Electron-beam testing station for detectors
The direct electron beam of the ELBE accelerator may be coupled out to air through a thin beryllium exit window and then used for detector tests:
In order to meet a broad range of user requirements the ELBE accelerator provides a unique combination of variability in time structure, intensity and applicable electron number. Three operation modes of the ELBE accelerator can be used.
Photo: 2 m long MRPC detector at the ELBE timing detector test station.
The most important features of the ELBE detector testing facility are listed in the following table.
|Electron beam energy||All||Ekin ≤ 31 MeV||This energy is near the minimum of ionization for several typical detector materials (freon RF-134 and vinyl toluene based plastic scintillators, e.g.)|
|Number of bunches / s||General (cw)||
26 MHz / 2n
(n = 1...7)
|The repetition rate of 13 MHz is downscalable as needed.|
|Single-pulse||1 - 1.3∙107||In case of bunch trains, the bunch repetition rate within the train can be set a for the general mode.|
|Single-electron||1 - 105||The repetition rate of single electron bunches can be set as for the single-pulse mode.|
|Number of electrons / bunch||General (cw)||
|The maximum permissible beam intensity in the detector testing electron beam line (0.1 mA) is lower than the maximum beam intensity of ELBE (1.6 mA) provided in other beam lines.|
~1 fC -
|The maximum bunch charge is limited by electron transport through detector testing beam line.|
1 - 20 electrons
|The number of electrons inside one bunch is limited to one or a few, as requested by the user. This allows to test a detector by just one or a very few minimum ionizing particles, if needed.|
|Jitter of timing reference||Single-electron||~35 ps||The radio-frequency signal of the accelerator (~2 ps) is provided as time reference inside the detector testing cave. The measured jitter of this signal provided to the user is tyically 30-35 ps.|
Some examples for recent timing detector tests are listed in the following:
- Test of high-rate resistant ceramic resistive plate chambers (RPCs) for their efficiency and timing resolution. The current world record for rate resistivity of a ceramic RPC was set at ELBE: Naumann et al. 2011
- Test of a multigap resistive plate chamber (MRPC) based neutron detector of 2 m x 0.5 m size. Using ELBE it was demonstrated that even such a large device including also massive steel converters can fulfill the usual excellent detection efficiency and time resolution characteristics: Yakorev et al. 2011, Elekes et al. 2013 .
- Test of a 2.7 m long plastic scintillator based neutron detector. This device is intended for the NeuLAND detector in the R3B setup at FAIR. Using ELBE, it was shown that even such long scintillator bars read out on both ends can read time resolutions of σ < 120 ps: Marko Röder, PhD thesis 2014.
Single-electron mode of ELBE
For timing detector tests, it is sometimes of benefit to inject only a very small amount of ionization into an ionization-based detector. This can be done exceedingly well using the single-electron mode of ELBE, where one or just a few electrons are accelerated in one bunch.
The plot shows the charge spectrum, as observed in a thin plastic scintillator serving as efficiency reference. Two different modes of operation of ELBE are shown, as controlled by the electron gun gate voltage:
Both the single-electron (bottom panel) and the multi-electron (top panel) modes of operation are available for detector tests.
The single-electron mode was patented (Kotte et al. 2010).
Single-user or parasitic use of ELBE
If just the use of the one electron per bunch capability is requested and trigger rates of 102 - 103 Hz are sufficient, the detector testing facility can be used in parasitic mode to other ELBE beam lines that usually require long running times for statistics, i.e. the nELBE and EPOS beam lines. This greatly facilitates the availability of beam time. In parasitic mode, some initial tests may even be scheduled on short notice, depending on the nELBE and EPOS beam time.
If instead several electrons per bunch are required, or if the rates to be tested should be in excess of 103 Hz, a dedicated beam time with the detector test as single user is required.
High-rate capability of RPC detectors tested at ELBE
Using the scalability of the electron flux in single-electron mode, ceramic RPC detectors that are capable of sustaining fluxes up to several times 105 electrons / cm2 s have been proven to show satisfactory efficiency and time resolution (Naumann et al. 2011).
This high rate capability represents the current world record for ceramic RPCs.
In addition to the timing detector tests discussed above, the unique combination of variability in time structure, intensity and applicable electron number of the ELBE accelerator allows a number of other detector tests. Some examples are listed in the following:
Recombination loss in gas- and liquid filled ionization chambers
Using the variability in time structure and intensity of the electron bunches in single-pulse mode, charge carrier recombination effects in pulsed radiation fields have been determined for gas-filled ionization chambers as radiation detectors. For short (~µs) pulses the existing theory on saturation correction was confirmed, whereas for longer pulses a reduced influence of ion recombination was observed (Karsch et al. 2011 Z Med Phys 21:4-10).
Absolute response of imaging plates
The dependence of the sensitivity of different imaging plates to picosecond electron bunches of varying charge of up to 60 pC was determined. No saturation effects have been detected, although the peak intensity exceeds values of previous studies by three orders of magnitude (Zeil et al. 2010 Rev Sci Instrum 81:013307).
New detectors for fast measurement of 3D dose distributions
A new detector based on scintillation blocks in combination with optical tomography has been characterized for enabling fast measurement of 3D dose distributions. The delivery of intense single electron bunches by the ELBE accelerator approximated the beam characteristics of new radiation sources (Kroll et al. 2013 Med Phys 40:082104).
Absolute charge calibration of scintillating screens
Charge calibration and linearity tests with high dynamic range have been performed for eight different scintillating screens typically used for the detection of relativistic electrons from laser-plasma accelerators (Buck et al. 2010 Rev Sci Instrum 81:033301).
Dose rate dependence of different detectors and dosimeters
Using the variability in time structure and intensity of the electron bunches in single-pulse mode, the dose rate dependence of four clinical dosimeters and two diamond detectors has been studied up to a maximum dose rate of 5∙1010 Gy/s (Karsch et al. 2012 Med Phys 39:2447-2455).
In addition to the direct electron beam, also intense high-energy bremsstrahlung from thick targets has already been used for testing. Some examples are listed in the following:
Performance tests for medical imaging detectors
The components for a Compton camera consisting of a CZT double sided strip detector and a pixelated scintillator intended to be used for monitoring proton irradiation of patients have been tested at ELBE. The bremsstrahlung spectra has similarities with the shape expected from prompt gamma rays in the clinical environment, the flux is bunched with the accelerator frequency. The charge sharing effect of the CZT detector has been studied qualitatively for different energy ranges. The LSO detecor pixel discrimination resolution has been analyzed (Hueso-Gonzales et al. 2014 JINST 9 P05002).
Proving the feasibility of in-beam PET at hard photon beams
In-beam PET measurements during high energy photon phantom irradiation have been performed with pulsed 34 MV bremsstrahlung. The measurements have been conducted with a dedicated double head positron camera made of position sensitive BGO detectors. In an inhomogeneous target consiting of tissue equivalent materials and plastic a high contrast for materials differing in carbon or oxygen concentration more than 10% could be shown (Kluge et al. 2007 Phys. Med. Biol. 52 N467–N473).
Information for external users
Infrastructure and electronics provided
Internal and external users can benefit from a well-developed infrastructure for detector testing:
- Gas mixing system for three gases (usually freon R134a, SF6, and iso-butane) with bottle depot outside the experimental hall and tube and bubbler inside the experimental hall.
- Patch panel (10 x SHV, 10 x BNC) connecting experimental hall and counting room
- Cable-LAN internet access inside the experimental hall
- VME TDC (25 ps) and QDC (25 fC) can be provided upon request, GSI MBS (multi-branch system) DAQ with RIO IV available, FPGA available
- High voltage for detectors and setup can be provided, controllable from counting room
- Remote controlled movable column (up/down, right/left) to scan various areas of a detector.
- Video link to experimental hall can be provided upon request
- Kitchen with microwave oven for shift personnel
Points of contact for external users
The ELBE accelerator and its beam lines, including the electron beam detector testing station, can be accessed for scientific use based on proposals evaluated twice a year by the ELBE Scientific Advisory Committee. Prior to submitting a proposal, it is recommended to discuss the technical feasibility of a proposal with a scientific point of contact. Points of contact for the electron beam detector testing station include:
- For users from the AIDA and NUSTAR communities: D. Bemmerer
- For users from the CBM and HADES communities: L. Naumann
- For users from the medical and medical imaging communities: J. Pawelke, F. Fiedler
Also a completely different beam line of ELBE giving high energy bremsstrahlung and fast neutrons from a photo neutron source has already been used for detector tests. Please contact R. Schwengner for the bremsstrahlung facility or R. Beyer for the photo neutron source.