The beta ray absorption method is a measurement method that utilizes the principle that beta rays are attenuated in proportion to the mass of a substance when the substance is irradiated with beta rays, a type of radiation.
Beta rays are high-speed electrons (charged particles) ejected by the decay of unstable atomic nuclei. When beta rays pass through a substance, they collide with the atoms of the substance, causing ionization and excitation (electron orbitals rise to a higher energy level) of the atoms with beta ray absorption and emission of electromagnetic waves (X-rays) with beta ray orbital change, resulting in attenuation of the intensity of the beta rays. This attenuation of beta rays is proportional to the mass (thickness) of the substance passing through.
The mass concentration＊1 of ambient particulate matter＊2 can be measured using this principle. The following sections describe the measurement of ambient particulate matter using the beta ray absorption method.
*1 : Mass concentration: Mass of particulate matter per unit volume of atmosphere. The unit is μg/m3.
*2 : Ambient particulate matter: Particulate matter suspended in the atmosphere.
Figure 1: Measuring principle of the mass concentration of ambient particulate matter by beta ray absorption
In the beta ray absorption method, ambient particulate matter is collected on a filter (filter paper).
The collected ambient particulate matter is irradiated with beta rays, and the intensity of the beta rays passed through the filter is measured. (Figure 1)
Beta rays have the property of attenuating exponentially with increasing mass (thickness) of the substance they penetrate. As a result, the mass of collected ambient particulate matter can be calculated using Equation 1. Since the mass absorption coefficient "μm" is almost constant, the mass of ambient particulate matter "Xm" on the filter can be obtained from the ratio of "I" and "I0".
Xm = ln(I0/I)/μm (Equation 1)
Xm: Mass of ambient particulate matter on the filter
I: Beta ray intensity passed through the filter and the collected ambient particulate matter
I0: Beta ray intensity passed through the filter only
μm: Mass absorption coefficient
Equation 1: Calculation of the mass of ambient particulate matter by beta ray absorption
The mass concentration of ambient particulate matter is calculated from "Xm" and the volume of sample ambient air when collecting the ambient particulate matter.
In the following descriptions, analyzers that measure the mass concentration of particulate matter in ambient air using the beta ray absorption method will be described simply as ambient particulate monitors or ambient dust monitors.
Figure 2: Structure of ambient particulate monitor
Overall structure of ambient particulate monitor
The ambient particulate monitor automatically measures the mass concentration using beta rays attenuated by continuously collected particulate matter on paper (filter). The ambient particulate monitor consists of an air inlet, a particle size separator (ex. impactor, cyclone), a filter collection mechanism, a beta ray source, a scintillation detector, a flow sensor, and a signal processing section. (Figure 2)
In the sampling section, the sample air flows to a particle size separator after passing through an air inlet that prevents large dusts, insects, and rain from entering the ambient air. The particle size separator sorts the particulate matter in the sample air according to the diameter of the particulate matter to be measured. The sorted particulate matter is collected on a filter at a constant flow rate by the filter collection mechanism.
Graph 1: Relationship between diameter of particulate matter and collection efficiency
Particle size separator
Ambient particulate matter has different behavior in the body and health effects depending on its size, so it is generally divided into the following three groups of particulate matter diameter.
PM2.5 (Particulate Matter 2.5): Commonly referred to as fine particulate matter. Fine particulate matter suspended in the air with the diameter of approximately 2.5 μm. Specifically, particulate matter with a diameter of 2.5 µm is collected at a collection efficiency of 50%.
PM10 (Particulate Matter 10) ： Fine particulate matter suspended in the air with the diameter of approximately 10 μm. Specifically, particulate matter with a diameter of 10 µm is collected at a collection efficiency of 50%.
SPM (Suspended Particulate Matter): Generally called suspended particulate matter. Particulate matter of 10 μm or less in diameter (different from PM10) that are suspended in the air.
The collection efficiency is an important performance indicator of a particle size separator and represents the percentage of particles of a given diameter that are collected. Graph 1 shows the relationship between the diameter particulate matter and collection efficiency.
For example, the 50% collection efficiency PM2.5 particulate size separator described above has the performance that if some 2.5 µm diameter particles are fed into the device, half of them will be collected and the other half will not. Particles other than PM2.5 are collected according to its own diameter (corresponding to the diameter of the aerodynamic particulate matter) and collection efficiency (corresponding to the content rate) on the PM2.5 curve (red curve) in Graph 1.
Typical types of particle size separators are impactors, cyclones, and multistage sieves. There is a correlation between diameter and mass for particulate matter and these particle size separators use a variety of forces for the mass of particulate matter to collect the particulate matter with a same diameter. The impactors use inertial force, cyclones use centrifugal force, and multistage sieves use gravitational settling. Impactors and cyclones are the primary particle size separators used to measure ambient particulate matter. Here is a brief principle of impactors (Figure 3) and cyclones (Figure 4).
Figure 3: Impactor structure and principle
The impactor uses inertial force to separate particles by mass. A sample air stream (blue arrow) generated through a rectangular nozzle hit on an impact plate ((blue arrow) and is changed direction (red arrow). Heavy particulate matter hits the impact plate and is collected on this plate. Light particulate matter flows downstream (red arrow) with the sample air.
Figure 4: Cyclone structure and principle
The cyclone uses centrifugal force to separate particles by mass. The sample air flow generated through the cyclone inlet becomes a swirling flow on the inner wall of the cyclone and is accelerated by the cone (blue arrow). The flow is then reversed and becomes a reversed upstream flow (red arrow). Particulate matter is given centrifugal force by the swirling flow, and due to the relationship between centrifugal force and drag force, heavy particulate matter moves along the inner wall of the cone and is collected in a grit pot. Light particulate matter is separated by the reversing upstream flow due to the relationship between centrifugal force and drag force.
Photo 1: Actual sampling section
HORIBA collects PM2.5 in ambient air by combining impactors and cyclones. (Photo 1)
Insects, large dusts and rain are removed at the air inlet, PM10 in the sample air is separated by the impactor, and PM2.5 is further separated and collected by the cyclone from the PM10 by the impactor.
Photo 2: Example of ambient particulate matter collected on a filter tape
The sample air containing the divided particulate matter by the particle size separator passes through a filter, and only the separated particulate matter is collected on the filter. (Photo 2) For continuous automatic measurement of particulate matter, it is necessary to have a mechanism that winds up a roll of tape-like filter (filter tape), or a mechanism that prepares several filters and changes them automatically. HORIBA uses the filter tape to collect perticulate matter in ambient air. (Photo 2)
The mesh size of the filter must be smaller than the selected diameter of the particulate matter and allow air to pass smoothly through the filter. The collection efficiency of particulate matter on the filter is changed by the mesh size of the filter. The collection efficiency of ambient particulate matter should be at least 99.7%. Filters are typically made of fiberglass or PolyTetraFluoroEthylene(PTFE)-based materials.
Photo 3: Filter tape
In addition, the material must be as thin as possible to minimize the absorption of beta rays in the filter itself. For example, the average thickness (film thickness) of HORIBA filter tapes is 140 μm. (Photo 3)
When the automatic filter tape rewinding mechanism is used, the collected particulate matter on the filter tape must not adhere to the back of the rewound filter tape, in case the collected particulate matter on the rewound filter tape needs to be analyzed again.
Beta ray source
HORIBA uses 14C* as a beta ray source, which is a safe sealed radiation source with a source strength of less than 10 MBq, and can be used without special handling qualifications or notification.
*14C is naturally occurring, and is also used in applications such as radiocarbon dating, due to its long half-life of 5,700 years.
Detector (scintillation detector)
A scintillation detector consists of a scintillator and a PMT (photomultiplier tube). The scintillator is a fluorescent material that absorbs radiation and emits light immediately. The beta rays that pass through the filter tape with the collected particulate matter enter the scintillator, emitting lights, and which are detected by the PMT. This PMT detection value is used in Equation 1 to calculate the mass of the collected particulate matter. The mass concentration (μg/m3) of the collected particulate matter is then calculated from the value of the calculated mass and the measured value of the flow sensor.
Photo 4: Standard attenuating film
Since it is difficult to check the sensitivity of the measurement section using the particulate matter actually collected on the filter tape, a thin film that attenuates beta rays equivalent to the particulate matter collected on the filter tape is used. This thin film (standard attenuating film) is made of mylar, polyimide, or other materials. (Photo 4)
Figure 5: Standard attenuating film to check the sensitivity of the measurement section
This standard attenuating film makes it easy to maintain the measurement accuracy of the measurement section by periodically checking the measurement sensitivity. (Figure 5)
The particulate matter in the air inside the monitor while waiting for measurement may adhere to the thin, electrically charged the PTFE-based filter tape. This can be reduced by using the filter tape with the lowest possible charge. HORIBA has developed a proprietary filter tape that combines PTFE-based materials and non-woven fabrics. Compared to PTFE-only filter tapes, this filter tape has lower hygroscopicity and lower electrostatic charge, thus reducing the factors that influence measurements. (Tables 1 and 2)
Table 1: Comparison of hygroscopicity for filter tapes
Table 2: Comparison of electrification for filter tapes
Ambient particulate matter is composed of various substances such as inorganic organic elements, and is classified into SPM, PM2.5, PM10, etc. according to diameter of the particulate matter. The smaller particulate matter, the more likely it is to penetrate deep into the body and cause respiratory diseases. The monitors using beta ray absorption method are used to measure the particulate matter in the ambient air in a variety of environments due to their ease of use.
PM sampling filter tape/filter
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