WO2009087104A1

WO2009087104A1 - Dosimeter based on monocrystalline synthetic diamond

Info

Publication number: WO2009087104A1
Application number: PCT/EP2009/000081
Authority: WO
Inventors: Marco Marinelli; Gianluca Verona Rinati; Enrico Milani
Original assignee: Universita'degli Studi Di Roma 'tor Vergata'
Priority date: 2008-01-09
Filing date: 2009-01-08
Publication date: 2009-07-16
Also published as: ITRM20080013A1

Abstract

Dosimeter based on monocrystalline synthetic diamond comprising a first synthetic CVD monocrystalline diamond layer and a second synthetic CVD monocrystalline diamond layer, intrinsic or doped, having electrical conductivity much lower than the first layer and defining a sensitive element of the device. The dosimeter, particularly suitable to define a portable device due to its complete energetic autonomy and useful for in-vivo dosimetry, turns out to be particularly stable in terms of response, drastically reduces memory effects and favours a very high repeatability of the production process of the device itself.

Description

DOSIMETER BASED ON MONOCRYSTALLINE SYNTHETIC DIAMOND

Field of the invention

The invention refers to a dosimeter based on monocrystalline synthetic diamond.

State of the art

Modern radiotherapy requires dosimetric systems which must be reliable and small in size, with a response precise, fast, stable in time, independent on the energy and on the dose rate and, possibly, tissue equivalent. Diamond shows very favorable properties for application as a dosimeter: it is nearly tissue equivalent, radiation-hard, has an extremely low dark current and a high mobility of the charge carriers, thus potentially leading to a very fast response time.

This produced a large scientific research activity in this field and led to the commercialization of diamond based dosimeters. However such devices, presently based on very high quality natural diamond, are extremely rare, expensive and characterized by a low uniformity of their performance.

For these reasons, the possible use of synthetic diamond was explored. The diamond grown by High-Pressure High-Temperature (HPHT), although monocrystalline, is not pure enough to overcome the problems above discussed for the natural diamond. On the other hand, diamonds grown by Chemical Vapor

Deposition (CVD) studied up to now are polycrystalline. This leads to some drawbacks such as a low signal stability and a very poor response time.

The remaining possibility is to use CVD diamond homoepitaxially grown on monocrystalline substrates, which can be poor in quality and consequently cheep.

A small number of detectors produced in this way has been studied and reported in literature for applications as particle detectors, e.g. for neutrons, charged particles and electromagnetic radiation such as X-rays and UV radiation.

For trying to produce a dosimeter based on CVD monocrystalline diamond, it was observed that the substrate deteriorates the response of the dosimeter in a non reproducible way. On the other hand, the removal of the substrate is a hard and expensive procedure, especially when thin films are required.

The diamond based dosimeters studied up to now or commercialized show one or more of the following drawbacks: 1. use of high quality natural diamond, extremely expensive and with a low availability;

2. presence of memory effects producing low stability and reproducibility of the response;

3. need of high bias voltage;

4. low repeatability of the device performance, due to the fabrication process or to the intrinsic properties of the used material;

5. response dependent on the delivered dose rate, making a correction of the raw measured data necessary.

In particular, the only diamond dosimeters presently used in clinical environments are based on natural diamond. Such diamonds must be accurately selected in order to match the requirements imposed by the international protocols. For this reason the availability of such dosimeters is extremely low, much lower than the market demand and consequently they are very expensive. Summary of the invention

The aim of the invention is to provide a multilayer monocrystalline diamond and a dosimeter based on such multilayer monocrystalline diamond. The present invention is therefore intended to achieve the above discussed aim by producing, according to a first aspect of the invention, a multilayer monocrystalline diamond which, according to claim 1 , comprises a first layer of synthetic monocrystalline CVD diamond, intrinsic or doped; a second layer of synthetic monocrystalline CVD diamond, intrinsic or doped, having electrical conductivity sufficiently lower than that of the first layer; a first contact, forming a junction with the second layer; a second contact, forming a junction with the first layer; said first and second contacts defining the contacts of the multilayered diamond for the connection to measurement means for measuring electrical signals; and by producing, according to another aspect of the invention, a dosimeter comprising, according to claim 12, the above mentioned multilayer monocrystalline diamond, wherein the second layer of synthetic monocrystalline CVD diamond layer, intrinsic or doped, defines a sensitive element of the dosimeter, and there are provided measurement means, as part of a control circuit, connected to said first and second contacts for measuring an electrical signal produced by a radiation incident on said sensitive element.

Said first and second contacts are connected through appropriate cabling to said electrical measurement means.

A further aim of the present invention is to provide a process for producing the above mentioned multilayer monocrystalline diamond.

The present invention is therefore intended to reach this aim through a process for producing a multilayer monocrystalline diamond that, according to claim 16, comprises the following steps of:

- providing the substrate for the epitaxial deposition of synthetic monocrystalline CVD diamond;

- depositing a first layer of synthetic monocrystalline CVD diamond, intrinsic or doped, on said substrate;

- depositing a second layer of synthetic monocrystalline CVD diamond, intrinsic or doped, on said first layer;

- making a first junction between the second layer of synthetic monocrystalline CVD diamond and a first contact;

- making a second junction between the first layer of monocrystalline synthetic CVD diamond and a second contact.

According to another aspect of the invention, such dosimeter can be applied in radiotherapy, and therefore in health frames equipped with radiotherapy treatment machines. In particular, it can be used both for monitoring the emission of the radiotherapic sources and planning the irradiation treatment, and for measuring the dose absorbed by the patient during the radiotherapic treatment. A further application is to monitor the ionizing radiation dose in potentially exposed environments or persons like, for instance, in scientific laboratories equipped with accelerators or radioactive sources, nuclear reactors, sites particularly exposed to environmental radiation such as mines, spacecrafts, etc.

Moreover, it can be used as a reference for the calibration of other dosimeters, and therefore in industries that produce or use such devices or in calibration and certification centers. Finally, a key property of the dosimeter, object of the present invention, is its capability to work powered by a common battery or even with no external power applied. This feature is particularly useful for in-vivo dosimetric application, and makes the realization of portable devices possible.

The dependent claims describe preferred embodiments of the invention.

Brief description of Figures

Further features and advantages of the invention will be more evident from the detailed description of a preferred, but not exclusive, embodiment of a dosimeter based on synthetic monocrystalline diamond, illustrated by way of non-limitative example with reference to the accompanying drawings, in which:

Fig 1 schematically shows a dosimeter according to the invention.

The numbers reported in the description below for the dosimeter elements refer to those indicated in Fig. 1.

Detailed description of a preferred embodiment of the invention

A multilayer monocrystalline diamond, according to the present invention, comprises: a) a layer 2 made of synthetic monocrystalline CVD diamond, intrinsic or doped; b) a layer 3 made of synthetic monocrystalline CVD diamond, intrinsic or doped, having electrical conductivity much lower than that of the layer 2 and defining a sensitive element of a dosimeter based on said multilayered diamond; c) a first contact or electrode 4, forming a junction with a surface of the layer 3; d) a second contact or electrode 5, forming a junction with the layer 2.

These electrodes or contacts 4, 5 define the electrical contacts of the multilayer monocrystalline diamond and allow the connection through appropriate cabling to measurement means, which are part of a control circuit, adapted to measure the voltage between the contacts or the current flowing through the contacts as a consequence of the incidence of a ionizing radiation on the sensitive element. The ensemble formed by the multilayer monocrystalline diamond, the contacts, cabling and the measurement means defines the dosimeter according to the invention. Preferably, the difference in the electrical conductivity between the first layer 2 and the second layer 3 is of at least about a factor 10000. In particular, the electrical conductivity of the first layer 2 is more than a factor 10000 higher than that of the second layer 3.

Moreover the thickness of the layer 3 is between 1 and 100 μm. In a preferred embodiment said thickness is between 5 and 40 μm.

Preferably the second contact 5 forms an Ohmic junction with the layer 2, while the first contact 4 can be made of any material forming an ohmic or non-ohmic junction with the diamond defining the layer 3. Contact 4 is preferably a non- patterned contact.

In a preferred embodiment of the multilayered monocrystalline diamond, there is provided a further substrate 1 made of monocrystalline diamond, which allows an easier homoepitaxial or epitaxial deposition of the synthetic diamond layers by supplying a monocrystalline crystallographic matrix. This substrate 1 does not have any electrical role and is electrically bypassed according to the procedures described below.

The substrate 1 is constituted by monocrystalline diamond and can be made, for example, of synthetic diamond grown by High-Pressure High-Temperature (HPHT) or constituted by a poor quality and low cost natural diamond. For the above described reason, the substrate 1 is an optional part of the dosimeter, which includes however layers 2 and 3.

The layer 2 can be doped with any element, for instance with Boron, or can have structural imperfections, as long as it has an electrical conductivity sufficiently higher than that of the layer 3.

The connection of the layer 2 with the contact 5 allows to electrically decouple the layer 3, defining the sensitive element, from the substrate 1 allowing to avoid the repeatability problems connected to it.

The ionizing radiation to be detected goes through the sensitive layer 3, and also through other elements of the device, producing electrical charges, electrons and holes. The adopted configuration of the crystalline layers advantageously produces a built-in electric field inside the device, which interacts with the said electrons and holes producing an electrical signal between electrodes 4 and 5. The built-in electric field can also be generated by applying an external bias voltage. The interaction of the ionizing radiation (photons, electrons, etc.) with the layer 3 produces a current proportional to the absorbed radiation dose rate and practically independent on the radiation quality, which can be measured by the measuring means.

Therefore the dosimeter of the invention advantageously allows to quantify in an optimal way the absorbed dose, i.e. the energy deposited in the tissue per unity mass. In fact, the output signal only depends on the energy deposited in the tissue. This implies that, given the absorbed dose, the dosimeter gives practically the same response regardless of the nature of the absorbed radiation. This effect is partly due to the tissue equivalence of diamond, that is the property of absorbing the ionizing radiation in a way very similar to the biological tissue.

The above mentioned control circuit including the measuring means may or may not include an external bias source. Preferably the circuit is operated using only the built-in potential difference established between electrodes 4 and 5 because of the configuration of the crystalline diamond layers in the device.

The control circuit with the measuring means, for instance conventional electrometers, can be integrated in the dosimeter or can be an external component.

The electrode 4 can be surrounded by a guard ring.

A procedure for the production of the multilayer monocrystalline diamond according to the invention provides the sequential growth of the layers 2 and 3. In this case, in order to make possible the contact between the electrode 5 and the layer 2, this procedure can include any one of the following steps:

A) a removal of the substrate 1 or a part of it, and/or a removal of a part of the second layer 3 or a removal of part of the diamond through mechanical and/or chemical and/or thermal treatment or any etching process, or by cutting or incision;

B) a selective deposition of the second layer 3, e.g. through the deposition in one or more places of the first layer 2 of any material adapted to prevent or modify the successive growth of the second layer 3;

C) deposition of the contact 5 on the layer 2 before the growth of the layer 3; D) creation of a conductive channel crossing the substrate 1 or the second layer 3 for instance by doping or damaging modifying part of the substrate 1 or of the second layer 3;

E) deposition of ohmic or injecting contacts on the first layer 2.

The dosimeter of the invention is compatible with the electrometers usually adopted in hospitals for the measurement of the output signal of conventional dosimeters, like for example the ionization chambers.

This dosimeter has completely new commercial opportunities.

It can be used, unlike already known ones, as a personal dosimeter, for in-vivo dosimetry applications, in radiotherapic treatments in which is useful to move the patient.

Moreover, it can be used in environments without electrical power.

Therefore, the proposed dosimeter advantageously allows to:

- use only synthetic diamond or, for the substrate 1 only, low-cost natural diamond;

- deposit relatively thin diamond layers 2 and 3, typically below 100 μm, reducing the cost and the production time;

- improve the stability of the response, drastically reducing the memory effects;

- use the device also without external bias and/or electrical power;

- obtain a high reproducibility in the production process;

- sensibly reducing the dependence of the measured signal on the dose rate. As a consequence through the dosimeter of the invention it is possible:

- to cut down the production costs;

- to reduce the production time thus allowing a larger scale production to better match the market demand;

- to simplify the acquisition electronics and to allow the production of portable diamond based dosimeters;

- to produce dosimeters suitable be certified for in-vivo applications.

The particular embodiments herein described do not limit the contents of this application covering all the variants of the invention defined in the claims.

Claims

1. Multilayered monocrystalline diamond comprising:

- a first layer (2) of synthetic monocrystalline CVD diamond, intrinsic or doped;

- a second layer (3) of synthetic monocrystalline CVD diamond, intrinsic or doped, having electrical conductivity sufficiently lower than that of the first layer (2);

- a first contact (4), forming a junction with the second layer (3);

- a second contact (5), forming a junction with the first layer (2); said first and second contacts defining the contacts of the multilayered diamond for the connection to measurement means for measuring electrical signals.

2. Multilayered monocrystalline diamond according to claim 1 , wherein the electrical conductivity of the first layer (2) is more than a factor 10000 higher than that of the second layer (3).

3. Multilayered monocrystalline diamond according to claim 1 , wherein said first contact (5) defines an ohmic junction with the first layer (2).

4. Multilayered monocrystalline diamond according to claim 1 , wherein said second contact (4) can be made of any material defining an ohmic or non ohmic junction with the diamond forming the second layer (3).

5. Multilayered monocrystalline diamond according to claim 1 , further comprising a substrate (1 ) adapted to provide a monocrystalline diamond matrix during the epitaxial deposition of said first and second layers (2, 3) of synthetic diamond.

6. Multilayered monocrystalline diamond according to claim 1 , wherein said first layer (2) can be without distinction doped with any element.

7. Multilayered monocrystalline diamond according to claim 6, wherein said first layer (2) is doped with boron.

8. Multilayered monocrystalline diamond according to claim 1 , wherein said first layer (2) is structurally imperfect.

9. Multilayered monocrystalline diamond according to claim 1 , wherein the first contact (4) is surrounded by a guard ring.

10. Multilayered monocrystalline diamond according to claim 1 , wherein the thickness of said second layer (3) is between 1 and 100 μm.

11. Multilayered monocrystalline diamond according to claim 10, wherein the thickness of said second layer (3) is between 5 and 40 μm.

12. Dosimeter comprising the multilayered monocrystalline diamond of claim 1 , wherein the second layer (3) of synthetic monocrystalline CVD diamond, intrinsic or doped, defines a sensitive element of the dosimeter and there are provided measurement means, which are part of a control circuit, connected to said first and second contacts (4, 5) for measuring an electrical signal produced by a radiation incident on said sensitive element.

13. Dosimeter according to claim 12, wherein the control circuit is adapted to work due to a potential difference between said contacts (4, 5).

14. Dosimeter according to claim 13, wherein said potential difference is of the built-in type.

15. Dosimeter according to claim 13, wherein said potential difference is provided by an external source.

16. Process for producing a multilayered monocrystalline diamond according to claims from 1 to 11 comprising the following steps of:

- providing the substrate (1 ) for the epitaxial deposition of synthetic monocrystalline CVD diamond;

- depositing a first layer (2) of synthetic monocrystalline CVD diamond, intrinsic or doped, on said substrate;

- depositing a second layer (3) of synthetic monocrystalline CVD diamond, intrinsic or doped, on said first layer (2);

- making a first junction between the second layer (3) of synthetic monocrystalline CVD diamond and a first contact (4);

- making a second junction between the first layer (2) of monocrystalline synthetic CVD diamond and a second contact (5).

17. Process according to claim 16, wherein said step of making a second junction provides a removal of the substrate (1 ) or a part of it, and/or a removal of a part of the second layer (3) or a removal of part of the diamond by mechanical and/or chemical and/or thermal treatment or any etching process, or by cutting or incision.

18. Process according to claim 16, wherein said step of making a second junction provides a selective deposition of the second layer (3) through the deposition in one or more places of the first layer (2) of any material adapted to prevent or modify the successive growth of the second layer (3).

19. Process according to claim 16, wherein said step of making the second junction is performed before the deposition of the second layer (3).

20. Process according to claim 16, wherein said step of making the second junction provides the creation of a conductive channel crossing the substrate (1 ) or the second layer (3) by doping or modifying part of the substrate (1 ) or of the second layer (3).

21. Process according to claim 16, wherein said step of making the second junction provides the deposition of ohmic or injecting contacts on the first layer (2).