A common procedure to evaluate the susceptibility of electronics with respect to radia-tion is to operate the electronics in a high-radiaradia-tion environment and measure radiaradia-tion effects under controlled conditions [Eng09, Rø09, Mar04, Xil14b, and many more]. Radia-tion environments are usually produced by either placing the device close to a radioactive source or by mounting it directly in the beam line of an accelerated particle beam.
Not all classes of radiation cause the same effects in electronics. SEU effects, for ex-ample, require a certain energy, and thus they are dominated by the level of high energy hadrons5(see section 2.2.2). The radiation environment has to be chosen according to the effect that one wants to evaluate. For example: using a beam of accelerated electrons to measure SEU effects would not work, because SEUs are hardly caused by electrons. The in-beam tests carried out for this thesis evaluate SEU effects. The beam particles were protons with a kinetic energy of about 2GeV.
For classification of measured SEU effects one needs to know the number of SEUs that occurred in the device under test. The number of SEUs in a device can be calculated from the number of beam particles and the SEU cross section.
no. of SEUs = no. of particles× device cross section (3.7)
= no. of particles× zbit cross section ×}|no. of bits in device{ (3.8) Mature techniques exist for measuring beam particle rates, many details can be found in literature, for example in [Kno00]. However, all of these techniques are hard to apply, an expert on-site is required for this task.
5For this reason, FLUKA simulations for CBM give also the flux of high energy hadrons [Sen11]. This allows for an estimation of SEU rates in the electronics.
For the SEU cross section, the situation is different. The SEU cross section depends on the particle type and on the energy of the particle. If the SEU cross section for the specific particle type at the given energy is not known, it has to be measured in a separate test. For such a test, it is very important to have precise knowledge about the internal structure of the device which is not always available. Fortunately, Xilinx publishes device specific bit cross sections for neutrons [Xil14b]. They give their results with the statement:
“Neutron cross sections are determined from LANSCE beam testing according to JESD89A/89-3A”. According to JESD89A, LANSCE neutrons are high energy neutrons (E > 10MeV) [JED06, pages 33 & 81].
The SEU cross section for protons and neutrons at higher energies do not differ much since only nucleus-nucleus collisions deposit enough energy in the silicon to induce SEUs. Figure 2.7 (page 28) shows that protons at high energies show only small vari-ations in their effective SEU cross section. For that reason the values published by Xilinx can be used directly to estimate the number of SEUs from the beam particle rate.
However, to be able to compare measurements under different conditions (different kind of particles, different spectrum of particle energies), normalization is required. A common method is to normalize the particle rate to 1MeVneq. 1MeVneqis the equivalent rate of neutrons with a kinetic energy of 1MeV that would cause the same radiation damage as the actual particle beam that was used for the test.
When testing electronics for radiation effects using a particle beam, it is standard pro-cedure to measure the particle flux and then calculate the SEU rate from the device cross section. Based on the SEU rate, the efficiency of radiation mitigation techniques can then be evaluated (see figure 3.5(a)).
Reliability of DUT SEU rate (beamtest)
exp. SEU rate (CBM)
exp. failure rate (CBM) In-Beam Test
DUT failure rate Dosimetry (ion chamber/
scintillator, ...)
Partile Flux (normalized to 1 MeV n )eq
Device Cross Section (measured by
Xilinx)
CBM Radiation Environment
(from Fluka simulations)
(a) Traditional flow
Reliability of DUT SEU rate
(beamtest)
exp. SEU rate (CBM)
exp. failure rate (CBM) In-Beam Test
DUT failure rate
Device Cross Section (measured by
Xilinx)
CBM Radiation Environment
(from Fluka simulations)
(b) SEU Counter flow
Figure 3.5.: Experiment flow to estimate failure rate of CBM detector (traditional and SEU Counter approach). The complicated and error-prone task of particle rate measure-ment is not needed when SEUs are directly counted.
3.3.1. SEU Counting
If only SEU effects are evaluated, the actual particle flux and the SEU cross section might not be required for classification of SEU effects. In many devices, SEUs can directly be counted by readback of the configuration memory. The number of SEUs sustained by
the device until it fails can be directly measured instead of getting it from complicated measurements and/or lengthy calculations. This has been done several times, see for example: [HSPG+08], [HDFC+11],[RBK+12], [Geb12].
Figure 3.5 shows a comparison between a traditional flow and the SEU Counter flow.
Instead of first measuring the particle rate and then calculating the SEU rate from that, which is a complicated and error-prone task, the SEU rate can be measured. With direct SEU counting it is still possible to evaluate the efficiency of radiation mitigation tech-niques.
Nevertheless, estimations based on device cross section and radiation environment are still required for determination of the expected failure rate of the device in the experi-ment, but this is a different task (see section 7.2).
Before going into implementation details in chapter 5, this chapter describes the approach for the implementation of a radiation mitigated CBM-ToF read-out controller and further-more the basic setup for in-beam testing of the mitigation efficiency is presented. As the previous chapter, this one is also organized in the three sections that represent the same three topics.
First, section 4.1 presents the GET4 read-out controller that follows the modular design approach of the CBM read-out controller family. The general advantages of this modular design approach are discussed. Then, in section 4.2, the choice of radiation mitigation techniques and the necessary adaptations for the techniques to fit for the CBM use case are discussed. Section 4.3 finally describes the basic ideas to verify the efficiency of the implemented techniques at in-beam tests.