Patent Publication Number: US-2017360322-A1

Title: Magnetic field measuring apparatus and cell array

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a magnetic field measuring apparatus and a cell array. 
     2. Related Art 
     There is a known optical-pumping-type magnetic sensor (magnetic field measuring apparatus) of related art that optically measures a weak magnetic field emitted from the heart or the brain. The optical-pumping-type magnetic sensor includes a cell in which a gas, such as an alkali metal, is encapsulated. An alkali metal atom is characterized in that the alkali metal atom irradiated with linearly polarized light changes the polarization rotation angle of the linearly polarized light in accordance with the magnitude of an applied magnetic field. The optical-pumping-type magnetic sensor is an apparatus that measures the intensity of the magnetic field by detecting the polarization rotation angle described above. In the field of the optical-pumping-type magnetic sensor, multi-channel measurement using a plurality of cells has been studied for expansion of the magnetic field measurement range and improvement in resolution of the magnetic field measurement. 
     For example, JP-A-2012-177585 proposes a magnetic field measuring apparatus in which partition walls are disposed between cells arranged in a matrix to prevent inter-cell optical crosstalk. 
     The magnetic field measuring apparatus described in JP-A-2012-177585, however, has a problem of a difficulty in improving easiness of maintenance of the cells and suppressing the inter-cell optical crosstalk at the same time. In detail, since the cells arranged in a matrix (cell array) are bonded to the partition walls and other components, for example, via low-melting glass, it is difficult to exchange a defective cell for a non-defective cell or remove the defective cell for repair. Similarly, also in the step of manufacturing the magnetic field measuring apparatus, it is difficult to discretely exchange a cell determined to be defective in finished product inspection for a non-defective cell. That is, a magnetic field measuring apparatus that allows suppression of optical crosstalk and improvement in easiness of maintenance of cells has been desired. 
     SUMMARY 
     An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples. 
     Application Example 
     A magnetic field measuring apparatus according to this application example includes a cell array including a first cell and a second cell that each accommodate a medium that changes a polarization rotation angle of probing light incident on the medium in accordance with an intensity of a magnetic field, a light source that emits the probing light, and shielding members with which the first cell and the second cell are provided. 
     According to this application example, since the first cell and the second cell have the shielding members independent of each other, easiness of maintenance of the cells can be improved with inter-cell optical crosstalk suppressed. In detail, the shielding members are not a partition wall integrated with the cells, unlike the related art, but are provided independently of each other on a cell basis. Any of the cells can therefore be readily individually removed from the cell array including the first cell and the second cell. As a result, in maintenance of the magnetic field measuring apparatus and post-manufacture/inspection correction of the magnetic field measuring apparatus, easiness of the maintenance of the cells is improved. Further, since each of the first cell and the second cell is provided with the shielding member, optical shielding capability between the first cell and the second cell is improved. Noise originating from the inter-cell optical crosstalk can therefore be reduced. A magnetic field measuring apparatus having not only detection sensitivity improved by the suppression of the inter-cell optical crosstalk but also the improved cell maintenance can therefore be provided. 
     In the magnetic field measuring apparatus according to the application example described above, it is preferable that the shielding members are provided on adjacent surfaces of the first cell and the second cell. 
     According to the configuration described above, in the first cell and the second cell, light (fluorescence) outputted from one of the cells toward the adjacent cell and light (fluorescence) incident on one of the cells from the adjacent cell are both reduced. Therefore, in the magnetic field measurement, noise resulting from the inter-cell optical crosstalk is further reduced. 
     In the magnetic field measuring apparatus according to the application example described above, it is preferable that each of the first cell and the second cell has a first chamber on which the probing light is incident, and that the corresponding shielding member is provided on an outer shell that forms the first chamber but in an area excluding an area through which the probing light passes. 
     According to the configuration described above, the shielding members do not prevent transmission of the probing light for the magnetic field measurement. In addition thereto, the amount of fluorescence emitted from the first chamber of each of the cells and acting as noise in the magnetic field measurement is reduced. 
     In the magnetic field measuring apparatus according to the application example described above, it is preferable that the shielding members have a light absorbing property. 
     In the configuration described above, the shielding members having a light absorbing property can suppress scattering of fluorescence. The optical crosstalk in the magnetic field measuring apparatus can therefore be further suppressed. 
     In the magnetic field measuring apparatus according to the application example described above, it is preferable that the shielding members each include a fabric. 
     According to the configuration described above, the shielding members can be readily processed and placed. Further, the function of protecting the cells against impact or any other type of external force can be imparted. 
     In the magnetic field measuring apparatus according to the application example described above, it is preferable that the shielding members each include a resin layer. 
     According to the configuration described above, the shielding members (resin layers) can be made of a liquid material, which can be applied onto the cells. The shielding members can thus be placed on the cells. Therefore, even though the cells each have irregularities, the shielding members can each be seamlessly formed over a desired application range, whereby fluorescence shielding capability can be ensured. 
     In the magnetic field measuring apparatus according to the application example described above, it is preferable that the medium contains an alkali metal. 
     According to the configuration described above, the polarization plane orientation of the probing light emitted from the light source can be changed in accordance with the intensity of the magnetic field. 
     In the magnetic field measuring apparatus according to the application example described above, it is preferable that each of the first cell and the second cell accommodates a buffer gas. 
     According to the configuration described above, the buffer gas restricts movement of the medium in each of the cells and therefore prevents the medium from directly colliding with the inner wall of the cell, whereby the period for which an excited state produced by the radiated probing light attenuates can be prolonged. The excited state of the medium is therefore maintained for a longer period than in a case where no buffer gas is present, whereby the detection sensitivity of the magnetic field measuring apparatus can be improved. 
     In the magnetic field measuring apparatus according to the application example described above, it is preferable that a paraffin film containing aliphatic hydrocarbon having a carbon number of 20 or more is provided on an inner surface of each of the first cell and the second cell. 
     According to the configuration described above, the excited medium is unlikely to directly collide with the inner wall of each of the cells, whereby the period for which the excited state of the medium attenuates can be prolonged. The excited state of the medium is therefore maintained for a longer period than in a case where no paraffin film is provided, whereby a decrease in the detection sensitivity of the magnetic field measuring apparatus with time can be suppressed. 
     In the magnetic field measuring apparatus according to the application example described above, it is preferable that each of the first cell and the second cell has a first chamber on which the probing light is incident and a second chamber that communicates with the first chamber, and that each of the first cell and the second cell is provided with the shielding member. 
     According to the configuration described above, the amount of fluorescence produced in the first chamber and leaking through the second chamber is reduced. The optical crosstalk in the magnetic field measuring apparatus can therefore be further suppressed. 
     Application Example 
     A cell array according to this application example includes at least a first cell and a second cell each accommodating a medium that changes a polarization plane orientation of probing light incident on the medium in accordance with an intensity of a magnetic field and shielding members with which the first cell and the second cell are provided, and the at least first cell and second cell are so disposed as to be adjacent to each other. 
     According to this application example, since the first cell and the second cell have the shielding members independent of each other, easiness of maintenance of the cells can be improved with inter-cell optical crosstalk suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a block diagram showing the configuration of a magnetic field measuring apparatus according to a first embodiment. 
         FIG. 2  is a schematic plan view showing the configuration of a cell array. 
         FIG. 3  is a schematic cross-sectional view showing the configuration of the cell array. 
         FIG. 4  is a schematic perspective view showing a cell. 
         FIG. 5  is a schematic perspective view showing a primary chamber and a secondary chamber of a cell according to a second embodiment. 
         FIG. 6  is a schematic perspective view showing a cell according to variation 1. 
         FIG. 7  is a schematic perspective view showing a cell according to variation 2. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Embodiments of the invention will be described below with reference to the drawings. In the following figures, each part and each member are so drawn at scales different from actual scales as to be large enough to be recognizable. 
     First Embodiment 
     Magnetic Field Measuring Apparatus 
     The configuration of a magnetic field measuring apparatus according to an embodiment of the invention will be described with reference to  FIG. 1 .  FIG. 1  is a block diagram showing the configuration of a magnetic field measuring apparatus according to a first embodiment. 
     The present embodiment will be described with reference to what is called a one-beam magnetic field measuring apparatus that measures a magnetic field by using nonlinear magneto-optical rotation (NMOR). In the one-beam magnetic field measuring apparatus, a cell is irradiated with laser light containing linearly polarized light to achieve a state in which medium (alkali metal) atoms accommodated in the cell are excited. The laser light having passed through the cell is then detected for magnetic field measurement. That is, the one beam method is a method in which one beam is used not only as pumping light for exciting the medium atoms but also as probing light (linearly polarized light) for detecting the polarization rotation angle changed by the excited medium atoms. 
     A magnetic field measuring apparatus  100  shown in  FIG. 1  includes a light radiator  101  and a cell array  120  including a plurality of cells  121 . The magnetic field measuring apparatus  100  further includes a splitting optical element  117 , polarization separation elements  103 , a light receiver  104 , a signal processor  105 , a display section  106 , and a controller  107 . 
     The light radiator  101  includes a light source  111  and a converter  112 . The light source  111  is a laser light generator that outputs laser light L containing linearly polarized light as the probing light and is, for example, a tunable laser. The laser light L is what is called CW (continuous wave) light, which is continuously radiated and has a fixed amount of light. The output of the light source  111  is so adjusted that the amount of laser light L incident on the cells  121  is about several tens of microwatts. The converter  112  is, for example, a polarizer and converts the laser light L emitted by the light source  111  into linearly polarized light having a polarization angle in a predetermined direction with respect to the optical axis. 
     The single laser light L outputted from the light radiator  101  is incident on the splitting optical element  117 . The splitting optical element  117  splits the laser light L incident thereon to cause the laser light L to be incident on each of the cells  121 . 
     Each of the cells  121  accommodates a medium that changes the polarization plane orientation of the laser light L incident on the cell in accordance with the intensity of a magnetic field. The medium is preferably an alkali metal, which can be vaporized at a relatively low temperature, specifically, potassium (K) and cesium (Cs). The alkali metal accommodated in each of the cells  121  is at least partially vaporized at the time of magnetic field measurement. In the present embodiment, cesium is used as the medium. The laser light L described above is therefore so adjusted as to have the wavelength according to an absorption line of cesium (894 nm corresponding to D 1  line, for example). 
     An outer shell of each of the cells  121  is made, for example, of quartz, which transmits the laser light L. The outer shell of each of the cells  121  can be made of any material that can transmit the laser light L but does not react with an alkali metal or any other medium, and an organic material can also be used in place of an inorganic material, such as quartz and borosilicate glass. The plurality of cells  121  are so arranged as to be adjacent to each other to form the cell array  120 . Use of the cell array  120  including a plurality of cells  121  allows expansion of the magnetic field measurement range and improvement in measurement resolution. Each of the plurality of cells  121  is provided with a spieling member  131  (see  FIG. 2 ), which will be described later. Among the plurality of cells  121 , one of cells  121  adjacent to each other corresponds to the first cell according to an aspect of the invention, and the other corresponds to the second cell according to the aspect of the invention. 
     The laser light L attenuates when it is reflected off the surface of the outer shell of each of the cells  121  and absorbed by the interior of the cell  121 . The alkali metal atoms as the medium described above, which absorb the linearly polarized light contained in the laser light L, repeats transition between the ground state and the excited state to form a specific energy distribution (spin polarization; alignment). When a magnetic field is applied to the cell in a state in which the energy distribution (spin polarization; alignment) is maintained, the atoms described above anisotropically absorbs the linearly polarized light. That is, the state of the spin polarization (alignment) changes. The linearly polarized light incident on each of the cell  121  is affected by the change in the spin polarization (alignment) so that the polarization plane orientation (polarization rotation angle) changes. As a result, the laser light L having the changed polarization plane orientation (polarization rotation angle) exits from the cell  121  and is incident on the corresponding polarization separation element  103 . 
     The polarization separation elements  103  are disposed in correspondence with the plurality of cells  121 . The polarization separation elements  103  transmit a linearly polarized light component (P-polarized light component) having the same polarization direction (polarization plane orientation) as that of the linearly polarized light component of the laser light L converted by the converter  112  but reflect a linearly polarized light component (S-polarized light component) having a polarization direction perpendicular to the polarization direction of the linearly polarized light component of the laser light L. Each of the polarization separation elements  103  can, for example, be a polarizing beam splitter or a Wollaston prism. In the polarization separation elements  103 , the light passing therethrough is called polarized light Lp, and the light reflected therefrom is called polarized light Ls. The polarized light Lp and the polarized light Ls are incident on the light receiver  104 . 
     The light receiver  104  includes light receiving devices  141  and  142 . One polarization separation element  103  is provided with one light receiving device  141  and one light receiving device  142 . The light receiving devices  141  and  142  are each a detector sensitive to the wavelength of the laser light L. The light receiving devices  141  are each disposed in a position where it can receive the polarized light Lp, and the light receiving devices  142  are each disposed in a position where it can receive the polarized light Ls. The light receiving devices  141  each output current (signal) according to the amount of received polarized light Lp and transmit the signal to the signal processor  105 . The light receiving devices  142  each output current (signal) according to the amount of received polarized light Ls and transmit the signal to the signal processor  105 . The light receiving devices  141  and  142  are preferably made of a nonmagnetic material that does not interfere with the measurement performed by the magnetic field measuring apparatus  100 . In the present specification, the “nonmagnetic” material means a material having no magnetism. 
     The signal processor  105  receives the signals described above and transmitted from the light receiving devices  141  and  142 . The signal processor  105  measures, from the signals described above, the amount of change in the polarization rotation angle having changed when the linearly polarized light contained in the laser light L passes through the cells  121 , that is, the angle of rotation of the polarization plane orientation. 
     The polarization separation elements  103 , the light receiver  104 , and the signal processor  105  have the function of detecting the angle of rotation of the polarization plane orientation having changed in the cells  121 . Since the angle of rotation of the polarization plane orientation changes in accordance with the magnitude (intensity) of the magnetic field applied to the cells  121 , the magnetic field measuring apparatus  100  can measure the magnitude of the magnetic field applied in a predetermined direction (measurement direction) to the cells  121  by detecting the angle of rotation of the polarization plane orientation described above. 
     The display section  106  is electrically connected to the signal processor  105 . The display section  106  displays, for example, the angle of rotation described above measured by the signal processor  105 . The display section  106  is, for example, a display apparatus using a liquid crystal panel or any other component. 
     The controller  107  includes a processing device (not shown), such as a CPU (central processing unit), and a memory (not shown). The controller  107  is electrically connected to the light radiator  101 , the signal processor  105 , the display section  106 , and other components and has the function of synthetically controlling the actions thereof. 
     Cell Array 
     The configurations of the cell array  120  and the splitting optical element  117 , which is associated with the cell array  120 , will next be described with reference to  FIG. 2 .  FIG. 2  is a schematic plan view showing the configuration of the cell array. In  FIG. 2 , the laser light L is drawn with a dotted line. 
     The cell array  120  shown in  FIG. 2  includes the plurality of cells  121 , each of which accommodates a medium that changes the polarization plane orientation of the laser light L incident on the cell in accordance with the intensity of the magnetic field, and the shielding members  131 , with which the cells  121  are provided. In the cell array  120 , the plurality of cells  121  are arranged in the form of a matrix in a cell container  151 . The cell array  120  is provided with the splitting optical element  117  via the cell container  151 . 
     The cell container  151  is made of a material that transmits the laser light L. The material of which the cell container  151  is made can, for example, be quartz. In  FIG. 2 , directions are defined as follows: The direction roughly parallel to the longitudinal arrangement of the cells  121  is called a Z direction (upward direction is called positive direction); the direction roughly parallel to the lateral arrangement of the cells  121  and perpendicular to the z direction is called an X direction (rightward direction is called positive direction); and the direction perpendicular to the Z direction and the X direction is called a Y direction. The traveling direction of the laser light L passing through the cell array  120  is roughly parallel to the Y direction. 
     The cell array  120  includes 16 cells  121  in total that form 4 rows and columns, each of which is formed of 4 cells  121 , in the X and Z directions, as shown in  FIG. 2 . Adjacent cells  121  are so disposed as to be separate from each other by a predetermined distance. Any of the cells  121  can therefore be individually removed from the magnetic field measuring apparatus  100 , and maintenance, such as exchange and repair, can be readily performed on the removed cell  121 . The predetermined distance described above ranges, for example, from about 0.1 to 10 mm. 
     The cells  121  have a roughly cubic shape and include the shielding members  131 . Each of the shielding members  131  is so provided as to be in contact with the surfaces of the corresponding cell  121  that are roughly parallel to the Y direction (surfaces excluding surfaces roughly perpendicular to Y direction). The surfaces of each of the cells  121  other than the surface on which the laser light L is incident or the surface through which the laser light L exits (surfaces roughly parallel to Y direction) are hereinafter also referred to as “side surfaces.” 
     The splitting optical element  117  has the function of splitting the laser light L incident thereon to cause the laser light L to be incident on each of the cells  121 , as described above. The splitting optical element  117  includes mirrors  115 A to  115 C,  116 , and  118 A to  118 D (see  FIG. 3 ). The laser light L incident on the splitting optical element  117  travels in the X direction (positive direction thereof) and first reaches the mirror  116 . The mirror  116  reflects part of the laser light L in the Z direction (positive direction thereof) and transmits the remainder. 
     The mirrors  115 A,  115 B, and  115 C are provided along the traveling direction of the laser light L reflected off the mirror  116  (Z direction). The three mirrors  115 A,  115 B, and  115 C are disposed in the positions corresponding to the three rows formed of cells  121  and arranged in the Z direction. The laser light L reflected off the mirror  116  first reaches the mirror  115 A. The mirror  115 A reflects part of the laser light L in the X direction (positive direction thereof) and transmits the remaining laser light L. Similarly, the mirrors  115 B and  115 C reflect or transmit the laser light L to cause the laser light L to travel in the X direction (positive direction thereof) in correspondence with the rows of cells  121 , which are roughly parallel to the X direction. The mirror  115 C may be assumed to totally reflect the laser light L. 
     The configurations of the cell array  120  and the splitting optical element  117 , the polarization separation elements  103 , and other components associated with the cell array  120  will next be described with reference to  FIG. 3 .  FIG. 3  is a schematic cross-sectional view showing the configuration of the cell array.  FIG. 3  is a cross-sectional view of the cell array  120  taken along the line A-A shown in  FIG. 2  and shows the splitting optical element  117 , the polarization separation elements  103 , and other components. It is assumed that the upward direction of the Y direction in  FIG. 3  is called a positive direction. 
     The laser light L having passed through the mirror  116  travels in the X direction (positive direction thereof). The mirrors  118 A to  118 D are provided in the positions corresponding to the four columns formed of cells  121  and arranged in the X direction. The laser light L having passed through the mirror  116  first reaches the mirror  118 A. The mirror  118 A reflects part of the laser light L in the Y direction (positive direction thereof) to cause the laser light L to be incident on the corresponding cell  121 . The mirror  118 A transmits the remaining laser light L to cause the laser light L to reach the mirror  118 B. Similarly, the mirrors  118 B,  118 C, and  118 D reflect or transmit the laser light L to cause the laser light L to be incident on the corresponding cells  121 . The mirror  118 D may be assumed to totally reflect the laser light L. 
     In the cell array  120 , the configurations in the X direction excluding the configuration in the A-A cross section are the same as the configuration in the A-A cross section described above except that the mirror  116  is replaced with the mirrors  115 A to  115 C. The laser light L reflected off the mirrors  115 A to  115 C in the X direction (positive direction thereof) is therefore radiated to the corresponding cells  121  via the mirrors  118 A,  118 B,  118 C, and  118 D arranged in the X direction, as in the case of the laser light L having passed through the mirror  116  described above. 
     Each of the mirrors  116 ,  115 A to  115 C, and  118 A to  118 D can, for example, be a partial polarizing beam splitter or a non-polarizing beam splitter having constant transmittance irrespective of the polarization plane orientation. It is preferable that the same amount of laser light L is incident on the 16 cells  121 . To this end, the transmittance and reflectance of the mirrors  116 ,  115 A to  115 C, and  118 A to  118 D at which the mirrors transmit and reflect the linearly polarized light contained in the laser light L are so adjusted that the same amount of light described above is achieved. 
     In the configuration described above, the single laser light L is split by the splitting optical element  117 , and each of the 16 cells  121  is irradiated with one corresponding laser light L. 
     Each of the shielding members  131  is provided on the side surfaces (surfaces roughly parallel to Y direction) of the corresponding cell  121 . Since each of the shielding members  131 , which will be described later, is a light absorbent member that absorbs light, such as the laser light L, optical crosstalk between the cells  121  is suppressed. 
     The laser light L passes through the cells  121 , where the polarization plane orientation of the laser light L changes in accordance with the intensity of the magnetic field, as described above. The laser light L having passed through the cells  121  is separated by the polarization separation elements  103  into the polarized light Lp and the polarized light Ls, which are received with the light receiving devices  141  and  142 , respectively. As described above, the laser light L radiated from the light radiator  101  passes through the cells  121 , is then received with the light receiving devices  141  and  142 , and undergoes the magnetic field measurement. 
     In  FIG. 3 , the magnetic field measuring apparatus  100  has been described as an apparatus based on what is called a single-pass method, in which the laser light L passes through the cells  121  only once, but the method is not necessarily employed. For example, the magnetic field measuring apparatus  100  may be an apparatus based on a multi-pass method, in which the laser light L passes through the cells  121  multiple times via mirrors and other components. Further, the laser light L does not necessarily pass through the cells  121  roughly in parallel to the Y direction. 
     Cell 
     The configuration of the cells in the present embodiment will be described with reference to  FIG. 4 .  FIG. 4  is a schematic perspective view showing a cell. 
     The cell  121  shown in  FIG. 4  has a primary chamber  122 , which is an example of a first chamber, and the shielding member  131  in the present embodiment. The primary chamber  122  is an internal space surrounded by a roughly cubic outer shell. The length of one side of the outer shell described above is, for example, about 2 cm. The laser light L enters the primary chamber  122 . The outer shell described above is formed of 6 surfaces, surfaces a, b, c, d, e, and f each having a roughly square shape. The primary chamber  122  at least partially encapsulates gaseous cesium as the alkali metal (medium), and the outer shell maintains airtightness of the cell. 
     The surface a is the surface on which the laser light L is incident, and the surface b is the surface through which the laser light L having passed through the cell  121  exits. The surfaces c, d, e, and f are surfaces (side surfaces) of the outer shell that are roughly parallel to the Y direction and include surfaces via each of which two cells  121  are adjacent to each other when the entire cells  121  are arranged to form the cell array  120 . 
     When the laser light L is incident on the cells  121 , the alkali metal atoms in the cells  121  (primary chambers  122 ) are brought into the excited state, as described above. When the excited state returns (transitions) to the ground state, fluorescence is emitted. The fluorescence that accompanies the transition is so produced that the type of polarization thereof is determined by the energy levels in an eigenstate that causes the transition. In the case of the transition to the ground state described above, since the transition occurs at the same probability for all energy levels in the eigenstate, the emitted fluorescence contains linearly polarized light and circularly polarized light mixed with each other. 
     The fluorescence described above could enter adjacent cells  121 , resulting in optical crosstalk. In particular, in measurement of a weak magnetic field in biological tissue, such as the heart and the brain, the detection sensitivity (resolution) decreases in some cases due to noise originating from the optical crosstalk. To suppress the emission and entry of the fluorescence described above, the surfaces c, d, e, and f may be provided with the shielding member  131 . That is, the surfaces a and b are provided with no shielding member  131 . The shielding member  131  is so placed as to seamlessly cover the surfaces c, d, e, and f. Further, the shielding member  131  is preferably provided in the area excluding the area through which the laser light L passes. That is, covering the outer shell of each of the cells  121  over a wide range thereof to the extent that the shielding member  131  does not block the transmission of the laser light L allows further suppression of the optical crosstalk between the cells  121 . Therefore, in addition to the side surfaces of each of the cells  121 , the surface a, on which the laser light L is incident, and the surface b, through which the laser light L exits, may be provided with the shielding member  131  except the area through which the laser light L passes. As a result, the optical shielding capability of the cells  121  is further improved. 
     The shielding members  131  are each a light absorbent member. That is, each of the shielding members  131  is preferably a light absorbent member that appropriately absorbs the fluorescence from cells  121  adjacent to the shielding member  131 . For example, in a case where the alkali metal atoms of the medium are cesium atoms and the D 1  absorption line is used, the transmittance of the shielding members  131  at which they transmit light having wavelengths close to the wavelength of 894 nm is preferably set to be lower than or equal to 0.01%. When the shielding members  131  are each the light absorbent member described above, a situation in which the fluorescence described above incident on the shielding members  131  is emitted as transmitted light or scattered light is avoided. The optical shielding capability of the cells  121  is therefore be improved. 
     Further, the shielding members  131  are each preferably a nonmagnetic member. When the shielding members  131  are each a nonmagnetic member, influence of a magnetic field originating from the shielding members  131  on the alkali metal accommodated in the cells  121  decreases. The detection sensitivity of the magnetic field measuring apparatus  100  can therefore be improved. 
     The shielding members  131  each preferably contain a fabric. The material of which the fabric is made is not limited to a specific material and may, for example, be cotton, hemp, and other vegetable fibers, silk, wool, and other animal fibers, polyester, acetate, polyamide, and other synthetic fibers, polylactic acid and other biodegradable fibers. The shielding members  131  can be formed by processing at least one of the materials described above into a fabric, such as a woven fabric, a knitted fabric, and a nonwoven fabric. The fabrics described above are preferably colored, for example, black by a coloring agent for an increase in the light absorbing capability of the fabrics. The shielding members  131  in the present embodiment are formed of a fabric produced by coloring a polyester nonwoven fabric black by using a coloring agent primarily made of carbon black. 
     In the case where a fabric is used as each of the shielding members  131 , a method for placing the fabric on each of the cells  121  can, for example, be a method in which the cell  121  is covered with the fabric by using the elasticity thereof or a method in which an adhesive layer is provided between the outer shell of the cell  121  and the fabric. The thickness of the fabric used as each of the shielding members  131  is not limited to a specific value and may be any value that ensures the light absorbing capability. For example, the thickness can range from about 0.1 to 0.5 mm. The shielding members  131  are not necessarily in contact with the outer shells of the cells  121 . 
     Instead of using a fabric, the shielding members  131  each preferably include a resin layer. The material of which the resin layer is made is not limited to a specific material and can, for example, be an acrylic resin, a urethane-based resin, a polyolefin-based resin, a polyester-based resin, a polyamide-based resin, an epoxy-based resin, and a vinyl-chloride-based resin. At least one of the resins described above can be used as the material of the resin layer. A coloring agent, such as a black coloring agent, is preferably added to the resin layer to increase the light absorbing capability of the resin layer. 
     In the case where the resin layer is used as each of the shielding members  131 , a method for placing the resin layer on each of the cells  121  can be a method in which a liquid raw material of the resin layer is applied onto the outer shell of the cell  121  followed by solidification of the liquid raw material. Instead, a resin layer formed in the shape of a sheet in advance may be placed on the outer shell. The thickness of the resin layer used as each of the shielding members  131  is not limited to a specific value and may be any value that ensures the light absorbing capability. For example, the thickness can range from about 0.05 to 0.1 mm. 
     Further, the shielding members  131  may each be formed of the combination of the fabric and the resin layer described above. To improve the light absorbing capability, both the fabric and the resin layer may be superimposed on each other and placed as the shielding members  131  on the cells  121  (outer shells). Instead, the fabric and the resin layer may be properly used in accordance with the portion of the outer shell on which the shielding member  131  is placed. The cells  121  arranged along the outer edge of the cell array  120  may be so configured that no shielding member  131  is placed on the side surfaces of the cells  121  except the surfaces adjacent to each other. 
     A paraffin film (not shown) containing aliphatic hydrocarbon having a carbon number of 20 or more may be provided on the inner surface of each of the cells  12  (primary chambers  122 ). The paraffin film containing paraffin in the form of aliphatic hydrocarbon having a carbon number of 20 or more causes the atoms in the excited medium (alkali metal) to be unlikely to directly collide with the inner wall (inner surface) of the cell, whereby the period for which the excited state of the medium attenuates can be prolonged. The excited state of the medium is therefore maintained for a longer period than in a case where no paraffin film is provided, whereby a decrease in the detection sensitivity of the magnetic field measuring apparatus  100  with elapsed time can be suppressed. 
     The cells  121  (primary chambers  122 ) may accommodate an inert gas, such as a rare gas, as a buffer gas. When each of the cells  121  accommodates the buffer gas, the buffer gas restricts movement of the medium in the cell  121 , preventing the medium from directly colliding with the inner wall of the cell. As a result, the period for which the excited state produced by the radiated laser light L attenuates can be prolonged. The excited state of the medium is therefore maintained for a longer period than in a case where no buffer gas is present, whereby a decrease in the detection sensitivity of the magnetic field measuring apparatus  100  with elapsed time can be suppressed. 
     As described above, the magnetic field measuring apparatus  100  and the cell array  120  according to the embodiment described above can provides the following effects. 
     According to the embodiment described above, since the plurality of cells  121  include the shielding members  131  independent of one another, easiness of maintenance of the cells  121  can be improved with optical crosstalk between the cells  121  suppressed. In detail, since the optical shielding capability of the cells  121  is improved, optical crosstalk between adjacent cells  121  can be suppressed. Further, since the cells  121  are independently provided with respective shielding members  131 , any of the cells  121  can be readily individually removed from the cell array  120  and exchanged to a new cell. As a result, the maintenance of the cells  121  can be performed more readily than in related art. Therefore, in the magnetic field measuring apparatus  100  and the cell array  120  provided by the embodiment, the detection sensitivity improved by suppression of optical crosstalk, and easiness of the maintenance of the cells  121  are improved. 
     At least the adjacent surfaces of the cells  121  are provided with the shielding members  131 , which reduce the amount of fluorescence emitted through the side surfaces of the cells  121 . Therefore, both the fluorescence emitted from a cell  121  toward the adjacent cells  121  and the fluorescence incident on a cell  121  from the adjacent cells  121  can be reduced. As a result, optical crosstalk between the cells  121  can be suppressed. 
     Use of the fabric (polyester nonwoven fabric) colored black as each of the shielding members  131  further improves the capability of absorbing the fluorescence and further suppresses the optical crosstalk. Further, when the shielding members  131  each include the fabric, the function of protecting the cells against impact (external force) acting when the apparatus is handled can be imparted in addition to the easiness of processing and placement of the shielding members  131 . Moreover, use of a nonmagnetic fabric allows improvement in the detection sensitivity of the magnetic field measuring apparatus  100 . 
     Second Embodiment 
     Cell 
     The configuration of cells according to the present embodiment will be described with reference to  FIG. 5 .  FIG. 5  is a schematic perspective view showing a primary chamber that is an example of the first chamber and a secondary chamber that is an example of a second chamber in a cell according to the second embodiment. The same configuration portions as those in the first embodiment have the same reference characters, and no redundant description will be made. 
     A cell  221  shown in  FIG. 5  has a primary chamber  222 , which serves as the first chamber on which the laser light L is incident, and a secondary chamber  223 , which serves as the second chamber that communicates with the primary chamber  222 . The internal space surrounded by outer shells of the primary chamber  222  and the secondary chamber  223  encapsulates cesium as the medium, as in the first embodiment. The outer shells of the primary chamber  222  and the secondary chamber  223  are each provided with a shielding member  231 . 
     The outer shell of the primary chamber  222  is formed of  6  surfaces, surfaces g, h, i, j, k, and l (alphabet) each having a roughly square shape. The secondary chamber  223  has a roughly cylindrical shape, is in contact with an outer edge portion of the surface j, and protrudes from the outer shell of the primary chamber  222 . The outer shells of the primary chamber  222  and the secondary chamber  223  maintain airtightness of the interior of the cell  221 . The outer shells of the primary chamber  222  and the secondary chamber  223  can be made of the same material as that in the first embodiment. In the present embodiment, the material of which the outer shells of the primary chamber  222  and the secondary chamber  223  are made is quartz. 
     Among the surfaces g, h, i, j, k, and l, the surface g is the surface on which the laser light L is incident, and the surface h is the surface through which the laser light L having passed through the cell  221  exits. The surfaces i, j, k, and l are surfaces (side surfaces) of the outer shell of the primary chamber  222  that are roughly parallel to the Y direction and include surfaces via each of which two cells  221  are adjacent to each other when the entire cells  221  are arranged to form a cell array  220  (not shown). 
     The secondary chamber  223  is provided as an introduction portion for introducing cesium (medium) or any other substance into the cell  221  (primary chamber  222 ) in the step of manufacturing the cell  221 . After cesium or any other substance is introduced into the primary chamber  222  via an opening provided in the outer shell of the secondary chamber  223 , the opening described above is closed, for example, with a heated sealing material. According to the method described above, the thermal burden imposed on the accommodated cesium or any other substance when the opening is closed can be reduced, as compared with the case where the opening is provided in the primary chamber  222 . 
     Further, providing the secondary chamber  223  allows employment of a manufacturing method in which an ampule containing cesium, a buffer gas, or any other substance is introduced via the opening described above into the secondary chamber  223 . In the manufacturing method, after the ampule described above is introduced into the secondary chamber  223 , the opening described above is closed. Thereafter, the ampule is so irradiated, for example, with laser light as to be unsealed, and the cesium or any other substance can transpire across the primary chamber  222  and can be accommodated therein. The manufacturing method also allows reduction in the thermal burden imposed on the cesium or any other substance. 
     The shielding member  231  is so provided as to cover the entire outer shell of the secondary chamber  223  and the surfaces i, j, k, and l of the primary chamber  222 . That is, the surfaces g and h are provided with no shielding member  231 . The shielding member  231  can suppress the fluorescence emitted from the cell  221  and the fluorescence incident on the cell  221 . The shielding member  231  includes a resin layer that is a light absorbent, nonmagnetic layer. The resin layer can be made of any of the materials described in the first embodiment. In the present embodiment, the shielding member  231  is made of an acrylic material, and a coloring agent primarily made of carbon black is added to the acrylic material for improvement in the light absorbent capability. 
     A method for placing the resin layer can be any of the methods described above. In the present embodiment, an emulsion which is formed of a medium primarily made of water and in which fine particles made of the acrylic resin and the carbon black pigment described above are dispersed is used as the resin layer. The emulsion described above, which is a liquid, is applied onto the cell  221 , and the shielding member  231  can thus be placed. The thus applied emulsion described above is then dried to forma coating (resin layer). According to the method described above, the shielding member  231  can be readily placed also on the outer shell of the secondary chamber  223 , which protrudes from the cell  221 . That is, the method is suitable for placement of the shielding member  231  on an irregular area where it is difficult to place a fabric shielding member. 
     In the present embodiment, the aqueous acrylic resin emulsion is used to place the shielding member  231 , but not necessarily used. For example, a non-aqueous emulsion or a heat curable or energy ray curable coating material may be used to place the shielding member  231 . Still instead, a method in which a resin layer is formed in a sheet-like shape in advance and then the resin layer is placed on the cell  221  may be used. 
     In the present embodiment, the secondary chamber  223  is provided on a side surface (surface j) of the cell  221 , but not necessarily limited thereto. For example, the secondary chamber  223  may be provided on the surface g or h, through which the laser light L passes but in an area other than the area through which the laser light L passes. Further, the shape of the secondary chamber  223  is not limited to a roughly cylindrical shape. The secondary chamber  223  may have any other shape, such as a columnar shape or a pyramidal shape, or the cross-sectional shape of part of the secondary chamber  223  may differ from the cross-sectional shape of the remainder. 
     As described above, the second embodiment differs from the first embodiment in that the cells  221  each have the secondary chamber  223 , which is in contact with the primary chamber  222 , and that the outer shell of the secondary chamber  223  is also provided with the shielding member  231 . As described above, the cells  221  according to the embodiment described above can provide the following effects in addition to those provided by the first embodiment. 
     According to the embodiment described above, since the shielding member  231  includes the resin layer, the shielding member  231  can be made of a liquid material. The shielding member  231  can therefore be placed in an application process. The shielding member  231  can therefore be placed even on a placement area, for example, having irregularities, whereby the shielding capability of the cells  221  can be improved. Further, in the cell array  120  and the magnetic field measuring apparatus including the cells  221 , optical crosstalk is suppressed and detection sensitivity can be improved with easiness of maintenance of the cells  221  ensured. 
     Since the outer shell of the secondary chamber  223  is also covered with the shielding member  231 , a situation in which the fluorescence produced in the primary chamber  222  and leaking through the secondary chamber  223  acts as noise in the magnetic field measurement can be effectively avoided. 
     The invention is not limited to the embodiments described above, and a variety of changes and improvements can be made to the embodiments described above. Variations will be described below. 
     Variation 1 
     Cell 
     The first embodiment has been described with reference to the configuration in which no shielding member  131  is placed on the surface on which the laser light L is incident (surface a) or the surface through which the laser light L exits (surface b), but the configuration is not necessarily employed. The configuration of a cell according to the present variation will be described with reference to  FIG. 6 .  FIG. 6  is a schematic perspective view showing the cell according to Variation 1. The same configuration portions as those in the first embodiment have the same reference characters, and no redundant description will be made. 
     A cell  321  shown in  FIG. 6  has a primary chamber  322 , on which the laser light L is incident, and a shielding member  331 . The primary chamber  322  is an internal space surrounded by a roughly cubic outer shell. The laser light L enters the primary chamber  322 . The outer shell described above is formed of 6 surfaces, surfaces m, n, o, p, q, and r each having a roughly square shape. The primary chamber  322  encapsulates cesium, and the outer shell maintains airtightness of the cell, as in the first embodiment. 
     The surface m is the surface on which the laser light L is incident, and the surface n is the surface through which the laser light L having passed through the cell  321  exits. The surfaces o, p, q, and r are surfaces (side surfaces) of the outer shell that are roughly parallel to the Y direction and include surfaces via each of which two cells  321  are adjacent to each other when a plurality cells  321  are arranged to form a cell array  320  (not shown). 
     The shielding member  331  is provided on part of the outer shell (surfaces m, n, o, p, q, and r), which forms the primary chamber  322 , specifically, in the area excluding the area through which the laser light L passes (enters or exits). That is, the shielding member  331  is placed on part of the outer shell described above, specifically, in the area excluding an area  335  of the surface m through which the laser light L passes and an area  336  of the surface n through which the laser light L passes. The areas  335  and  336  each preferably have a size greater than or equal to the beam diameter of the laser light L. The areas  335  and  336 , through which the laser light L passes, each have a roughly circular shape, but not necessarily limited thereto. Further, the positions of the areas  335  and  336  do not necessarily roughly coincide with the centers of the surfaces m and n, respectively. 
     The shielding member  331  can be made of any of the materials described above. The shielding member  331  is a fabric produced by coloring a cotton velvet sheet black by using a coloring agent primarily made of carbon black. 
     As described above, the cell  321  according to the present variation can provide the following effects in addition to those provided by the first embodiment. Since the shielding member  331  is placed also on part of the surfaces m and n, through which the laser light L passes, specifically, in the area excluding the areas  335  and  336 , through which the laser light L passes, the shielding capability of the cell  321  is further improved. As a result, in the cell array  320  (not shown) and the magnetic field measuring apparatus including the plurality of cells  321 , optical crosstalk can be further suppressed and detection sensitivity can be further improved. 
     Variation 2 
     Cell 
     The configuration of a cell according to the present variation will be described with reference to  FIG. 7 .  FIG. 7  is a schematic perspective view showing the cell according variation 2. The same configuration portions as those in the first embodiment have the same reference characters, and no redundant description will be made. 
     A cell  421  shown in  FIG. 7  has a primary chamber  422 , on which the laser light L is incident, and a shielding member  431 . The primary chamber  422  is an internal space surrounded by a roughly cubic outer shell. The laser light L enters the primary chamber  422 . The outer shell described above is formed of 6 surfaces, surfaces s, t, u, v, w, and α each having a roughly square shape. The primary chamber  422  encapsulates cesium, and the outer shell maintains airtightness of the cell, as in the first embodiment. 
     The surface s is the surface on which the laser light L is incident, and the surface t is the surface through which the laser light L having passed through the cell  421  exits. The surfaces u, v, w, and α are surfaces (side surfaces) of the outer shell that are roughly parallel to the Y direction and include surfaces via each of which two cells  421  are adjacent to each other when a plurality cells  421  are arranged to form a cell array  420  (not shown). 
     The shielding member  431  is provided on part of the outer shell (surfaces s, t, u, v, w, and α), which forms the primary chamber  422 , specifically, on the side surfaces (surfaces u, v, w, and α) and in the area excluding an area  436  of the surface t, through which the laser light L passes. In other words, the surface s or the area  436  is provided with no shielding member  431 . That is, Variation 2 differs from the embodiment 1 in that the shielding member  431  is placed also on the surface t, through which the laser light L exits. The area  436  described above preferably has a size greater than or equal to the beam diameter of the laser light L. The shielding member  431  is made of the same material described in the first embodiment. 
     As described above, the cell  421  according to the present variation can provide the following effects in addition to those provided by the first embodiment. Since the shielding member  431  is placed also on part of the surface t, through which the laser light L exits, specifically, in the area excluding the area  436 , through which the laser light L passes, the shielding capability of the cells  421  is further improved. Further, since no shielding member  431  is provided on the surface s, on which the laser light L is incident, the structure of the cell  421  is simplified as compared with the structure in Variation 1 described above, whereby the cell  421  is readily manufactured. As a result, in the cell array  420  (not shown) including the plurality of cells  421  and the magnetic field measuring apparatus, optical crosstalk can be further suppressed in the simplified configuration. 
     Variation 3 
     Cell 
     A cell according to the present variation includes a shielding container as the shielding member in place of the fabric or the resin layer described above. That is, a single cell is accommodated in a single shielding container, and a plurality of the shielding containers each including a cell are arranged to form a cell array. To allow the laser light 
     L to pass through the cells, a corresponding area of each of the shielding containers has an opening. The shielding containers are not necessarily made of a specific material and may be made of any material that is a light absorbent, nonmagnetic material. Specifically, examples of the material may include a rubber material and a resin material with which carbon black or any other substance is kneaded. As a result, after the cell accommodated in a shielding container is exchanged to a new cell, the shielding container is reusable, whereby easiness of maintenance of the cell array and the magnetic field measuring apparatus can be further improved. 
     The entire disclosure of Japanese Patent Application No. 2016-118613 filed Jun. 15, 2016 is expressly incorporated by reference herein.