Production of tunable, monochromatic X-rays by the Vanderbilt
free-electron laser
Frank E. Carroll *a , James W. Waters b, Robert H. Traeger a, Marcus H. Mendenhall c, WeiWei Clark b,
and Charles A. Brau b
a
Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Nashville,
TN 37232
b
c
Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235
W. M. Keck Foundation Free-Electron Laser Center, Vanderbilt University, Nashville, TN 37235
ABSTRACT
Sustained/long duration X-ray output has been demonstrated emanating from the monochromatic X-ray beamline of
the Vanderbilt Free-Electron Laser. Tunable, pulsed monochromatic X-rays ranging in energy from 14 - 18 keV are produced
by inverse Compton scattering created by the counter propagation of the FEL e-beam and its own infrared beam. These
beams are focused and optimized at an interaction zone between the linac and the wiggler where they are brought into
coalignment. The X-rays produced exit the beamline through a beryllium window and are directed onto mosaic crystals
which divert the beam to an imaging laboratory on the floor above the vault.
The initial application of these X-rays is directed toward human imaging, specifically for the diagnosis of breast
diseases including cancer. The characteristics of the X-rays are such that they can be used in standard geometry
monochromatic imaging, CT like images of the breast using a rotating mosaic crystal “optic”, time-of-flight imaging and
phase contrast images.
Eventual extension to other portions of the body, cell biology and material sciences are already anticipated.
Keywords: Monochromatic X-rays, Free-Electron Lasers, Breast Cancer, Inverse Compton Scattering
1. INTRODUCTION
Development of a source of pulsed, tunable, near-monochromatic X-rays deliverable in a beam geometry large
enough to encompass a sizable body part, would open the door to diagnostic imaging techniques heretofore unavailable.
The Free-Electron Laser (FEL) can be configured to deliver just such a beam by use of inverse Compton scattering. The
X-rays are created by the counter propagation of the FEL electron beam and its own infrared beam producing photons with
energies from 14-18 keV.1,2
1.1 The free-electron laser
The W. M. Keck Foundation Free-electron Laser Center at Vanderbilt University has been producing tunable
infrared photons since 1991. In its original form the laser has been documented many times. 3,4,5 In 1993 and 1994, it was
reconfigured to facilitate this project, as well as to rebuild the infrared beam transport system. The basic design of the
machine was not altered, but several segments were moved. The FEL consists of a few basic elements: a radio frequency
power system, an electron gun, a linear accelerator, an electron beam transport system, a wiggler, an electron beam dump, and
an infrared beam transport system. The power system consists of many elements, the most important being the klystron, the
thyratron, the pulse forming network, and the wave-guide. The RF power can be divided in any desired ratio, with the power
going to the gun and the linear accelerator.6 The facility is housed in a four-story building, with the laser itself in a vault set in
the underlying limestone. Separated by 2 meters of concrete is the second story, primarily for the control room and
experimental laboratories. The third story, added in 1995 along with the fourth, contains administrative rooms and two
experimental areas. The top floor is equipped for human surgery.
1.2 The electron gun
The electron beam is generated in the section known as the gun. The electron source is a cathode consisting of a LaB 6
button mounted on two rhenium wires. The electrons pass through an accelerating cavity, where they reach a kinetic energy
of about 1 MeV, and after passing through several focusing and steering magnets, they are bunched by an alpha magnet.
1.3 The linear accelerator
After leaving the gun, the electron beam passes through a linear accelerator which raises the electron beam energy to a
value between 25 and 43 MeV. The accelerator, about 3 meters long, is of the S-band type and is powered by the klystron.
1.4 The electron beam transport system
For normal FEL operations, the electrons are transported through a system of eight quadrupole and eight dipole magnets,
the last arranged in two chicanes of four magnets each. The present experiment uses two more pairs of quadrupoles, the first
pair of which focuses the electrons at the X-ray production point, denoted hereafter as the IZ. After the IZ, the electrons must
be transported around the X-ray transport system by the first chicane, refocused by the second pair of quadrupoles for this
experiment, and transported to the wiggler, being steered around the upstream mirror of the FEL optical cavity by the second
chicane.
1.5 The wiggler
The wiggler is an array of 53½ pairs of SmCo permanent magnets arranged in two parallel rows of alternating polarity,
with the top and bottom rows having opposite sequences. The distance between the rows can be changed by motors driven by
the computer in the control room. Over a small range of wavelength, the laser is tuned by varying the spacing between the
two rows of magnets, thereby changing the magnetic field seen by the electrons.
1.6 The optical system
Light produced in the wiggler is trapped by a system of mirrors before and after the wiggler. Infrared light of the desired
wavelength escapes from the cavity and is transported to the users. For normal operations the IR light is transported from the
vault to one of the laboratories above, but for this experiment the light is picked off and directed to an optical bench located
near the middle of the vault. Through a system of mirrors, the light is brought into a vacuum chamber and deflected by a
beryllium mirror so as to meet the focused electron beam head-on. 7
1.7 The dump
The dump is designed to transport the spent electron beam into the earth some 20 feet below. It has a 90º -bend dipole
magnet, which deflects the horizontal electron beam vertically through pipe to a copper target far below the vault floor. This
magnet also serves as a downstream chicane, deflecting the electrons around the downstream optics of the free-electron laser.
2. BEAM CHARACTERISTICS
2.1 The electron and infrared beams
The electron and infrared photon beams have a pulse structure, which determines the pulse structure of the X-rays. The
beams have a macropulse structure, which is further divided into a micropulse structure.
2.2 The electron beam time structure
The RF system is run with a frequency of 2.856 Gigahertz. The electrons are generated in micropulses, which are
separated by the period of the RF, about 350 ps. Autocorrelation measurements8 give a micropulse length of about 1 ps.
The micropulses are grouped into macropulses which are several microseconds long. The macropulses are produced with a
repetition rate of 10 to 30 Hz.
2.3 The IR beam time structure
The laser does not produce coherent light at the very beginning of a macropulse; while the electron beam is typically 6 or
7 µs, the IR pulse is 2 or 3 µs long, coming at the end of the electron beam pulse.
2.4 The fluxes of the beams
The electron beam pulses are characterized by the average current per macropulse, usually between 100 and 200
milliamperes. Because the accelerator is beam loaded, the maximum current is a function of the electron beam energy at a
given klystron output power. This FEL has produced 200 milliamperes at the 43 MeV beam energy required for this
experiment. The size of the electron beam is limited by its emittance, which is proportional to the product of the maximum
extent and the maximum divergence. As the beam is shrunk in the transverse dimensions, the divergence of the beam
increases and the X-ray beam loses its monochromaticity because photons produced near the beam axis actually come from
electrons with a range of angles from the axis. The fluxes of the beams are estimated by the FEL operating crew in units of
milliamperes averaged over the electron macro pulse and millijoules of photon energy averaged over the infrared macropulse.
Typical fluxes at the time of this experiment were 150 mA of electron current and 30 mJ of photon energy per macropulse.
The size of the electron beam is estimated by examining the spots produced by optical transition radiation as the electrons
pass through the beryllium screen. An image captured by frame grabber software yields the full width at half maximum from
which one calculates a radius. Until now the electron spot size has exceeded the design value by a factor of at least three.
The measurement of the photon beam size has so far yielded only an upper limit. The infrared beam struck an old IBM card
coated with Aerodag, and the hole burned in the carbon was 300 µ in diameter. This hole is considerably wider than the full
width at half-maximum.
In practice it is necessary for two reasons to accept electrons in a cone of half-angle c of a few milliradians
measured from the axis of the electron beam, producing a spectrum of X-ray energies. One reason is geometric: the element
of solid angle for such a cone, , is 0 at c = 0. The other reason is the divergence of the electron beam, which in this
experiment will be on the order of a few milliradians.
The IR photon pulses are characterized by the average energy per macropulse, measured in millijoules. The
maximum optical output is very wavelength-dependent, approaching 80 mJ at some wavelengths. At the wavelength which
originally produced monochromatic X-rays, 2.4 µ, the maximum macropulse energy has been as high as 30 mJ, but in the first
experiment varied between 15 and 22 mJ. Higher power can be produced when the focusing magnets for this experiment are
turned off.
3. CREATION OF THE X-RAYS
3.1 Inverse Compton scattering
Inverse Compton scattering in the regime of the W. M. Keck Free-electron Laser Center at Vanderbilt University
produces a nearly monochromatic beam of X-rays in a narrow cone of several milliradians. The term inverse Compton
scattering refers to photon scattering by an electron moving at relativistic speeds. Compton scattering as treated in elementary
texts is the process in which a photon scatters off an electron at rest, in which case the photon loses energy to the electron and
its wavelength is lengthened. In inverse Compton scattering, the electron is moving and gives up energy to the photon.
Figure 1 - Inverse Compton Scattering. Incoming electron (e-1 ) from the
accelerator “collides with” the IR photon, converting it to an X-ray photon which
follows a path almost collinear with the electron beam. The angle is
exaggerated here for clarity.
The inverse Compton scattering of a beam of low energy photons
backwards by an antiparallel beam of electrons has been used to produce a narrow beam of high energy photons. 9 . In the
case of scattering of the photon through 180°, its energy is increased by several orders of magnitude. The steps in the
backward scattering process are shown in Fig.1.
The production rate of X-rays by inverse Compton scattering is governed by two factors: the probability of scattering
an infrared photon by an electron, which depends on the cross section, and the intensities of the two, which is expressed as the
luminosity of the beams.
The first factor is obtained by integrating the differential cross section over the angular range of the narrow cone
(~ .005 rad) containing the high energy X-rays. The general solution of the photon-electron scattering yields the KleinNishina formulas, which, in the case that the photon energy in the electron rest frame is small compared to that of the electron
rest mass, reduce to the Thomson scattering formulas. The electron velocity is relativistic, characterized by  = 85, where  is
the ratio of the electron's energy to its mass.
In this experiment the shortest photon wavelengths are about 2µ, which correspond to an energy in the electron rest
system of .52 eV, small compared to mec2 of .511 MeV. The total Thomson cross section is given by
T 
8 2
re ,
3
where re is the classical electron radius.
2
Due to the relativistic electron motion, which has a Lorentz factor   Ee / me c , the scattering angle in the electron rest
frame is related to the half-angle of the X-ray cone in the laboratory frame by  S  2 C
The cross section for scattering into the forward cone is
  S

 re 2 (1  cos 2  S ) sin  S d S
For a half-angle of .005 rad, the cross section is .21 of the total Thomson cross section of .66 barn (=6.6×10-29m2).
As seen by the electron, the photon energy is increased by a factor of 2 to ~102 eV. This energy is so small compared to the
electron rest mass that the Compton shift of wavelength is negligible. The photon is scattered nearly elastically through some
angle S. Near S = 180º the energy of the scattered photon as seen in the laboratory system gains another factor of ~
2, reaching a maximum of ~17.9 keV.
The second factor is the luminosity, which for colliding beams is
L  N e  N  f / A ,
where Ne is the number of electrons per micropulse, N is the number of photons per micropulse, f the frequency of
micropulses, and A the area of overlap of the two beams. The area can be gotten by integrating the product of the Gaussian
distribution of the particles. If the two beams have the same size, the area is related to the width of the beams by
1
A   (2 ) 2 . For different radii, the area is A   (re 2  r 2 )
2
Table I: Typical figures for the FEL electron and IR beams
Electron Beam
Energy
Macropulse Duration
Micropulse Duration
Repetition Rate
Radius at IZ
Maximum current
Typical current
43 MeV
7 µs
1 ps
20-30 Hz
110 µ
200 mA
150 mA
IR Beam
Wavelength
Macropulse Duration
Micropulse Duration
Repetition Rate
Radius at IZ
Maximum energy at 2.4 µ
Typical energy at 2.4 µ
2.4 µ
2.5 µs
1 ps
20-30 Hz
150 µ
30 mJ
10-20 mJ
3.2 Counterpropagation of the beams
The electron beam of the FEL is focused to a diameter typically between 100 and 200 microns at a point designated
the IZ, which lies within the e-beam vacuum line, roughly at the halfway point between the linear accelerator and the wiggler.
Likewise the IR output of the FEL is intercepted in the vault, where it is simultaneously focused to the same diameter as the
electron beam and reinserted into the electron beam vacuum line. There the beams are counterpropagated against one another
at the IZ point.
These beams are brought into coalignment, first by visualization of the transition radiation produced by the electron
beam hitting a beryllium screen and secondly by focusing HeNe and diode lasers (that have been pre-aligned through the
wiggler cavity) onto an aluminum screen. The e-beam and IR screens are machined from a single Al plug, so that their
surfaces are at 90º to one another and centered to the electron beam using actuators in the X, Y and Z directions. (Fig. 2)
Both beams are observed through a common CaF window via a CCD TV camera with a remotely controlled and adjustable
zoom/focus/iris lens. Beam alignment takes approximately three minutes, following initialization and adjustment of the FEL
itself.
Figure 2 - The IZ screen. First, the “alignment
screen” assures centering of the device within the vacuum
beamline pipe. Next the “electron viewing screen” is used to
delineate the location, size and shape of the e-beam from the
transition radiation generated by the beam striking the screen.
Lastly, the “IR viewing screen” is used to steer the pointing
lasers to the e-beam’s center.
Timing of the arrival of the IR and e-beam pulses
at the IZ point is adjusted using an optical trombone.
Settings were determined at an earlier time by RF phase
Interferometry measurements using the same IZ screen.
Each of the beams was reflected in turn from the IZ screen
onto a 6 GHz visible/IR photoreceiver-Model 1537 (New
Focus, Inc. Santa Clara, CA).
3.3 The X-ray detector
An X-ray detector consisting of two thin silicon surface-barrier detectors was designed and built by one of the
authors (MM). It is placed outside of the beamline on the optical table adjacent to a .010 inch beryllium window used as an
exit port for the X-ray beam. These detectors are used as calorimeters, which are separated by an aluminum absorber. The
front detector sees both the intense high energy background radiation emanating from the FEL, plus the low energy X-rays
produced by the inverse Compton scattering. The rear detector sees only the high energy background. Subtraction of one
signal from the other using a balanced differential amplifier chain allows us to separate the signals and display the X-ray
signal as a time-resolved voltage overlying the timing signals generated by the e-beam and IR beam pulses (Fig. 3).
Figure 3. - Signals monitored during X-ray
production. The e-beam macropulse is shown as
a 7.7 µsec long downward deflection. The IR
pulse lasting approximately 3 µsec is seen
toward the end of the e-beam trace as a positive
amplitude. The X-ray beam traces (both
smoothed and unsmoothed data) are represented
by the last two signals, slightly delayed by
monitoring circuitry.
4. RESULTS
4.1 First generation of X-rays
With the FEL linear accelerator operating at 43 MeV and producing 20mJ/macropulse of 2.45 µ infrared light, the
monochromatic X-ray flux measured was 4.2 × 104 photons/second at the exit window of the beamline. The X-ray beam was
sustained initially for 90 minutes and extinguished deliberately. Subsequent attempts to create X-rays have been successful
with the FEL generating as little as 8 mJ/macropulse at similar operating parameters. The energy of the X-rays first generated
was 14.5 keV. These diverge at a half-angle of .005 rad from the IZ point and exhibit a pulse structure as seen in Figs. 3&4.
To confirm that the signal being displayed truly represents X-rays generated by the inverse Compton scattering
process, the IR beam can be blocked near the point at which it is introduced into the steering/focusing mirror system of the
inverse Compton beamline. By repeatedly and deliberately blocking the IR beam, one can easily turn the X-ray beam on and
off at will.
4.2 Sustained operation
Sustained production of hard X-rays has been reproducible on an hourly, day-to-day and week-to-week basis. The
stability and flux of the X-ray beam is directly related to the stability of the FEL itself. The beam geometry is that of a large
circular area beam at the imaging laboratory, but can be changed by use of variable mosaic crystal designs.
While the flux was originally less than that predicted, an improvement of one order of magnitude has been possible
with only minimal readjustment of various FEL parameters.
Figure 4 - Inverse Compton signal. Signal obtained by X-ray
detector results from about 2000 photons/micropulse.
5. DISCUSSION
Currently, standard X-ray tubes deliver a much
broader spectrum of radiation than what is either needed or
desired to make an image. Production of pulsed, tunable
monochromatic X-rays allows one to select a photon energy
best suited to the imaging task at hand. Monochromatic X-ray
imaging can simultaneously reduce radiation dose to a patient
and reduce scattered radiation from high energy photons not
needed for the image in the first place.
This can be useful in several ways. Cancerous breast tissues, for example, exhibit higher linear attenuation
characteristics than do normal tissues, when studied with monochromatic X-rays.10,11 This property can be exploited to
improve the sensitivity and specificity of breast imaging in a number of ways. The ability to alter the beam geometry makes it
ideal for imaging in humans as well as in materials science, molecular biology and cell biology. Standard geometry
monochromatic imaging, CT imaging using new X-ray optics made from mosaic crystals, phase contrast imaging, and timeof-flight imaging are just a few examples of their potential uses.
The monochromaticity and narrow divergence angle of this X-ray beam not only enables the mosaic crystals to divert
the beam from the vault to an imaging laboratory above the vault, but also allows the redirection of the beam in a circular
fashion creating CT images using conebeam backprojection algorithms.
The fact that the X-rays can be pulsed in 2 ps bursts would permit them to be used for time-of-flight imaging,14
where data is collected by imaging only ballistic photons up to 180 ps from the initiation of the exposure and ignoring scatter
exiting over many nanoseconds. This alone can improve conspicuity of lesions by ten times.
The small effective spot size of this X-ray beam enables the performance of phase contrast imaging using
information traditionally discarded in conventional imaging15 These improvements in imaging are not restricted to the breast
but apply to any body part and to materials science as well. The energy of the X-ray beam is not limited to 14-18 keV using
this technique but rather is restricted in these experiments to that range due to the large angle by which the beam must be
diverted by the mosaic crystals to exit the vault.12,13 Higher energy beams (up to 40-50 keV) are achievable using smaller
angles of reflection from the mosaics and using light of shorter wavelengths. All of these techniques can be effected while
reducing radiation dose to a patient and decreasing scatter due to the tunability of the beam and the limited bandwidth/narrow
energy range delivered to the imaged part.
Further development of small sources is currently underway in an effort to scale them to the size of a standard X-ray
room in a hospital or clinic.
ACKNOWLEDGMENTS
This work was supported by grants from The Office of Naval Research: ONR-N00014-94-1-1023 and by a grant
from the Eastman Kodak Corporation, Health Sciences Division, Rochester, New York. The Vanderbilt University W. M.
Keck Foundation Free-Electron Laser Center is supported by Vanderbilt University and by grants from the Office of Naval
Research, and the W. M. Keck Foundation. The authors would also like to acknowledge the excellent technical and
secretarial assistance of Ms. Barbara Sammons throughout this project.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
US patent #4,598,415. A.U. Luccio, B. A. Brill. “Method and apparatus for producing X-rays.” Dated Jul 1, 1986.
US Patent Office.
F.E. Carroll, J.W. Waters, R. R. Price, C.A. Brau, C.F. Roos, N. H. Tolk, D. R. Pickens, and W. H. Stephens. “Nearmonochromatic X-ray beams produced by the free-electron laser and Compton backscatter,” Invest Radiol. 25: pp.
465-471, 1990.
C.A. Brau, "The Vanderbilt university free-electron center," Nucl. Instrum. & Methods A 319, p.47, 1992.
C.A. Brau and M.H. Mendenhall, "The Vanderbilt university free-electron center”, Nucl. Instrum. & Methods A 331,
p.4, 1993.
C.A. Brau and M.H. Mendenhall, "Medical and materials research at the Vanderbilt university free-electron center,"
Nucl. Instrum. & Methods A 341, p.21, 1993.
J. M. Madey, “Stimulated emission of bremsstrahlung in a periodic magnetic field.” J. Appl. Phys. 42: pp. 19061913, 1971.
W. Dong, C.A. Brau, J.W. Waters, P.A. Tompkins, F. E. Carroll, R. R. Price, D. R. Pickens, and C. F. Roos,
“Current status of the VU MFEL Compton X-ray program” Journal of X-ray Science and Technology 4, pp. 346352, 1994.
F. Amirmahdi, C. Brau, M. Mendenhall, J.R. Engholm, and U. Happek, “Electron bunch length measurements at the
Vanderbilt FEL”, Nucl. Instrum & Methods A 375, p 427, 1996.
D.L. Burke, et al, "Positron production in multi-photon light-by-light scattering." Phys. Rev. Lett. 79: pp. 1626-1629,
1997.
P.C. Johns, M.J. Yaffe. “X-ray characterization of normal and neoplastic breast tissues.” Phys Med Biol 32: pp. 675695, 1987.
F.E. Carroll, J. W. Waters, W. W. Andrews, R. R. Price, D. R. Pickens, R. Willcott, P. Tompkins, C. Roos, D. Page,
G. Reed, A. Ueda, R. Bain, P. Wang, and M. Bassinger. “Attenuation of monochromatic X-rays by normal and
abnormal breast tissues.” Invest Radiol. 29: pp. 266-272, 1994.
C. J. Sparks. “Mosaic crystals for obtaining larger energy bands and high intensities from synchrotron radiation
sources.” SSRL Report No. 78/04, May 1978.
P.A. Tompkins. “Application of graphite mosaic monochromator crystals for X-ray transport.” Journal of X-ray
Science and Technology. 4: pp. 301-311, 1994.
C. L. Gordon, Y.Yin, B.E. Lemoff, P.M. Bell, and C.P.N. Barty. “Time-gated imaging with an ultrashort pulse,
laser-produced plasma X-ray source.” Optics Letters. Vol 20, No.9: pp. 1056-1058, May 1995.
T. Takeda, A. Momose, Y. Itai, J. Wu, and K. Hirano. “Phase-contrast imaging with synchrotron X-rays for
detecting cancer lesions. Preliminary investigation.” Acad Radiol 2: pp. 799-803, 1995.
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* Correspondence: Email: frank.carroll@mcmail.vanderbilt.edu ; Telephone 615-322-0864 ; FAX 615-322-3764