RARAF: Microbeams, Broad Beams, and Beyond

Andrew D. Harken, Guy Y. Garty, Malek Haj Tahar, Gerhard Randers-Pehrson, David J. Brenner Radiological Research Accelerator Facility 136 S. Broadway, PO BOX 21 Irvington, NY 10562 [email protected]

ABSTRACT

The Radiological Research Accelerator Facility (RARAF) at Columbia University is dedicated to the delivery of known quantities of to biological samples and the effects on these samples from the cellular to systemic level. Over the past 25 years, RARAF has been instrumental in the development of single-, single-particle ion microbeam irradiation systems. We have progressed from a pin hole aperture, to focusing electrostatic quadruplets, to a superconducting solenoid system with subsequent reduction in beam spot sizes through these developments. The RARAF facility has broad beam irradiation technologies that allow for large cell population and tissue irradiation simulation for comparison to the single cell irradiations from the microbeam. System intercomparisons allow for the elucidation of the effects of radiation from the cellular response to tissue response, ultimately, to systemic responses to radiation injury and treatments. We are developing a ‘heavy’ ion microbeam with future applications toward the simulation of heavy ion therapy using fixed field alternating gradient (FFAG) accelerators.

KEYWORDS microbeam, heavy-ion therapy, biological irradiation, FFAG

1. INTRODUCTION

The Radiological Research Accelerator Facility (RARAF) was founded in 1967 as a collaboration between Brookhaven National Labs and Columbia University [1]. RARAF, through the years, has developed many irradiation modalities to study the effects of radiation on biological samples. These modalities include , charged particles, and photons. The development of the microbeam facilities at RARAF have been ongoing for the past 25 years. These microbeams include charged particles, x-rays, UV and neutrons. Microbeams allow for sub-micrometer spatial and temporal placement of known quantities of radiation in a tissue. The RARAF microbeam has been used for cellular studies, tissue studies and small animal irradiation studies.

RARAF is based at the of Columbia University in Irvington, New York. We have approximately 10,000 square feet of laboratory space centered around our 5 MV Singletron Accelerator from High Voltage Engineering Europa (HVEE). Our laboratories are spread out across 3 floors and include full cell biology facilities as well as the beam lines and end stations of an accelerator facility. We are dedicated to and radiation effects on living organisms.

What is presented here is a review of the RARAF microbeam facilities with an emphasis on the charged particle microbeams, a comparison to our broad beam irradiation systems, and a discussion as to the future direction of the microbeam systems and irradiation technologies at RARAF.

AccApp '17, Quebec, Canada, July 31-August 4, 2017 13 2. MICROBEAMS

2.1 Charged Particle Microbeams

RARAF has designed, built and operated several variations of charged-particle microbeams: an aperture microbeam, an electrostatically focused microbeam, a permanent magnetic microbeam focused microbeam and a superconducting solenoid focused system. The primary use of the microbeam is the investigation of cell cultures where the delivery of particles to single cells is known, down to a single particle per cell. The microbeam also allows for studies where a certain percentage of the cells in culture, say 10%, are irradiated and guarantee that the remaining 90% do not receive any irradiation energy allowing for the basic study of non-targeted effects, i.e. bystander or abscopal effects. Each of the microbeams have their own characteristics and opportunities with their associated beam sizes in Table I.

Table I. RARAF microbeam systems and corresponding beam sizes

Type of microbeam Beam Size Aperture Microbeam 8 μm Permanent Magnet Quadrupole Triplet 5 μm Electrostatic Quadrupole Quadruplet 5 μm Electrostatic Double Quadrupole Triplet 0.6 μm Superconducting Solenoid 0.4 μm Superconducting Solenoid with Electrostatic Double Triplet 0.1 μm

2.1.1 Aperture microbeam

The aperture microbeam was our first microbeam. The aperture system collimated the beam using a 5 μm diameter collimator followed 300 μm above by a 6 μm anti-scatter collimator [2]. This provided a main beam (~92%) 6 μm in diameter with a ‘‘penumbra’’ (~7%) 8 μm in diameter due to particles scattered by the first collimator that were transmitted through the second collimator. The beam size was measured using the knife edge occlusion method we still use today [3].

2.1.2 Electrostatic quadrupole quadruplet

To eliminate the penumbra scattering and further decrease the beam size, RARAF designed and built a pair electrostatic focusing systems. The first focusing lens was an electrostatic quadrupole quadruplet design by Drs. Randers-Pehrson and Alexander Dymnikov [4] constructed of four ceramic rods with four electroplated gold sections with insulating separations as electrodes. The rods were held equally spaced from one another, clamped with v-blocks and precision ground spacers at the insulating sections, forming quadrupoles of the plated electrodes among the four rods. The lens was run with a ‘‘Russian’’ symmetry (potentials +A, –B, +B, –A) to produce a circular beam spot of minimum size. This system achieved a beam spot size of 5 μm.

AccApp '17, Quebec, Canada, July 31-August 4, 2017 14 2.1.3 Electrostatic double quadrupole triplet

Our second focused microbeam system was based on our electrostatic quadruplet system, but designed for sub-micron focusing [3]. Optics calculations for the design of a multiple quadrupole system to focus the beam to submicron size determined that two quadrupole triplets with a Russian symmetry (potentials +A, –B, +C, –C, +B, –A) would be required. This electrostatic double triplet quadrupole system consisted of two lenses of similar construction to the electrostatic quadrupole, but with only three elements per lens. The two lenses were separated by 2 m with the first lens providing the object for the second lens. The overall focusing system length from the object aperture to the focal point was just over 4 m. This system has a beam spot size on 0.6 μm.

2.1.4 Permanent magnet microbeam

Development of a microbeam using permanent magnets began in 2004 [5]. The focusing design was the same as for the electrostatic double triplet microbeam: a pair of permanent magnet quadrupole triplets where strengths of the permanent magnets themselves are fixed and adjustments in quadrupole field strength are made by moving magnets slightly in or out relative to the quadrupole center. This permanent magnet microbeam (PMM) system was originally designed to be used as a stand-alone microbeam with a radioactive alpha source (210Po) as the ion source, however, it became politically untenable to get the amounts of polonium needed for the ion source after the poisoning incident in London. The object aperture was redesigned and the PMM was attached to our Singletron beamlines as the source for He++ ions. This microbeam system has a focused beam spot size of 5 μm and is used primarily as a development end station for our microfluidics projects [6-8].

2.1.5 Superconducting solenoid microbeam

We are currently developing a microbeam based around a superconducting solenoid system for focusing our ion beams to ~100 nm. This system will use a combination of the double triplet electrostatic lens as the first elements for the solenoid focusing lens. The solenoid alone, as a first development step, has focusing capabilities of less than 0.4 μm. This microbeam will replace the double triplet electrostatic microbeam as the primary microbeam system at RARAF. The solenoid microbeam system is undergoing final testing and will be more fully reported on in a future manuscript which is in preparation.

2.2 X-ray Microbeam

A second type of microbeam that we have developed is an X-ray microbeam. This microbeam is designed using the principle of PIXE to generate soft x-rays [9]. Unlike electrons, protons generate relatively no bremsstrahlung and therefore an essentially monoenergetic X-ray beam is produced. A proton beam is focused by an electrostatic quadrupole quadruplet (from our previous microbeam incarnations) into an elliptical spot on a titanium target (4.5 keV Kα X rays) with an angled surface, which is embedded in a copper rod for cooling. X rays emitted in the vertical direction are focused using a 120-μm diameter zone plate to a 5 μm diameter beam spot determined, as for the charged particle microbeams, using a knife- edge occlusion method; however, in this case, only the amount of transmitted beam is measured as the occluded beam is absorbed. A beam current of 2 μA on the target yields an X-ray dose rate of 0.10 mGy/s (approximately 10 photons absorbed/targeted /sec).

2.3 Microbeam

In 2009, RARAF began development on a neutron microbeam, which is particularly difficult since neutrons have no charge and therefore cannot be focused and neutron collimation is limited to much larger beams due to scattering. Near the threshold for the Li(p,n)7Be reaction at 1.881 MeV incident

AccApp '17, Quebec, Canada, July 31-August 4, 2017 15 proton energy, low-energy neutrons (~60 keV) are emitted in a very narrow forward angular distribution. At the back of a very thin lithium target, the diameter of this cone can be tens of microns [10].

A quadrupole quadruplet focuses the 1.886 MeV proton beam onto a gold foil 20 μm thick with a 1 μm thick coating of 7LiF. In 2013 the proton beam was focused to a 10 μm diameter and a 36 μm diameter neutron beam was measured. This is the first neutron microbeam in the world. The neutron dose rate provided by the 10 nA proton current is estimated to be 270 mGy/min.

3. BROAD BEAM SYSTEMS

3.1 Track Segment Irradiator

In 1976 the ‘‘track segment’’ facility was developed by Colvett and Rohrig to irradiate cell monolayers with charged particles having a narrowly defined LET [11]. The ion beam is defocused, passes through a slit-shaped collimator, exits the vacuum system through a thin metallic window and penetrates thin samples plated on dishes with 6-lm thick Mylar surfaces. The dishes are rotated across the beam for uniformity. By varying the ion energy and type LETs from 10 to 180 keV/μm can be achieved with a wide range of dose rates. Three dosimeters are used: a proportional counter to determine the average LET, an ion chamber to measure the dose rate and a solid-state detector that is movable to determine dose uniformity along the length of the beam. This facility is very versatile. It has been in use essentially without modification since its initial construction and continues to be used frequently even today [1].

In addition to cell cultures [12], the facility has been used to irradiate spores, thin layers of DNA in solution [13], 3D artificial tissue [14], TLDs [15], track etch dosimeters, and microchips. One result of irradiations using this facility was the measurement of oncogenic cell transformation as a function of LET [16, 17].

Although the ions are delivered randomly, the facility is used for some of the ‘‘bystander’’ irradiations in which only a fraction of the cells are irradiated and the effect on unirradiated cells is examined [18]. A dish with narrow trips of Mylar 38 μm thick is inserted into a standard cell dish and cells are plated on the combination. Cells on the 6-μm thick surface are irradiated with 4He ions; those on the thick Mylar are unirradiated since the total Mylar thickness exceeds the range of the ions. This is particularly useful for low-yield experiments that require many more cells than can be readily irradiated with the microbeam.

3.2 FLASH Irradiator

We have recently designed and built a broad beam irradiation system that can irradiate a spot 1 cm in diameter with a therapeutic dose of protons (~20 Gy at 5 MeV) in less than 50 msec. This system was prompted by work with fractionation in therapy. There is evidence that hypofactionation, the giving of fewer fractions at higher doses, present fewer long term effects such at pneumonitis and fibrosis, especially in the treatment of lung [19]. This broad beam system is used to irradiate model skin and tissue systems to look for effects from millisecond fractions up to multiple low dose fractions simulating therapy settings. We work with tissue development programs and companies to generate tissues of interest that can be cultured for the longer terms needed for these late effects to be measured. This system is on-line and available for users.

3.3 Monoenergetic Neutrons

The original experiment for RARAF was the design and construction of a quasi-monoenergetic neutron source for neutron microdosimetry and biological effects [20-24]. This system uses targets of tritium or deuterium absorbed into a thin titanium layer which is then irradiated with protons, deuterons, or Helium-

AccApp '17, Quebec, Canada, July 31-August 4, 2017 16 3 to generate neutrons through nuclear reactions. These neutrons range in energy from a few tens of eV up to 14 MeV. The energy is quasi-monoenergetically chosen by selection the angle of the irradiation sample to the incident beam on target. The angle subtended by the sample gives the energy range of the possible neutrons coming out of the reactions in that direction. The lowest energies are at a back angler (up to 120 degrees) while the highest energies are at a forward angle. This system has been in continuous use for decades in microdosimetry, biological irradiations, and physics calibration experiments [25-27].

3.4 IND-Neutron Spectrum Irradiator

Development was begun in 2011 by RARAF of our Columbia IND-Neutron Facility (CINF), a fast neutron source with a broad energy spectrum which extends to 10 MeV and emulates that of the ‘‘Little Boy’’ atomic bomb at Hiroshima 1.5 km from ground zero [28]. A mixed beam of 5 MeV monatomic, diatomic and triatomic protons and deuterons is incident on a thick beryllium target producing neutrons from the 9Be(d,n)10B and 9Be(p,n)9B reactions. The diatomic and triatomic particles break up on contact with the target into individual ions with 2.5 and 1.67 MeV energies, respectively, enhancing the lower- energy portion of the spectrum. To produce this mixed beam a gas source with a ratio of hydrogen to deuterium of 1:2 is used in the Singletron RF ion source.

Neutron spectroscopy was performed and verified that the neutron spectrum created is a close match to that at Hiroshima [28]. Mice and human blood samples are placed in tubes and mounted on a ‘‘Ferris wheel’’ that rotates around the target to average out variations in dose rate around the target and to create a multi-lateral uniform irradiation.

4. RARAF FUTURE DIRECTIONS

4.1 Heavy(er) Ions

Our Singletron accelerator uses an RF plasma ion source that strips the electrons off a source gas and feeds them into the acceleration column. This system is limited in the capability of stripping electrons off heavier elements, such as Carbon. The RF plasma generator does not have the power to fully strip these ions, especially their most inner shells. We have recently purchased an electron beam ion trap system from DREEBIT in Dresden, Germany. This system, through a combination of magnetic and electrostatic trapping, uses an electron beam to fully strip gases of interest of their electrons. This allows for a range of different ions (C6+, N7+, Ne10+, O8+, B5+, Li3+) to be generated to study their effects on biological systems through our microbeams. As the interest in carbon ion therapy ramps up worldwide, we are interested in studying the underlying biological mechanisms of this type of ions. The current RARAF accelerator will allow the study of biological effects at the end of the ion track ranges (final 50 μm) seen in therapy. This system is in the final testing stages and heavy ions will be available to users in 2018.

4.2 Linac Booster

The RARAF Singletron will give fully stripped heavy ions an energy of 2.5 MeV/amu. This will give us sufficient range to do cell studies with these ions. For further studies into tissues and small animals, we will need higher particle energies. We will be purchasing a 2.5 m long IH-mode DTL Linac accelerator that will boost our energies to 6 Mev/amu on fully stripped ions giving us a range in tissue of 1 mm with deuterons and 200 μm with C6+ ions. These ranges will allow us to work on 3D tissue systems such as skin models, organoids, tumor spheres and small animal window systems. We plan on this installation happening in late 2019.

AccApp '17, Quebec, Canada, July 31-August 4, 2017 17 4.3 Racetrack Accelerator

The future for RARAF will be directed to the development of an accelerator system that can be used as a pre-clinical tests system for the irradiation of small animals using therapy methods for targeting and irradiation. The system in design is a fixed field alternating gradient (FFAG) racetrack accelerator [29]. The FFAG racetrack systems can be designed with overlapping beam paths on both sides allowing for injection and extraction of the beam at the desired number of acceleration steps for the chosen particle energy. We are designing the FFAG system so that, using our Singletron-Linac system as the injector at 6 MeV/amu, it will boost our C6+ particle energy to 60 MeV/amu, resulting in more than 1 cm of range for therapy simulations on mice. The variable extraction will allow us to tailor energies as needed to scan the irradiation volume without the use of degraders.

The new FFAG system will enable RARAF to perform pre-clinical heavy ion therapy mouse studies based on current human therapy protocols. We also will use the small animal irradiation system as a test bed for the enlargement of the FFAG system as a human therapy accelerator system. We will explore the full range of ions available to us from our DREEBIT ion source to research the effects from different ions between helium and carbon to look at effectiveness from ion mass and LET on therapy outcomes. If ions lighter than carbon but heavier than protons are as effective as carbon ion therapy, this would lead to smaller, cheaper accelerators for therapy needs making heavier ion therapy more accessible.

5. CONCLUSIONS

The Radiological Research Accelerator Facility (RARAF) has been dedicated to the irradiation of biological samples for 50 years. We continue to work on the development of our microbeam and broad beam systems to elucidate the effects of particles and photons on biological systems with a specific emphasis on cancer causes and therapies. We envision a future for RARAF as a pre-clinical heavy ion therapy research center that will enable the continued development of promise shown by heavy ion therapy for the effective treatment of currently therapeutic resistant cancers.

ACKNOWLEDGMENTS

This work was supported by the National Institutes of Health (NIH) through the Nation Institute of Biomedical Imaging and Bioengineering (NIBIB) grant 5 P41 EB002033. These views are the authors alone and do not represent the views of NIH or NIBIB.

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