Binary filters—also referred to as “degraders”—are devices designed to slow ions by a particular amount by passing them through a certain configurable amount of material. The binary filters at NSRL are constructed from many polyethylene sheets that can be pneumatically moved into or out of the path of the beam.
They are called “binary filters” because the thickness of each sheet increases in powers of two, where the next sheet is exactly twice the thickness of the previous one. This property allows the total thickness to be set to any value between 0 and the sum of the thickness of all of the sheets, with a step size of the thickness of the smallest sheet.
There are currently 3 binary filters in operation on the NSRL beamline, however only one of them—the “small” binary filter—is typically used for physics and electronics testing experiments.
Small Binary Filter
The small binary filter is predominantly used for physics and electronics testing experiments, and is an important part of the functionality of the SEE library.
The device is generally located just upstream of the electronics testing position and is designed for beam spots up to 20×20 cm2. The degrader material is polyethylene which has a density of 0.93 g/cm3.
There are a total of 7 remotely configurable sheets which have thicknesses of 0.1 mm, 0.2 mm, 0.4 mm, 0.8 mm, 1.6 mm, 3.2 mm, and 6.4 mm. This configuration the device to achieve any thickness between 0 and 12.7 mm in 0.1 mm steps.
The small binary filter also includes two additional sheets—a 12.8 mm thick sheet and a 25.6 mm sheet—which are too large to be driven with the pneumatic system but can be inserted or removed manually. The use of these sheets increases the total available thickness to 51.1 mm with the same resolution, albeit with the cost of needing to make an access to change settings. Generally these extra sheets are only used when measuring Bragg curves and not for normal operation.
The 5x speed video below shows the small binary filter in operation. Due to its compact geometry and the order of the sheets, it may be difficult to discern which sheets have been inserted.
The shape of the both the 7×7 cm2 and 20×20 cm2 beam spots can be seen “burned” into some of the sheets, appearing as a darker color than the surrounding polyethylene.
The small binary filter can be adjusted remotely and on the fly by NSRL operators at the request of users. When requesting a change to degrader settings, either the total thickness of polyethylene in millimeters, or the desired degraded energy, can be specified. In the latter case, the NSRL operator will calculate the total thickness needed to achieve that energy. Generally it takes only 3 or 4 seconds for the small binary filter to be reconfigured.
Large Binary Filter
The large binary filter is generally located at position in the middle of the rail system, downstream of the physics and electronics testing position. It is almost exclusively used for biology, but works on the same principle as the small binary filter for the same up to 20×20 cm2 beam format. Its use is generally not recommended for physics or electronics testing experiments.
The sheets are constructed out of high density polyethylene which has a thickness of 0.97 g/cm3.
The large filter has a coarser step size than the small filter but a much larger range of thicknesses. There are a total of 10 sheets with thicknesses of 0.25 mm, 0.5 mm, 1.0 mm, 2.0 mm, 4.0 mm, 8.0 mm, 16.0 mm, 32.0 mm, 64.0 mm, and 128.0 mm, allowing it to achieve thicknesses between 0 and 255.75 mm in 0.25 mm steps.
The 5x speed video below shows the large binary filter in operation.
Due to its more open construction, its operating principle is much simpler to ascertain through watching the video.
Because of its size, it is not usually possible to place the large filter upstream of the physics and electronics testing position on the beamline without moving the test article back behind the energy calibration position. Considerable time is needed to rearrange the equipment on the rails should that need to occur.
Moreover, for the heavy ions typically used in electronics testing, using a polyethylene thickness of much larger than 10 mm can cause significant fragmentation and straggling, which serves to broaden the LET distribution of the particles interacting with the test article. As such, there isn’t really any advantage to using the large binary filter as compared to using the small filter in conjunction with changes to the beam’s base energy.
Outside of biology, the large filter’s use should be limited to just the experiments that require the beam be passed through a large amount of material.
Nonstandard Setups
60×60 Binary Filter
The final binary filter, called the 60×60 filter is located at the furthest downstream position of the beamline, just before the end of the rail system. As the name suggests, it is designed to be used with the overfocused, 60×60 cm2 beams used for radiobiology. Its use is highly discouraged for physics and electronics testing experiments unless it is absolutely necessary.
The polyethylene used in the 60×60 filter has a density of 0.93 g/cm3.
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