Target gas loading for neutron generator
A method includes operating a neutron generator in a loading mode by ionizing ionizable gas within an ion source of the neutron generator to create a plurality of ions, and accelerating the plurality of ions by providing a first voltage to a target rod that supports the target to create a first ion beam that bombards a target of the neutron generator. The method also includes operating the neutron generator in a generating mode to generate a plurality of neutrons by accelerating the plurality of ions by providing a second voltage to the target rod to create a second ion beam that bombards the target. The second voltage is greater than the first voltage.
1 . A method, comprising:
operating a neutron generator in a loading mode by:
ionizing ionizable gas within an ion source of the neutron generator to create a plurality of ions; and
accelerating the plurality of ions by providing a first voltage to a target rod that supports a target of the neutron generator to create a defocused ion beam that bombards the target, wherein the first voltage has a first value ranging from approximately 10 kilovolts (kV) to 50 kV, wherein the defocused ion beam results from an increased number of charge exchange collisions at the first voltage that scatter ion trajectories, and wherein the defocused ion beam is incident on at least 80% of a surface area of the target; and
operating the neutron generator in a generating mode to generate a plurality of neutrons by accelerating the plurality of ions by providing a second voltage to the target rod to create a focused ion beam that bombards the target, wherein the second voltage is greater than the first voltage, wherein the second voltage has a second value ranging from approximately 60 kV and 130 kV, and wherein the focused ion beam is more focused than the defocused ion beam due to reduced charge exchange collisions at the second voltage compared to the first voltage.
2 . The method of claim 1 , further comprising:
making a determination that a neutron generation value, for a particular value of the second voltage, is less than a threshold neutron generation value; and
operating the neutron generator in the loading mode after operating the neutron generator in the generating mode responsive to the determination.
3 . The method of claim 1 , further comprising:
making a determination that the second voltage, for a particular neutron generation value, is greater than a threshold voltage; and
operating the neutron generator in the loading mode after operating the neutron generator in the generating mode responsive to the determination.
4 . The method of claim 1 , further comprising:
determining that an amount of time of operating the neutron generator in the generating mode is greater than a threshold; and
operating the neutron generator in the loading mode after operating the neutron generator in the generating mode responsive to determining that the amount of time is greater than the threshold.
5 . The method of claim 1 , further comprising stopping operating in the loading mode responsive to determining that a neutron generation value, for a particular value of the first voltage, is above a threshold neutron generation value.
6 . The method of claim 1 , further comprising:
deploying a logging tool including the neutron generator into a wellbore;
transmitting the plurality of neutrons from the neutron generator into a formation surrounding the wellbore; and
receiving a signal measurement related to the plurality of neutrons at one or more sensors in the logging tool.
7 . The method of claim 1 , further comprising operating the neutron generator in a pulsed mode to generate the neutrons in pulses according to a timing sequence.
8 . The method of claim 7 , wherein a duration of each pulse is between 10-80 microseconds.
9 . The method of claim 7 , wherein an idle time between the pulses is between 50 to 1000 microseconds.
10 . The method of claim 7 , wherein a duty factor of the pulsed mode is between 5-30 percent.
11 . The method of claim 1 , wherein the ionizable gas comprises deuterium gas.
12 . The method of claim 1 , wherein the ionizable gas comprises tritium gas.
13 . The method of claim 1 , wherein the target comprises a metal foil layer.
14 . The method of claim 1 , wherein the target comprises a titanium foil layer.
15 . The method of claim 1 , wherein the target comprises a zirconium foil layer.
16 . The method of claim 1 , wherein the target comprises a scandium foil layer.
17 . The method of claim 1 , wherein a diameter of the focused ion beam is smaller than a diameter of the target.
18 . The method of claim 1 , wherein the target has a diameter of approximately 8 millimeters and a thickness of approximately 3-5 micrometers.
19 . The method of claim 18 , wherein the defocused ion beam has a beam spot larger than the diameter of the target.
20 . The method of claim 1 , wherein operating the neutron generator in the loading mode results in the target achieving a gas saturation ratio of at least 1.5:1 of combined deuterium and tritium atoms per target metal atom.