JP2001116845A - Scintillation detector for reducing background radiation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the influence of a background radiation caused by β-rays from an incident face of a photomultiplier. SOLUTION: A β-ray absorber 3 transparent sufficiently for a scintillation- emitted light wavelength and for absorbing the β-rays from the incident face 2 is arranged between a scintillator 4 and the incident face 2 of the photomultiplier 1, in a scintillation detector for amplifying fluorescence by a radiation in the scintillator 4 by the one or plural photomultiplier 1 to be detected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a scintillation detector for reducing background radiation due to β-rays from an incident surface of a photomultiplier tube.

[0002]

2. Description of the Related Art Conventionally, in radiation measurement, a counting value is obtained by a measuring instrument, and the value measures the radioactivity of the background (BG) at the same time as the net radioactivity of the measured object. Before and after, the background count must be measured and subtracted from the count value, but when measuring a trace amount of radioactivity, it is important to reduce the influence of background radiation as much as possible.

[0003] Generally, the origin of the background is radiation such as radiation from cosmic rays or natural radioactivity, such as radiation from the soil, radiation from structures such as buildings, and radiation from dust contained in the air. Other examples include radiation from members constituting the radiation measuring instrument and shielding materials.

In the case of a scintillation detector, a photomultiplier (photomultiplier) is generally used.
The incident surface of the photomultiplier tube (photocathode, or glass that is used as an entrance window material), the contains ⁴⁰ K traces. For example, in a Na (Tl) scintillation detector,
Gamma rays (1.461 MeV, emission rate 11%) emitted from ⁴⁰ K may be a problem. However, since the energy is high and the transmission power is strong, the material of the incident surface of the photomultiplier tube is actually ⁴⁰ There is no effective countermeasure other than the means of changing to quartz glass having a low K content. In this case, Na (Tl) scintillator is tightly sealed because of the deliquescence, since the contact surface of the photomultiplier tube is also contacted via (such as Pyrex) glasses, released from ⁴⁰ K Attention has been paid only to the γ-ray component to be performed.

On the other hand, like a plastic scintillator,
Used, for example, in direct contact with the photomultiplier tube entrance surface.
Scintillation using a scintillator
In the detector, the β-ray from the entrance surface of the photomultiplier tube
Background radiation ( ⁴⁰K-1.314MeV, emission rate
89%) was larger than expected. However
However, conventionally, β-rays from the incident surface of this photomultiplier tube
The effects of background radiation due to
Was not affected by this background radiation
No attempt has been made to reduce this.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional scintillation detector, in which background radiation due to β-rays from the incident surface of the photomultiplier tube was not considered at all. It is an object of the present invention to provide a new scintillation detector which can reduce the influence of background radiation.

[0007]

SUMMARY OF THE INVENTION An object of the present invention, which has been made to solve the above-mentioned problems, is to provide a scintillation detector for amplifying and detecting fluorescence due to radiation in a scintillator with one or more photomultiplier tubes. In the vessel, between the scintillator and the incident surface of the photomultiplier tube, a β-ray absorber that is sufficiently transparent to the scintillation emission wavelength and absorbs β-rays from the incident surface may be arranged. It is a feature.

That is, the inventors of the present invention provide a photomultiplier in a scintillation detector, particularly a scintillation detector using a plastic scintillator which can be used in direct contact with the incident surface of a photomultiplier tube. Since the effect of background radiation due to β-rays from the entrance surface of the tube is larger than expected, in order to prevent this effect, an appropriate thickness of scintillation emission light is placed between the entrance surface of the photomultiplier tube and the scintillator. Β-ray absorber sufficiently transparent to the wavelength (for example, a plate or sheet of acrylic resin, Teflon, nylon, polyester, or the like, or
Glass with a low content of ⁴⁰ K, such as quartz glass or Pyrex glass), and β from the entrance surface of the photomultiplier tube.
It has been found that it is extremely effective to prevent the background radiation due to the rays from reaching the scintillator, and the present invention has been completed.

Incidentally, conventionally, an acrylic plate has been inserted between the scintillator and the incident surface of the photomultiplier tube. This is because the scintillator and the incident surface of the photomultiplier tube having different sizes and shapes are different from each other. The purpose is to provide a light guide for optical connection or mechanical support when the scintillator is extremely thin, and to reduce background radiation due to β-rays from the incident surface of the photomultiplier tube. It was not something.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a side view showing an essential part of an example of the scintillation detector of the present invention, and FIG. 2 is a graph showing the relationship between the thickness of the β-ray absorber and the background (BG) count.

In FIG. 1, reference numeral 1 denotes a photomultiplier tube (photomultiplier) of the scintillation detector of the present invention, 2 denotes an incident surface of the photomultiplier tube 1, and 3 denotes a β-ray disposed in front of the incident surface 2. The absorber 4 is a scintillator arranged in front of the β-ray absorber. Although one photomultiplier tube 1 is used here, two or more photomultiplier tubes, for example, a plurality of photomultiplier tubes such as a coincidence method may be used.

Although the plastic scintillator is used here as the scintillator 4, the scintillator 4 is not limited to the plastic scintillator, and is chemically stable in a normal atmosphere and is similar to the plastic scintillator. Any scintillator that can directly contact the incident surface 2 can be used. Incidentally, the plastic scintillator has a very long product life (semi-permanent) and can be suitably used.

Next, the β-ray absorber 3 has sufficient transparency to the emission wavelength due to the scintillation in the scintillator 4, and also absorbs the β-ray as the background radiation from the incident surface 2. As a material satisfying such a condition, a plate or a sheet of acrylic resin, Teflon, nylon, polyester, or the like, or a glass having a low content of ⁴⁰ K, such as quartz glass or Pyrex glass, may be used. Here, an acrylic plate having high transparency is used as the β-ray absorber 3.

The thickness of the β-ray absorber 3 may be 1 mm or more. However, in order to sufficiently reduce the background radiation, it must be 2 mm or more, preferably 3 mm or more, as described later.

The incident surface 2 of the photomultiplier tube 1 and the surface of the β-ray absorber 3, and the surface of the β-ray absorber 3 and the surface of the scintillator 4 are arranged with some space even if they are in direct contact with each other. May be.

An example of the scintillation detector of the present invention is as described above. The background (BG) count when the thickness of the β-ray absorber 3 in the detector is changed is shown in FIG.
Shown in The graph of FIG. 2 shows the results obtained by measuring the background count by exchanging three tubes to see the individual differences of the photomultiplier tubes. In the graph of FIG. 2, the horizontal axis represents the thickness (mm) of the acrylic plate as the β-ray absorber 3, and the vertical axis represents the background count (cpm). Among them, the photomultiplier 1 is indicated by a wavy line, the photomultiplier 2 is indicated by a solid line, and the photomultiplier 3 is indicated by a one-dot chain line.

Usually, a plastic scintillator (diameter 2)
inch (50.8 mm) x 1 mm thickness) and the incidence surface of the photomultiplier tube, when an acrylic plate (β-ray absorber) is not inserted, counting with background radiation of about 100 cpm in a normal atmosphere Although the rate is recognized, the background count rate is reduced to about 30 cpm, which is about 1/3 or less, by inserting an acrylic plate with a thickness of about 3 mm between the plastic scintillator and the incident surface 2 of the photomultiplier tube 1. (See FIG. 2).

[0018]

The present invention is as described above. According to the scintillation detector of the present invention, a β-ray absorber such as acrylic resin is disposed between the scintillator and the incident surface of the photomultiplier. The effect of background radiation due to β-rays from the incident surface of the photomultiplier tube can be reduced, and a special effect that a trace amount of radioactivity can be measured more accurately can be obtained.

[0019]

[Brief description of the drawings]

FIG. 1 is a side view showing a main part of an example of a scintillation detector of the present invention.

FIG. 2 Thickness and background (BG) of β-ray absorber
7 is a graph showing the relationship between counting.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photomultiplier tube 2 Incident surface 3 β-ray absorber 4 Scintillator

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroshi Nakahara 1642 Kumakawa, Fussa-shi, Tokyo F-term (reference) 2G088 EE06 FF04 FF05 GG11 GG18 JJ08 JJ09 JJ30 LL02 LL06 LL08 LL11

Claims (4)

[Claims] 1. A scintillation detector for amplifying and detecting fluorescence due to radiation in a scintillator with one or more photomultiplier tubes, wherein a scintillation emission wavelength is provided between the scintillator and an incident surface of the photomultiplier tube. A scintillation detector for reducing background radiation, wherein a β-ray absorber that absorbs β-rays from the incident surface is arranged sufficiently. 2. The β-ray absorber may be a plate or sheet of acrylic resin, Teflon, nylon, polyester, or the like, or
2. A scintillation detector for reducing background radiation according to claim 1, wherein one of glass having a low content of ⁴⁰ K such as quartz glass or Pyrex glass is used. 3. The scintillation detector for reducing background radiation according to claim 1, wherein the scintillator is chemically stable in a normal atmosphere. 4. The scintillation detector according to claim 1, wherein the scintillator is a plastic scintillator.