DK172585B1 - Determining density profile in plate-form material - moving two detectors in direction towards different measurement volumes to obtain scanning density profile, and both detectors are located on same side of endless plate - Google Patents

Abstract

Wood-based plates are particularly involved and X-rays or gamma-rays are used from a source located on one side of the plane. Two directionally determined detectors are used, both of which are directed towards a desired measurement volume. The two detectors can be moved so that they are directed towards different measurement volumes, whereby with a scanning a density profile can be procured. The two detectors are located on the same side of the endless plate moved forward longitudinally during measurement. They are on the same side as the source or can be positioned on the opposite side to the source or the one detector can be positioned on one side of the plate whilst the other detector is on the other side of the plate.

Description

in DK 172585 B1

The invention relates to a method for determining the density profile of a plate-shaped material, by means of X-rays from a source whose radiation is directed at the plate, and at least three detectors.

Such a detection is envisaged, for example, but not exclusively, used in the production of 5 wood-based sheets produced by gluing together wood particles of varying sizes, ie. plywood (made of single layers), OSB (Oriented Strand Boards, made of large strips of wood), particle board (made of a layered mat made of large and small chips usually separated into layers) and fibreboard made of wood fibers.

In making these sheets, the particles are applied to a thermosetting adhesive which is laid out or sprinkled on sheets or strips as a layered or homogeneous mat and then pressed / cured in a continuous or discontinuous heat process in a continuous or discontinuous process. typical properties of the finished sheet.

This is most evident in the density profile of the plate, ie. the density variation over the thickness, which is thus an indicator of both the operating state of the production plant and the application properties of the plate IS.

The density profile can be determined destructively on laboratory specimens, either gravimetrically or by layer milling and weighing, or by isotope transmission scanning on a specimen in the plane of the plate. Based on these results, the process can be adjusted, but with a time delay of at least 1-2 hours.

However, for a measurement system according to patent application No. 0723/94 supplemented with a detector for measuring the unimpaired radiation intensity of the x-ray tube, it is necessary to reduce the direct radiation intensity to the detectors if non-linear effects are to be avoided. At the same time, no or least possible attenuation of the far less intense scattered radiation is desired for one detector. In practice, therefore, it is not possible to select a combination of filters that act uniformly for the three detectors upon changes in the radiation energy spectrum.

The object of the invention is therefore to indicate how to prevent pre-filtration in connection with the detectors.

2 DK 172585 B1

According to the invention, a method of the kind mentioned initially is characterized in that the detectors are located outside the direct beam, two of the detectors being positioned to measure scattered radiation from reference plates interposed in the direct beam, respectively. above and below the plate-shaped material, and the third detector is positioned to measure scattered radiation from a desired sub-volume, and all detectors measure radiation scattered at substantially the same angle.

This allows all detectors to operate in pulse mode within the same dynamic range without 10 pre-filtering, while the energy spectra of the three detectors are almost identical.

Furthermore, according to the invention, a method for determining the density profile of plate-shaped material, the density of which varies discretely or continuously over the plate thickness, whereby the density at a certain depth of the plate is preferably assumed to be constant, including, for example, wood-based plates, by means of x-ray or gamma rays. a source located on one side of the plate is characterized by the use of two directional detectors, both pointing to a desired measurement volume, the two detectors being movable to be directed at different measurement volumes, thereby a scan can provide a density profile.

The invention will be explained in more detail below with reference to the drawing, in which 1 shows a measurement array with one detector; FIG. 2 shows a measurement array with two detectors, both pointing to the desired measurement volume; FIG. 3 shows a measurement array with one detector for measuring backward scattering.

FIG. 4 shows a measuring array with two detectors, both pointing to the desired measuring volume and used for measuring backward scattering, DK 172585 B1 3 fig. 5 shows a combined measurement arrangement FIG. 6 shows a measurement array for simultaneous measurement of forward and backward scattering by means of two detectors, the two detectors pointing to the same measuring point, and fig. 7 is an arrangement for obtaining the same spectral sensitivity in the three detectors

A measurement array for measuring backward scattering using only one detector is shown in FIG. 1. The scattered radiation from a volume element Vj at depth Xj detected by means of a detector F is given by

i-l) N

W *) = ^ oexP · Σ pP St exp -µ, · Δχα, Σ p, p = l) p = i

10 where the scattering factor S, = σ (θ, Ε) · p, · V, · eD

The desired density p, cannot be calculated without knowledge of the attenuation up to Xj and the attenuation from x {and out of the plate.

By scanning the detector F's detection direction from A to B, a density profile can be successfully obtained from a progressive knowledge of part-densities.

4 DK 172585 B1

If the neighboring point xi + 1 to κ · is considered the ratio exists.

(“° ° IFS (X,) W S, '' (2.a)

As mentioned in the previous patent application 0723/94, it is possible to select the angle of incidence and angle of departure such that μα = μχαν

For a constant intensity Ig of the incident radiation there is / Æ,) p ,, = p, (2-1- *) 5 The successful construction of the density profile according to formula 2.1 .a or 2.1 .b assumes that the intensity Ig is constant. and that the surface density of the plate does not change during the measurement. For X-ray tubes where Ig varies, a separate measurement of Ig must be made, and for non-stationary situations, the plate weight of the plate must be continuously measured and used to correct the measured values of IFS.

10 The density profile should not necessarily be provided from measured values in neighboring points.

Formula 2.1.b can be rewritten to

/ 'CO

p. = pref · -2 -! - (2.1.c) rtf l ^ ref) 5 DK 172585 B1 where the density of the measured volume element Pj is given in relation to a reference density pref.

Similarly, formula 2.1.a can be rewritten as (i / (X) p- · = 'exp (µα "µ! Αι>' ^ Σ Pp C2.1.d)) (P = i>

Formula 2.1.c with compensation for attenuation of the incident and scattered radiation leads to a variant of the method according to 0723/94. If the reference plate is permanently disposed in the incident beam (above or below the plate material whose density is desired to be measured) and if an additional detector continuously measures IFS (ref), while the scanning detector F measures IFS (Xi), then the density profile of formula 2.1.c be correct, regardless of any variations in the intensity Ig and the surface weight IFS (ref) thus replacing the transmission measurement according to 0723/94.

Optionally, the scanning detector F can be replaced by a detector array which performs simultaneous measurement of scattered radiation from several subvolume nes.

Optionally, a detector array can be used in conjunction with a powerful X-ray source.

FIG. 2 shows a measurement array with two detectors, both pointing to the desired measurement volume. It is possible to take into account variations in ^ and flatweight variations during a measurement by using two detectors that scan line AB but at two different scattering angles, all of which are always pointing to the same point on line AB.

The relationship between the two terms of type 2.1.a. for different scattering angles Eye and Eye are available 6 DK 172585 B1

Seeds, ι + Ι, Ο G0 + 1 · 0 = ·:, Λ = exP (^. “In 'µ2α2 ^ P, the ratio of release factors lo, scattering factors S; and damping factors βχρ (-μαΔχρί) being similar for Θ, and θ2.

Contrary to 2. l.a, (μ, α, - ^ a2) must be as large as possible for measurement sensitivity.

Immediately 2.2.a leads to a simple method for successively building the density profile, using ί'ίθ ,, ι '+ υΟ. . FXi + 1).

--- can be rewritten to -where

Seed + 2.0 FXO

F (0 = ——1—1i- and F (i + 1) correspondingly: P '= ln ^ y · j (Μιαι - (2-2-6) or I ^ ^)', 11 p '7 DK 172585 B1 where F (l) can be a measurement of a reference in the same measurement geometry, cf. formula 2.1.d.

However, the sensitivity is very small. Of formula 2.2.a. are found P, = toG / (μ1ο1 - μ2a2.) Δχ and

and 1 AG

Δρ = - · -

(µ1αι - ^ a2) Ax G

The expression (μ, α, - μ2α2) Δχ is at most 0.01 cm3 / g for Δχ = 1 mm. A relative error of 1% o on G therefore gives an uncertainty in Pj of at least 0.1 g / cm3, which is relatively high, since p 5 for wood or plastic based composite materials is of the order of 1 g / cm3. Furthermore, G represents count numbers in detectors from Compton scattered radiation, where the relevant error is rather 1-3% at realistic measurement times (seconds).

FIG. 3 shows a measurement array with one detector for measuring back-scattered radiation.

The scattered radiation from a volume element V (in depth Xs is given by i-1 µl IBS (X) = / „exp -μαΔχ Σ pp · p, exp -μ, α, Δχ Σ pp or l P = 1 J l P = l (il 'jbs (χ) =; o si e * P Δχ · Σ pp l P = 1 8 DK 172585 B1 where S, = σ (Ø.etø / ^

If the ratio of scattered radiation from natu elements is considered, IBÅXi + 1) BO '· * · 1.0 = —- =

W

= - εχρΚμα + μ, α ^ Δχρ,.) (3.1.α) Unlike the measurement geometry of forward scattering radiation, independence of the correction point (exponential factor) cannot be obtained but the term 3.1.a can be rewritten to IBJX.), _1 p! = Qref · exp (μα * μ <χ) Δχ Σ p (3.1.4)

{p = 1 J

5 In measurement situations where both sides of the sheet material are not accessible or where only surface properties are desired, the backscatter geometry is preferred.

FIG. Figure 4 shows a measurement array with two detectors, both pointing to the desired measurement volume and measuring back-scattering.

As before, variations in] and surface weight can be circumvented by using two detectors 10, both pointing to the same point on line AB, scanning along this line.

9 DK 172585 B1

You have the following 5 (6., / + 1.0 +, · = βχρ (- (μ, α, - μ2α2) Δχ · ρ) (3.2.a) 5 (0 ^, / +1.0) The measurement sensitivity expressed by (μ, α, - μ ^) are generally lower than for forward radiation, and therefore the measurement principle is not attractive, cf. Fig. 5.

FIG. Figure 6 shows a measurement array for simultaneous measurement of forward and backward estimation by means of two detectors, the two detectors pointing to the same measuring point.

Measured both forward and backward radiation are found by the terms 2.1 .a and 3.1 .a for neighboring elements ™. , „F (/ + 1.0 βχρΚμα-μ, α, ΙΔχ · p,) F5 (/ + l, l) = -— = --- = 5 (/ + 1.0 εχρ (- (μα + μ2α2 ) · Δχ · p,.) Βχρ ((μ, α, + μ2α2) · Δχ · ρ;) where α2 refers to backscatter geometry.

10 By rewriting to F (/ + l, Q _ ^^ ι «ι) t W _ F5Q + 1) 50 + 1.0 = / # i (1) ~ FB (i) 10 DK 172585 B1 exists ·> < "Fm.oi'a" * PA) ** or p <'(, n * ^) ΔΧ) - £ »r In relation to the use of two detectors for diffuse scattering or two detectors for backscattered radiation, the measurement sensitivity is typically 5- Variations in Iq are correctly compensated, but no flat-weight variations in the plate material 5 are measured during measurement, as previously mentioned, two detectors on both sides of the plate material that "see" the same sub-volume are required.

In practice, the determination of the density profile in measuring scattered radiation is possible only by using an X-ray tube as a source, the source having to be sufficiently strong.

The radiation from an X-ray tube is not monoenergic and the mean energy is increased by transmission through growing material layers. Furthermore, the spectral distribution and thus the mean energy fluctuate over time. Measured transmission and scattering intensities therefore depend on the instantaneous function, construction, materials and measurement object of the X-ray tube.

For wood and plastic composite materials of thicknesses of 10 to 50 mm, the power is low for 15 normally pre-filtered X-rays at 100 kW or more, while the power is more apparent for structural materials such as aluminum, iron or brass.

For a measurement system according to Patent Application No. 0723/94 supplemented with a detector TK for measuring the unimpaired radiation intensity of the X-ray tube, it is necessary to reduce the direct radiation intensity of the detectors T and TK if non-linear effects are to be avoided.

At the same time, no or at least possible attenuation of the much less intense scattered radiation is desired for the detector T. In practice, therefore, it is not possible to select a combination of filters that act uniformly for the three detectors by changes in the radiation's energy spectrum.

DK 172585 B1 π

However, according to the invention, a method has been provided for obtaining the same spectral sensitivity in the three detectors, and reference is made to FIG. 7, which shows a measurement array for obtaining the same spectral sensitivity in the three detectors required for the practice of the invention according to patent application No. 0723/94.

The detectors TK and T are both located outside the direct beam so that they only measure scattered radiation from two reference plates 5 interposed in the direct beam, 10 respectively above and below the plate-shaped material M. The detectors TK and T are placed so that they measure radiation scattered at the same angle Θ as the radiation from the plate-shaped material measured by the detector F.

This allows all detectors to operate in pulse mode within the same dynamic range without pre-filtering, while the energy spectra of the three detectors TK, T and F are almost identical.

15 There is a slight increase in the energy of the transmitted spectrum between the measurement object M and the two reference plates 5 and the necessary radiation windows 6 in the measuring array, but for measuring objects such as wood or plastic-based materials and for thin reference plates 5 and radiation windows of low atomic element elements. as mentioned earlier, the effect is minimal.

Claims (2)

A method for determining the density profile of a plate-shaped material M by means of x-rays from a source whose radiation is directed to the plate, and at least three detectors, characterized in that the detectors are located outside the direct beam of beam, two of the detectors T TK is positioned to measure scattered radiation from reference plates (5) interposed in the direct beam, respectively, above and below the plate-shaped material M, and the third detector F is positioned to measure scattered radiation from a desired subvolume, and all detectors measure radiation scattered below 10 at substantially the same angle (Fig. 7). A method according to claim 1 for determining the density profile of a plate-shaped material, the density of which varies discretely or continuously over the plate thickness, whereby the density at a certain depth of the plate is preferably assumed to be constant, including, for example, wood-based plates using X-rays or gamma rays. a source positioned on one side of the plate, characterized in that two directional detectors are used, both pointing to a desired measurement volume, the two detectors being movable to be directed at different measurement volumes, scanning can be provided a density profile. 20