Production of MeV Gamma Rays through Inverse Compton

Production of MeV Gamma Rays through Inverse Compton

Production of MeV Gamma Rays through Inverse Compton Scattering
Keegan Behm1, J.Cole2, E. Gerstmayr2, S.P.D. Mangles2, J.C. Wood2, C. Baird3, C. Murphy3, K. Krushelnick1, A.G.R.
Thomas1
1
Center for Ultrafast Optical Science, University of Michigan, Ann Arbor
2
Plasmas Group, Imperial College of London, London, UK
3
The University of York, York, UK

Abstract

Experimental Setup

Research for high energy photon sources has been continuing since the discovery of
X-rays in 1895. Here we present data showing the production of gamma rays as high
as 100 MeV through inverse Compton scattering of a laser wakefield accelerated
(LWFA) electron beam. One of the reasons for studying high energy photon sources
on an all-optical device is because they have a high degree of tunability and it is
possible to eliminate timing jitter between various arms of the experiment. At the
Astra-Gemini laser system at Rutherford Appleton Labs (RAL), we collided an 800
MeV electron beam with a counter-propagating ultra-short pulse with a maximum
a0 of 20 [1]. The goal for this experiment was to measure a radiation reaction due to
the immense energy radiated away by the electron beam [2]. A CsI crystal array
positioned parallel to the photon beam was used to detect the high energy gamma
rays and provide information about the penetration depth of the gammas and the
vertical divergence. Figure 1 shows an example of the data obtained from the
fluorescing CsI crystals within the detector array. With this detector we can analyze
correlations between vertical divergence of photon flux and characteristics of the
electron beam such as charge or maximum electron energy.

Objectives

f/2 scattering
beam

Probe be
am

15 mm gas jet

f/40 drive wakefield
driver

Spatial and temporal
alignment tools

47 x 33 CsI
crystal array
used as primary
gamma ray
diagnostic.
CsI crystals are
5 x 5 x 50 mm.
Beam incident
on side of
spectrometer to
measure
penetration
depth.

Results and Discussion

1. Measure a radiation reaction in the electrons by colliding them with a
strong counter-propagating beam.
2. Produce MeV-level gamma rays through inverse Compton scattering
of the electron beam.
3. Develop a gamma ray spectrometer by measuring the penetration
depth of photons in the crystal array.

Inverse Compton Scattering
This is a method of high energy photon production (on the MeV)
level by scattering an electron beam with a counter-propagating
laser. In this experiment, we accelerated electrons from a 15 mm
gas jet to 800 MeV and collided them with a counter-propagating
laser with an a0 of 20. The intense electric field of the scattering
laser causes the electrons to wiggle in the field, thus releasing very
high energy photons. The critical energy of the photons is
proportional to both the energy of the electrons ( in the equation
below) and the intensity of the laser. For the ideal scenario in this
experiment (a0 = 20 and 1 GeV electrons), the critical energy of the
produced gamma rays would be over 300 MeV.
Coun
te

r-pro
paga
ti

ng pu
lse

rays

Laser wakefield
accelerated electrons

Actual Signal
Calculated Signal

Gamma Spectrum

An iterative algorithm was used to create a sample spectrum
and calculate what the resulting CsI signal would look like.
Starting with a flat spectrum, small perturbations were made to
gradually form an input spectrum that can match the signal data.
The calculated signal was then checked against the actual
experimental data and perturbations to the spectrum were kept
if the matching was improved and thrown away if it was not
improved.
The simulations cannot match the first few bricks of the signal
very well resulting in an overestimated energy in the gamma ray
spectrum.
A cause for this is likely a nonlinear relationship between energy
deposited and light yield in the CsI crystals.
Calculations of perfect laser beam overlap with the electrons
suggest that scattering could produce up to 300 MeV gamma
rays.

Conclusions
The raw data obtained from the CsI crystal is shown on the left.
The image is 1024 x 1024 pixels with the dark spots due to the Al
face plate blocking the light. The data within each crystal was
averaged together into a single data point to make it possible to
analyze with an iterative algorithm and MCNP simulations.

In an attempt to maximize the signal on the CsI scintillator, a
raster scan was performed to try and improve the overlap
between the electron beam and the counter-propagating laser.
On the left shows an area of highest signal was found in the
middle. To turn the figures above into a spectrum, several MCNP
simulations were performed of monoenergetic photon beams
entering a simulated CsI block, results shown on the right.

Successfully beam overlap between the electrons and counterpropagating f/2 heater beam.
There was no evidence of a radiation reaction in the electron
beam on the electron spectrometer.
Produced gamma rays of 100 MeV or greater through inverse
Compton scattering.
The simulation curves struggle to match up with the data in the
low energy regime due to the sharp rise in signal at the start of
the CsI array.
The iterative algorithm is currently not producing a very accurate
spectrum. It is likely that the light yield from the CsI scintillation
is not linearly proportional to energy deposited.

References
[1] Sarri, G., et al. "Ultrahigh brilliance multi-MeV -ray beams from nonlinear
relativistic thomson scattering." Physical review letters 113.22 (2014): 224801.
[2] Di Piazza, A., K. Z. Hatsagortsyan, and Christoph H. Keitel. "Quantum radiation
reaction effects in multiphoton Compton scattering." Physical review letters 105.22
(2010): 220403.
[3] Corde, Sbastien, et al. "Femtosecond x rays from laser-plasma
accelerators." Reviews of Modern Physics 85.1 (2013): 1.

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