5.3. In-Beam Tests
5.3.3. Beam Diagnostics
In this section common methods for beam diagnostics and their qualification for COSY beam tests are briefly discussed.
Scintillation Counter
A very common device to measure particle rates is the scintillating counter. A particle passing through the active volume of the scintillator generates a small amount of light
that can be converted to an electrical signal, amplified, and the resulting signal can then be counted.
However, the following issues prevent the usage of scintillation counters for our single event upset tests.
High Rates Scintillation counters cannot be used at particle rates of above 105−106 particles per second and per active area of the scintillator. At such high particle rates the dead time of the scintillaton counter is longer than the time between two particles. The scintillation counter then approaches saturation and the counting rate reaches a plateau.
The measured particle rate is then less than the real particle rate.
To be able to achieve enough statistics, we aim for rather high particle rates of not less than 106s−1cm−2 (see section 5.3.2). This is already in the area, or even slightly above, where scintillators start to become unreliable.
Not only will they become unreliable, the scintillator will also show degradation effects caused by the high particle rate. Eventually, the device will be damaged.
Pulsed Beam On top of the required high rates comes another effect that is considerably vivid at COSY. Within one spill, the beam intensity is not constant but forms bunches.
Figure 5.10 shows the response of a scintillator mounted in COSY beam. The bunched structure of the beam is clearly visible.
Figure 5.10.: Oscilloscope measuring the response of an in-beam scintillator at COSY.
The bunched beam structure is clearly visi-ble, the cycle time is about 6.5 ms.
During ∼5/6 of the time there is no beam, and in consequence, during the remaining
∼1/6 the beam intensity is six times higher than it would be with a constant intensity beam. With such a temporal beam structure it is impossible to use a scintillator, already at much lower rates than we require for single event upset tests. A scintillator would mea-sure no beam for∼5/6of the time and operate completely in saturation during the remaining
∼1/6of the time period. This is especially del-icate because the counting rate is usually inter-preted as the average over a much longer time period than the cycle time of the bunch struc-ture. This misleadingly suggests that the
scin-tillator is not yet operated in saturation which would mean the measured particle rate is reliable. In reality however, during the relevant time window when particles are actually measured, the scintillator is operated completely in saturation, and the measurement is not reliable at all.
In consequence, a scintillator as particle rate monitor is not an option for single event upset tests at COSY.
Self-Developing Dosimetry Film
A method for dosimetry, especially popular in biological and in medical science, is to install self-developing dosimetry film [Int] in the radiation exposed environment. The film darkens when exposed to radiation. The level of darkening can be used to determine the amount of radiation it was exposed to. Knowing the time of exposition, the average particle rate can be calculated.
Because there is no online information available and because it is cumbersome to deter-mine the rate information from the darkening self-developing was not used for dosimetry film for the purpose of rate measurement.
However, it was very useful to determine position, opening angle, and shape of the beam by installing the film on the electronics in-beam. Figure 5.11 shows how self-devel-oping film is used during the 2012 beam test at COSY. It proved to be a very straight-forward, convenient and reliable method to “aim” the beam directly on the target and to choose a distance from the exit window where the beam is wide enough to fully cover the electronics to be tested, while it is still is focused enough to hit the target with high intensity.
Figure 5.11.: Usage of self-developing dosimetry film during 2012 beam test. The pic-ture on the left shows the exit window of the beam pipe in JESSICA cave. The picpic-ture in the middle shows the film directly mounted on the electronics setup. The picture on the right shows the widened beam at the end of the beam line.
Estimates from COSY Data
As already mentioned in section 4.3.2, COSY provides online information of the syn-chrotron status on their homepage. Figure 5.12 shows exemplary screenshots of the two relevant plots.
Figure 5.12(a) illustrates, in blue, the measured beam-current transformer signal and, in red, the calculated number of particles per spill. Shown in figure 5.12(b) is the history of approximately one day of “beam intensity”. “Beam intensity” means the number of particles in the synchrotron, sampled once per spill. The trigger time is determined by Beam Position Monitors (BPMs) [BBG+94].
Knowing the number of particles per spill and the duration of extraction, the particle rate can easily be calculated:
(a) The “Beam Current Transformer” (BCT) signal in blue, and, derived from BCT and synchrotron frequency, the number of par-ticles in red. The green value at the top refers to the sample value at position of the green triangle on the red curve.
(b) One day history of “intensity” (particles in the storage ring). The samples are taken from the red curve in figure 5.12(a) at time the Beam Po-sition Monitors (BPMs) [BBG+94] trigger, re-sulting in one sample per spill.
Figure 5.12.: Information of beam characteristics provided by COSY on their home-page: http://donald.cc.kfa-juelich.de/world. Figure 5.12(a) has been color-inverted and slightly optimized for better readability in print.
particle rate [1
s]=number of particles / extraction time [s]
This approach is not very accurate because it only takes into account the particles in the synchrotron and not the extracted particles reaching the experiment site. Potential par-ticle loss during extraction procedure is completely neglected. During the in-beam tests situations were observed that show signatures possibly related to particle loss during ex-traction. The particle rate at our experiment (measured by the SEU Counter approach described in section 3.3.1) continuously decreased over time while the measured number of particles from BCT signals remained constant. This effect might also be caused by a shift of the beam position.
However, COSY homepage information allows a valuable estimation of the particle rate (upper bound). This is e.g. be used to check for plausibility of the values measured by other beam diagnostics instruments.
In addition, the one-day-history of the intensity gives a very convenient overview of beam condition during the experiment. Interruptions of the beam are common, e.g. to grant access to the cave for adjusting the experimental setup.
Ionization Chamber
In higher radiation environments, a very common device for particle rate measurements is the ionization chamber. Unlike with scintillators, not a single signal per particle is detected but a flow of electric charge. A traversing particle ionizes the gas in the active
volume of the detector, the created ions are collected by a condenser and the resulting electric current is measured. The higher is the particle rate, the more ion pairs are created and the higher is the measured current.
Sven Löchner from GSI electronics department brought an elaborate setup for beam diagnostics to the COSY in-beam tests 2012 [LGWH13] and 2013 [LFG+14]. One amongst several components is an ionization chamber.
In 2013, the ionization chamber measured particle fluxes between 6·107cm−2·s−1and 6·107cm−2·s−1. Since the spill repetition rate is 22 seconds, but particles are extracted in only 7 of the 22 seconds and the ionization chamber only takes the 7 seconds into account, this translates to an average rate of∼(2−3)·107cm−2·s−1.
SEU Counter Approach
As already mentioned in section 4.3.2, the SEU Counter approach was used for particle rate measurements during the in-beam tests. A problem occurs when evaluating (blind) scrubbing. Scrubbing corrects SEUs and thus the number of SEUs does not accumulate.
As a result, a test that is comparing the situation when scrubbing is enabled to the situa-tion when scrubbing is disabled is not directly possible.
One solution is to perform a readback of the design before scrubbing the correct bits.
Thereby the number of SEUs can be counted. This has been done in [RBK+12]. The drawback is that the scrubbing cycle time is thereby more or less doubled, because it can be assumed that readback of the configuration lasts as long as writing the configuration.
For low SEU rates, this is not a problem. However, for high SEU rates, as expected (and required) for in-beam tests, this becomes relevant. In section 5.3.2 a SEFI was estimated every minute (for 107particles/s), according to equation 5.8 this means one SEU every sec-ond. A scrubbing cycle is approximately 80 milliseconds, only one order of magnitude faster as the SEU rate. When too many SEUs accumulate during a single scrubbing cycle, the chance for multi-bit upsets increases. For that reason readback of the FPGA configu-ration was not performed. Instead, blind scrubbing was used.
The setup for the in-beam test solves the problem of missing SEU counts by mounting a second board with an identical FPGA into the beam line. The setup combines the ad-vantages of fast scrubbing on the main board with precise measurement of SEUs on the counter board. Figure 5.13 shows the two-board setup.
It has to be cross checked if both boards indeed show the same SEU rate. The SEU rates on both boards may differ for the obvious reason that they might not be perfectly aligned in the beam line, i.e. one board might not be hit by full beam intensity, and hence shows less SEUs than the other.
There are also more complex reasons, like secondary particles that are created when beam particles pass through the first board then contribute to the particle rate in the second board. On the other hand, the material of the first board in the beam line might scatter a significant amount of beam particles out of the beam line, this would reduce the particle rate in the second board.
Figure 5.13: Two board setup used for the beam tests of section 6.3, here the 2012 in-stallation. With this trick it is possible to per-form SEU counting on one board while on the other board (DUT) the efficiency of blind scrubbing is evaluated. If executed on the same board, blind scrubbing would interfere with SEU counting.
Therefore, a comparison of SEU rates has been measured at the beginning of each run, see stepInit Readbackin figure 5.9. Both boards were left exposed to the beam for three minutes and the numbers of SEUs collected during this period in each board are recorded in the logfile.
In the main loop of the test procedure, only the SEU Counter board collected SEUs while the DUT board can safely execute scrubbing.
This chapter presents the results that are achieved within the scope of the thesis. For the sake of continuity, it is also organized in three sections covering the same topics as the chapters before, although the third section – results of the in-beam tests – is clearly dominating this chapter.
First, in section 6.1, the modular approach for the implementation of the GET4 read-out controller is presented briefly. Then, section 6.2 covers the results of the radiation miti-gation efforts that were not obtained during the in-beam tests. A particularly important achievement that is presented in this section is the very modest increase of resource con-sumption due toSelective TMR. However, since most of the results concerning radiation mitigation were achieved during in-beam tests, they are presented in section 6.3.