JP2015176814A - Ion beam purity monitoring device, method and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ion beam purity monitoring technique which makes possible to properly execute the purity monitoring in real time while continuing an output operation.SOLUTION: An ion beam purity monitoring device 30 comprises: an acquisition part 31 operable to take detection data 26 concerning ions emitted in a direction having an angle to a main axis X of an ion beam emitted from a target 12 irradiated with laser; a time counter 32 operable to repeat the counting of time according to the same cycle as that of laser oscillation; an allocation part 33 operable to allocate a signal value of the detection data 26 to a time of counting time; a setting part 35 operable to set a timing corresponding to each charge-to-mass ratio of impurity ions different from an intended ion forming the ion beam; a determination part 36 operable to make determination on whether or not the signal value of the detection data 26 in line with each timing of the impurity ions is above a threshold; and an output part 37 operable to output an abnormality signal on condition that the signal value is determined to be above the threshold.

Description

  The present invention relates to a technique for monitoring the purity of an ion beam taken out from an ion source.

An ion source using a laser condenses the laser to irradiate a solid target, and vaporizes and ionizes the target element by the energy of the laser to generate plasma.
Then, after the generated plasma is transported to the accelerator entrance as it is, only ions are drawn into the accelerator due to a potential difference and output as an ion beam (for example, see Patent Documents 1 and 2).

On the other hand, the acceleration of ions in the accelerator is more excellent as the ion mass is smaller and the valence is larger. And it is known that the ion source using a laser is advantageous for generation of multivalent ions.
Further, if impurity ions other than the target ions are mixed in the plasma generated by the laser, there is a concern that the beam quality may be deteriorated.
Therefore, an interlock that stops the operation of the accelerator when the purity of the ion beam extracted from the ion source is measured and a decrease in purity is observed has been studied (for example, Patent Document 3).

Japanese Patent No. 3713524 JP 2009-37764 A JP 2012-051577 A

Examples of the contamination source that decreases the purity of the ion beam include impurity elements contained in the target, moisture adhering to the target, residual gas components in the laser irradiation atmosphere, and the like.
These impurity element ions, water-derived hydrogen ions and oxygen ions, and residual gas component ions are mixed into the ion beam as impurity ions and transported downstream as they are.
For this reason, it is desirable to monitor the purity in real time during the output operation of the ion beam and to stop the operation immediately when a decrease in purity is observed.

However, since there are many kinds of impurity ions as described above and the mixing ratio with respect to the target element ions (target ions) is originally small, a decrease in detection sensitivity is inevitable.
For this reason, there is a possibility that the above-described interlock function may reduce the stability of the ion beam output operation due to erroneous measurement of impurity ions.

  The present invention has been made in consideration of such circumstances, and an object of the present invention is to provide an ion beam purity monitoring technique that can be accurately performed in real time while continuing output operation.

  In the ion beam purity monitoring device, an acquisition unit that acquires detection data of ions emitted in a direction that makes an angle with respect to a main axis of an ion beam emitted from a target irradiated with a laser, and the same oscillation cycle as the laser A time counter that repeats time counting in a cycle, an assigning unit that assigns the signal value of the detection data to the timing of the time count, and a timing corresponding to a mass-to-charge ratio of impurity ions different from target ions constituting the ion beam A setting unit for setting, a determination unit for determining whether or not a signal value of the detection data matching the timing of the impurity ions exceeds a threshold value, and an output for outputting an abnormal signal when the threshold value is determined to be exceeded And a section.

  According to the present invention, there is provided an ion beam purity monitoring technique that can be accurately executed in real time while continuing output operation.

The block diagram which shows embodiment of the purity monitoring apparatus of the ion beam which concerns on this invention. The timing chart which shows a laser oscillation and an ion detection period. The graph which shows the detection result of the element ion contained in an ion beam. The block diagram which shows the ion accelerator to which the purity monitoring apparatus of the ion beam which concerns on embodiment is applied. The flowchart explaining operation | movement of the purity monitoring apparatus of the ion beam which concerns on embodiment.

Embodiments of the present invention will be described with reference to the accompanying drawings.
As shown in FIG. 1, in the ion source 10, a target 12 is accommodated in a vacuum vessel 11, and the potential of the vacuum vessel 11 is set at the same level as the target 12.
A laser oscillating unit 13 is disposed outside the ion source 10, and the laser that has entered the interior through the transparent window irradiates the surface of the target 12.

The element of the target 12 is evaporated and ionized by the energy of the irradiated laser to generate the plasma 14.
The plasma 14 is in a state where the evaporated element of the target 12 is ionized into cations and electrons, and is electrically neutral as a whole.
When a carbon-based plate-like member is used as the target 12, the plasma 14 includes not only the desired polyvalent carbon ions but also impurity element ions, water-derived hydrogen ions and oxygen ions, and the vacuum vessel 11. In some cases, ions of residual gas components are mixed as impurity ions.

The plasma 14 radially diffuses inside the vacuum vessel 11 along the main axis X of the ion beam extending in the vertical direction from the incident point where the laser is incident on the target 12.
A passage hole 15 for plasma 14 is provided on the wall of the vacuum vessel 11 where the main axis X of the ion beam intersects.

A plasma transport tube 19 having the passage hole 15 as an open end is provided so that the main axis X of the ion beam is a concentric axis and further has the same potential as the vacuum vessel 11.
The plasma 14 is guided to the inside of the linear accelerator 17 by the plasma transport tube 19 without being diffused.
The passage hole 15 of the vacuum vessel 11 is provided with a collimator 18 that excludes the spread unnecessary plasma 14 from being transported to the plasma transport tube 19.

The vacuum vessel 11 and the linear accelerator 17 are connected by a communication path 16 that is an insulator, and the plasma transport tube 19 is arranged so as to coincide with the central axis of the communication path 16.
In this way, the plasma 14 generated in the ion source 10 passes through the communication path 16 and is introduced into the linear accelerator 17.

Since the ion source 10 and the linear accelerator 17 have different potentials, the positive ions from which electrons are separated from the plasma 14 introduced into the linear accelerator 17 are introduced into an acceleration channel (not shown) and accelerated to form an ion beam.
The ion beam that has been increased in energy from the ion source 10 in this way is used, for example, in a heavy particle beam irradiation apparatus used for cancer treatment.

The vacuum vessel 11 is provided with a branch hole 21 through which ions emitted in a direction that forms an angle with the main axis X of the ion beam from the target 12 pass.
The branch holes 21 guide ions from the plasma 14 diffused inside the vacuum vessel 11 to the outside. The ions introduced from the branch hole 21 to the outside of the vacuum vessel 11 are guided to the analysis unit 20 that performs qualitative / quantitative analysis of the ion species.

  Since the composition of the ion species of the ions extracted from the branch hole 21 is the same as the composition of the ion species of the ion beam extracted from the passage hole 15, the analysis unit 20 monitors the ion species included in the ion beam in real time. Is possible.

  The analysis unit 20 detects an ion deflected by applying an electric field to the flight trajectory 25 of ions extracted from the branch hole 21 and deflecting the flight trajectory 25, and detected data 26. And a detection unit 24 that outputs

Here, assuming that the deflection radius r of the electrostatic deflector 23 and the electric field E are the flight speed v, valence q, mass m, and elementary quantity e of the ion, the equation (2) is derived from the relationship of the following equation (1). It is burned.
1/2 · mv ² = 1/2 · qerE (1)
v ² ｑ q / m (2)

  As shown in Equation (2), the flight speed v of ions is proportional to the square root of the mass-to-charge ratio q / m of the ions. Thereby, the flight time of the flying ions is uniquely determined with respect to the mass-to-charge ratio q / m. Then, ions having different mass-to-charge ratios q / m enter the detection unit 24 after different flight times and are detected separately on the timeline.

  The ion beam purity monitoring device 30 includes a detection data acquisition unit 31 that acquires detection data 26 of ions emitted in a direction that forms an angle with respect to the main axis X of the ion beam emitted from the target 12 irradiated with the laser; A time counter 32 that repeats time counting at the same period T as the laser oscillation period T (FIG. 2), a detection data assigning unit 33 that assigns the signal value of the detection data 26 to the timing of the time count, and target ions constituting the ion beam A timing setting unit 35 that sets a timing corresponding to a mass-to-charge ratio of impurity ions different from, a determination unit 36 that determines whether or not the signal value of detection data 26 that matches the timing of impurity ions exceeds a threshold value, And an abnormal signal output unit 37 that outputs an abnormal signal when it is determined that the threshold value is exceeded.

As shown in the timing chart of FIG. 2, the laser oscillation unit 13 oscillates the laser pulses 13a, 13b, and 13c intermittently at regular intervals.
When one laser pulse 13a is irradiated onto the target 12, all the ions emitted from the target 12 fly along the flight trajectory 25 during the period until the next laser pulse 13b is irradiated. Detected.

Therefore, in the detection data acquisition unit 31, the detection data is acquired at an earlier timing as the ion species has a larger mass-to-charge ratio (q / m).
The time count reset flag 38 by the time counter 32 shown in FIG. 2 is set at the same period T as the laser oscillation period T, and the detection data 26 is assigned the signal value to the timeline at the acquired timing. It is done.

  As shown in FIG. 3, the graph creating unit 34 can update a graph in which the horizontal axis is set to time (timeline) and the vertical axis is set to a signal value at the same time interval as the oscillation period T of laser oscillation. . Thereby, the purity of the ion beam can be monitored in real time.

In the graph of FIG. 3, detection peaks of carbon ions from hexavalent to monovalent are observed.
The setting unit 35 sets an indicator 39 on the graph for the timing corresponding to the mass-to-charge ratio of target ions and impurity ions.
The setting of the indicator 39 for various element ions having different valences can be made in advance by calculation based on the deflection radius r, electric field E, ion flight distance, etc. of the electrostatic deflector 23 (FIG. 1).

The generation source of the impurity ion species is impurities contained in the target 12 and residual gas in the vacuum vessel 11. For this reason, since the impurity element which is highly likely to be mixed into the ion beam can be grasped from the impurity data of the target 12 or the like, the corresponding indicator 39 can be set.
For example, as shown in FIG. 3, the indicator 39 can be set by predicting the detection timing of ions having different valences of nitrogen (N) and oxygen (O) which are impurity elements.

Here, when the final constituent ions of the ion beam output from the ion source 10 are C ⁶⁺ , the impurity ions N ⁷⁺ and O ⁸⁺ both have a mass-to-charge ratio q / m of 1 / 2 are the same, the detection peaks in the timeline coincide with each other and separation detection cannot be performed.
Therefore, by determining the presence or absence of detection peaks of N ^{6+ to} N ¹⁺ and O ^{7+ to} O ¹⁺ (excluded because O ⁴⁺ is not separated from C ³⁺ ) that can be separated and detected from carbon ions. , N ⁷⁺ and O ⁸⁺ ions can be indirectly estimated.

In addition, since the amount of impurity ions is originally very small, the S / N ratio of the detection peak may be small.
In this case, the detection data assigning unit 33 (FIG. 1) improves the S / N ratio of the detection peak by adding the signal value of the detection data assigned at the common timing on the time counts of different periods. be able to.

That is, as shown in FIG. 2, the sensitivity can be improved by adding the detection peaks of ions emitted by the laser pulses 13a, 13b, and 13c at a plurality of timings at the same timing.
In this case, the update interval of the graph of FIG. 3 increases in proportion to the number of additions, but does not degrade the real-time property of monitoring.

The determination unit 36 determines whether the signal value at the impurity ion timing (indicator 39) or the integrated value of the detected peaks exceeds a threshold value.
Thereby, the purity of the ion beam can be evaluated by dividing various impurity ions contained in the ion beam by valence.
Or the determination part 36 determines whether the signal value which added all the detection data in the timing (indicator 39) of an impurity ion exceeds a threshold value.
Thereby, all the various impurity ions contained in the ion beam can be identified and the purity of the ion beam can be evaluated.

  The abnormal signal output unit 37 outputs an abnormal signal 27 indicating that the purity of the ion beam has decreased when the determination unit 36 determines that the threshold value is exceeded.

  As shown in the block diagram of FIG. 4, the ion accelerator to which the ion beam purity monitoring device 30 according to the embodiment is applied includes an ion source 10, a radio frequency quadrupole (RFQ) linear accelerator 41, It consists of a drift tube linear accelerator (DTL) 42 and a circular accelerator 43, which accelerates the ion beam and outputs it as a heavy particle beam.

  The RFQ linear accelerator 41 is connected to the subsequent stage of the ion source 10 and includes four electrodes (not shown) that form a quadrupole electric field by high frequency. The RFQ linear accelerator 41 simultaneously accelerates and converges the ion beam from the ion source 10 with a quadrupole electric field.

The DTL 42 is connected to the rear stage of the RFQ linear accelerator 41, and includes an electrode (not shown) that forms an electric field along the central axis by a high frequency, and a drift tube (not shown) that is arranged apart from each other along the central axis. I have.
The DTL 42 accelerates the ion beam from the RFQ linear accelerator 41 in a period in which the electric field is in the traveling direction parallel to the central axis, and passes the ion beam into the drift tube in a period in which the electric field is in the direction opposite to the traveling direction. As a result, the ion beam is gradually accelerated.

The circular accelerator 43 is connected to a subsequent stage of the DTL 42 and includes an electrode (not shown) that forms an electric field along a circular orbit by a high frequency.
The circular accelerator 43 circulates and accelerates the ion beam from the DTL 42 along the circular orbit by an electric field, and outputs a heavy particle beam 45 for irradiating a site to be irradiated.

The gate valve 44 is installed at any one of the input of the RFQ linear accelerator 41, the input of the DTL 42, and the input of the circular accelerator 43 (illustration shows the input of the circular accelerator 43).
The gate valve 44 stops the output of the ion beam by closing in response to the abnormal signal 27 output from the ion beam purity monitoring device 30.
That is, when the purity of the ion beam decreases due to the mixing of impurity ions, the ion accelerator closes the gate valve 44 and stops operation.

The operation of the ion beam purity monitoring apparatus according to the embodiment will be described based on the flowchart of FIG. 5 (see FIG. 1 as appropriate).
Impurity ions contained in the ion beam are estimated from the impurity elements of the target 12, adhering moisture, residual gas components in the vacuum vessel 11, and the like, and the detection timing corresponding to the mass-to-charge ratio of these impurity ions is set in the setting unit 35. (S11).

  Information on the laser oscillation period T of the laser oscillation unit 13 is acquired (S12), the reset flag 38 is set at a time interval of the period T, and counting by the time counter 32 is started (S13).

Laser irradiation is started, and detection data of ions flying from the target 12 to the detection unit 24 is acquired by the acquisition unit 31 (S14).
And the signal value of the acquired detection data is allocated with respect to a vertical axis | shaft on a timeline (FIG. 3: horizontal axis) according to the timing of a time count (S15).

In the process in which one cycle of laser oscillation elapses, detection data is acquired in order from the largest mass to charge ratio q / m and assigned to the timeline (S16: No).
Then, when one cycle of laser oscillation has elapsed and the next cycle T has been entered (S16: Yes), the time counter 32 is reset (S17: No, S13), and the flow up to (S16) described above is repeated. . Each time the flow from (S13) to (S17: No) is repeated, the S / N ratio of the detection peak improves.
When the number of additions reaches a predetermined value (S17: Yes), it is determined whether or not the detection peak that coincides with the timing of the impurity ions exceeds the threshold value (S18).

If the determination of exceeding the threshold is not made (S19: No), the time counter 32 is reset (S13), the flow up to (S19) described above is repeated, and the operation of the ion accelerator is continued.
If it is determined that the threshold value is exceeded, it is determined that the purity of the ion beam has decreased, an abnormal signal is output, and the operation of the ion accelerator is urgently stopped (S20).

  According to the ion beam purity monitoring apparatus of at least one of the embodiments described above, the qualitative / quantitative analysis of the ion species emitted in synchronization with the oscillation cycle of the laser irradiating the target is performed in a stable manner in real time. In addition, it is possible to monitor the presence or absence of impurity ions.

Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, changes, and combinations can be made without departing from the scope of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.
The constituent elements of the ion beam purity monitoring apparatus can be realized by a processor of a computer, and can be operated by an ion beam purity monitoring program.

  DESCRIPTION OF SYMBOLS 10 ... Ion source, 11 ... Vacuum container, 12 ... Target, 13 ... Laser oscillation part, 13a, 13b, 13c ... Laser pulse, 14 ... Plasma, 15 ... Pass-through hole, 16 ... Communication path, 17 ... Linear accelerator, 18 ... Collimator, 19 ... plasma transport tube, 20 ... analyzer, 21 ... branch hole, 23 ... electrostatic deflector, 24 ... detector, 25 ... flight trajectory, 26 ... detection data, 27 ... abnormal signal, 30 ... purity of ion beam Monitoring device 31 ... detection data acquisition unit (acquisition unit) 32 ... time counter 33 ... detection data allocation unit (allocation unit) 34 ... graph creation unit 35 ... timing setting unit (setting unit) 36 ... determination unit 37 ... Abnormal signal output unit (output unit), 38 ... Reset flag, 39 ... Indicator, 41 ... High frequency quadrupole linear accelerator (RFQ), 42 ... Drift tube linear accelerator (D L), 43 ... circular accelerator, 44 ... gate valve, 45 ... heavy ion.

Claims (5)

An acquisition unit for acquiring detection data of ions emitted in a direction that forms an angle with respect to a main axis of an ion beam emitted from a target irradiated with a laser;
A time counter that repeats time counting at the same cycle as the laser oscillation cycle;
An assigning unit for assigning the signal value of the detection data to the timing of the time count;
A setting unit for setting timing corresponding to a mass-to-charge ratio of impurity ions different from target ions constituting the ion beam;
A determination unit that determines whether a signal value of the detection data that coincides with the timing of the impurity ions exceeds a threshold;
An ion beam purity monitoring device comprising: an output unit that outputs an abnormal signal when the determination of exceeding the threshold value is made.   2. The ion beam purity monitoring apparatus according to claim 1, wherein the assigning unit adds a signal value of the detection data assigned to a common timing on the time count of different periods.   The purity of the ion beam according to claim 1 or 2, wherein the determination unit determines whether or not a signal value obtained by adding all the detection data at the timing of the impurity ions exceeds the threshold value. Monitoring device. Obtaining detection data of ions emitted in a direction that makes an angle with respect to a main axis of an ion beam emitted from a target irradiated with a laser;
Repeating time counting at the same cycle as the laser oscillation cycle;
Assigning the signal value of the detection data to the timing of the time count;
Setting a timing corresponding to a mass-to-charge ratio of impurity ions different from target ions constituting the ion beam;
Determining whether a signal value of the detection data matching the timing of the impurity ions exceeds a threshold;
Outputting an abnormal signal when it is determined that the threshold value is exceeded, and a method of monitoring the purity of the ion beam. On the computer,
Obtaining detection data of ions emitted in a direction that makes an angle with respect to a main axis of an ion beam emitted from a target irradiated with a laser;
Repeating time counting at the same cycle as the laser oscillation cycle,
Assigning the signal value of the detection data to the timing of the time count;
Setting timing corresponding to the mass-to-charge ratio of impurity ions different from target ions constituting the ion beam;
Determining whether the signal value of the detection data that coincides with the timing of the impurity ions exceeds a threshold;
An ion beam purity monitoring program that executes a step of outputting an abnormal signal when it is determined that the threshold value is exceeded.

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