1. .

2. New Experimental Method for a Precise Measurement of the Proton Charge Radius at the S-DALINAC *
I. Pysmentska, P. von Neumann-Cosel, S. Rathi,
A. Richter, G. Schrieder and A. Shevchenko
Institut für Kernphysik, TU Darmstadt
*Supported by the DFG under contract SFB 634

3. Introduction Proton charge radius –
one of the fundamental quantities in physics!
Its precise determination is very important -
to understand its structure in terms of quark and
gluon degrees of freedom of Quantum Chromodynamics.
for high-precision tests of Quantum
Electrodynamics using Lamb shift measurements .

4. Proton charge radius – the data

5. Differential cross-section H(e,e)pì æ 2 + t 2 ö ü ï G G q ï q 2 ç ÷ F ( q 2 ) = í E M + 2 t G 2
tan 2 ý ,
with t ç ÷ ï 1 + t M 2 ï 4 M 2 î è ø þ
At low ,
q 0

6. Experimental method e- E Ep Previous New Spectrometer qe qp
Si Detectors Ep qp qe
Drp/rp < 1%

7. S-DALINAC

8. Set-up
Si-Detectors
QCLAM
Spectrometer
DW = 35 msr

9. Setup
472 mm e
28 Detectors

10. Experimental energy spectrum and background

11. A typical spectrum with pulsed beam
Energy (Channel no.)
65° p e-
50 ns e- p
Linearised spectrum
Time (Channel no.)

12. Projected spectra p e-
Time Resolution ~ 5 ns

13. Measured energy spectra
0.22
0.31
0.38
0.44
q (fm-1)
1/E2 fit

14. Outlook
Proton charge radius
Measurements at higher beam energies !

15. Rosenbluth Separation
Separation of el. and mag. contributions:
Same q, different kinematics (Ee,q)
e-p Scattering

16. Spectrometer spectra

17. .

18. Test Experiment

19. Effect of beam spot on the angle defination of the detector:x
target
90-q q r dq a c d
Defining detector
aperture q`
Beam dia. =
a = 2mm
Detector
Distance=
r = 20cm
tan dq = c/ r
= a*cos()/r

20. Si Detectors Reichweite qp° Ep(MeV) Dicke (mm)
Si – Detectors 2.5 x 2.5 cm2
qp°
Ep(MeV)
Reichweite
(mm)
Dicke
20°
8.90
570
800
30°
7.57
440
600
40°
5.92
290
380
50°
4.16
160
250
60°
2.52 70
100
70°
1.18 22 50
80°
0.3 3 12