conversion (e + /e - ): fit d = 1.7 X 0 Fitzgerald et al., NIM 192 (1982)

14 Analysing Power D = 15 m P L I e = 10 W = 100 ma d conv = 1.7 X 0

15 Beam Parameters 2 D ε 2 σ + ( ε β) + f D γ β

16 Beam Parameters FCC 2 D ε 2 σ + ( ε β) + f D γ β D = 500 Z: β x 16m β z 100m ε x 90pm ε z 1pm

17 Shift of Spatial Distribution FCC-ee D = 500 m detector size: 2 cm x 2cm infinite detector finite detector

18 Required Pitch Strip detector with pitch p: spatial resolution: p zdet = 12 achievable position error: 2 z = 2 i σ N z i 2 + z 2 2 det N N n z P 1% z 0.7µm p 50µm

19 Counting Microstrip Detector detector amplifier, shaper, discriminator counter Developed in close collaboration with ATLAS pixel-detector group of N. Wermes, PI Bonn

20 Compton ELSA Si microstrip detector 768 channels, 50 µm pitch

21 Detector Design silicon microstrip detector (BABAR 1) 768 strips with 50 µm pitch (42.4 x 41.3 mm 2 ) resolution 14 µm front-end chip high rate acceptance ( MHz) digital part built in LVDS technology FPGA controlled

22 Detector Performance a) raw data Channel

23 Detector Performance b) corrected data Channel

24 Summary and Outlook Precise Compton Polarimetry: Compton backscattering of ~515 nm photons circularly polarized photons transverse polarized e-beam measurement of shift of photon intensity distribution counting silicon microstrip detector with p = 50 µm Achievable precision: ELSA (3.5 GeV, distance 15 m): P 1% FCC-ee (< 90 GeV, distance 500 m): P < 0.1% FCC-ee (175 GeV, distance 500 m): P 0.2%