Ruggedized neutron shields
A shield for a well logging instrument includes an outer layer including a reinforcing fiber disposed in a matrix and an inner layer including a thermal neutron absorbing material disposed in a matrix. The thermal neutron absorbing material is selected to emit capture gamma rays having energy outside a selected energy range.
1. A shield for a well logging instrument disposed in a formation, comprising:
an outer layer including a reinforcing fiber disposed in a matrix, the reinforcing fiber comprising a carbon fiber;
an inner layer including a thermal neutron absorbing material disposed in a matrix in a graduated manner with a greater concentration of thermal neutron absorbing material disposed in a direction away from the outer layer, the thermal neutron absorbing material selected to emit capture gamma rays having energy outside a selected energy range of gamma rays of interest resulting from neutron interaction with the formation; and
a structural layer disposed between the inner layer and the outer layer.
2. The shield of claim 1 wherein the thermal neutron absorbing material comprises boron-10.
3. The shield of claim 1 wherein the outer layer matrix comprises at least one of nitrile rubber, polyetherketone, polyetheretherketone and polyetherketoneketone.
4. The shield of claim 1 wherein the thermal neutron absorbing material comprises neutron absorbing particles deposited on a fiber, the deposited fiber disposed in a matrix.
5. The shield of claim 4 wherein the deposited fiber comprises at least one of glass fiber and carbon fiber.
6. The shield of claim 1 wherein the structural layer comprises polyetheretherketone.
7. The shield of claim 1 wherein the inner layer and the outer layer are configured as annular cylinders.
8. The shield of claim 7 wherein the annular cylinders are disposed on an exterior surface of a tool mandrel, the tool mandrel having a radiation detector therein.
9. A well logging tool, comprising:
a tool mandrel configured to be moved along an interior of a wellbore drilled through subsurface formations;
at least one radiation detector disposed in an interior chamber defined by the tool mandrel;
a radiation shield disposed on an exterior surface of the tool mandrel proximate a position therein of the radiation detector, the shield including an outer layer including a reinforcing fiber, comprising a carbon fiber, disposed in a matrix, and an inner layer including a thermal neutron absorbing material disposed in a matrix in a graduated manner with a greater concentration of thermal neutron absorbing material disposed in a direction away from the outer layer, the thermal neutron absorbing material selected to emit capture gamma rays having energy outside an energy range of gamma rays of interest resulting from neutron interaction with the subsurface formations, and a structural layer disposed between the inner layer and the outer layer.
10. The tool of claim 9 wherein the thermal neutron absorbing material comprises boron-10.
11. The tool of claim 9 wherein the outer layer matrix comprises at least one of nitrile rubber, polyetherketone, polyetheretherketone and polyetherketoneketone.
12. The tool of claim 9 wherein the thermal neutron absorbing material comprises neutron absorbing particles deposited on a fiber, the deposited fiber disposed in a matrix.
13. The tool of claim 12 wherein the deposited fiber comprises at least one of glass fiber and carbon fiber.
14. The tool of claim 9 wherein the structural layer comprises polyetheretherketone.
15. The tool of claim 9 wherein the inner layer and the outer layer are configured as annular cylinders.