WO2019072319A1

WO2019072319A1 - A method to determine the type of ionising radiation using a semiconductor diode and a circuit for carrying out this method

Info

Publication number: WO2019072319A1
Application number: PCT/CZ2017/000076
Authority: WO
Inventors: Martin KÁKONA; Václav Štěpán; Pavel Krist
Original assignee: Ústav Jaderné Fyziky Av Čr, V.V.I.
Priority date: 2017-10-12
Filing date: 2017-12-04
Publication date: 2019-04-18
Also published as: CZ307570B6; CZ2017649A3

Abstract

A method to determine the type of ionising radiation using a semiconductor PIN diode connected in the reverse direction, where a PIN diode with a low/zero negative bias from zero to a few volts is used and its output contains a charge proportional to energy transferred to the diode by the incoming ionising radiation. Subsequently, the charge is amplified and converted to a voltage pulse whose area corresponds to energy transferred to the diode by the ionising radiation; the signal is processed using shape discrimination to determine the incoming radiation type, where the response of a positive ion has a higher amplitude than that of a photon for the same pulse area, i.e., the same transferred energy. The output of the semiconductor PIN diode connected in the reverse direction, is connected to an amplifier which is connected to a computer via an A/D converter.

Description

A method to determine the type of ionising radiation using a semiconductor diode and a circuit for carrying out this method.

Technical field

The invention is used to detect ionising radiation using a semiconductor PIN diode. Background art

Semiconductor PIN diodes or identical structures with an intrinsic semiconductor layer with low conductivity (the same as in a PIN diode) are currently used in various ionising radiation detectors. In addition to a conventional PIN diode consisting of a doped semiconducting P/N material and a non-doped or very lightly doped I material, metal-semiconductor junctions are used as a replacement of the P or N layer. Each of these structures has features enabling construction of the below described ionising radiation detector. Using a PIN structure enables the creation of diodes thicker than common PN diodes, with lower requirements on its reverse bias. The intrinsic layer expands the depleted diode area even for zero reverse bias. If the intrinsic layer were fully neutral, the depleted area width would be at least equal to the intrinsic area's width; that would imply that the PIN diode would then need no reverse bias. Due to defined parameters of factory-made semiconductor materials, only the cleanest, very lightly doped semiconductor crystals, mostly of the N-type, are available on the market. If a PIN diode with an I layer manufactured using a very lightly doped material is left without a reverse bias, the electric field in this layer will be asymmetric, with a small gradient. However, this feature is not used in today's ionising radiation detectors.

Most detectors use a PIN diode in the reverse direction with a high negative bias to fully deplete a diode area, as wide as possible. Such design does not enable the determination (or in a very limited way) of the type of incoming ionising radiation, be it directly or indirectly ionising radiation. The high-negative-bias setup does not enable determination of the ionising radiation type based on the type of ionising radiation interaction with the diode material. Only the penetration depth of the particle that has transferred all its energy to the material can be, in a limited way, determined. This causes substantial functional disadvantages. To determine the equivalent dose for humans, or in general to evaluate radiation exposure levels and to quantify the risk increase for living organisms in connection with higher ionising radiation exposure, we have to know the type of the incoming ionising radiation. If we use a PIN diode in a dosimeter in current designs with a high negative bias, we have to know the type of the ionising radiation in the field we measure to determine the effective dose. The effective dose cannot be determined this way in a mixed radiation field without prior knowledge of its composition.

Summary of the invention

The presented invention's task is to distinguish ionising radiation consisting of positive ions, e.g. alpha particles or protons, from photons, e.g. gamma photons, and to enable designing devices for radiation dosimetry in mixed fields that contain both positive ions and photons.

In order for the invention's underlying principle to determine the ionising radiation type using a semiconductor PIN diode connected in the reverse direction, a PIN diode with a low/zero bias of zero or just a few volts is used which has an output charge proportional to energy transferred by the ionising radiation; the charge is, in turn, amplified and converted to a voltage pulse whose area corresponds to the energy the ionising radiation has transferred in to the diode; the signal is evaluated using shape discrimination to determine the ionising radiation type, where the response from a positive ion has higher amplitude than the response from a photon for the same pulse area, i.e., for the same transferred energy.

To determine the particle type, the pulse area is calculated by integrating the voltage pulse signal over time using an analogue/digital method and comparing pulse amplitudes for the same pulse area.

Alternatively, the particle is specified from the pulse area calculated by integrating the voltage pulse signal using an analogue/digital method and comparing pulse areas for the same pulse amplitude.

Determining the particle type is advantageously performed comparing the pulse duration over a previously selected discrimination level against the pulse amplitude, or comparing the pulse duration over a previously selected discrimination threshold against the pulse area. The invention principle also contains a circuit used to implement the above

mentioned method, where a semiconductor PIN diode is in the reverse direction and connected to an amplifier that is, in turn, connected to a computer via a filter and A/D converter.

The invention enables to distinguish between ionising radiation consisting of positive ions (e.g. alpha particles, protons or heavier charged ionising particles) and photons (e.g. gamma photons). It enables designing devices for ionising radiation dosimetry in mixed fields that contain both positive ions and photons with an unknown ratio of each radiation type. It enables the separate quantification of contributions from photons and positive ions in such fields using a PIN diode as a detection element, and consequently determination of an equivalent dose without prior knowledge of the field composition.

Brief description of the drawings

Fig. 1 shows an example of block diagram of a system used to determine the ionising radiation type using a semiconductor PIN diode; Fig. 2 shows a specific circuit, implemented using the diagram in Fig. 1.

Fig. 3 shows an output from the A/D converter from Figs. 1 and 2; an example of signal discrimination for particles with the same response amplitude. The red curve represents a proton response; the green curve is a photon response. Seven protons and three photons are overlaid in the chart.

Fig. 4 displays the output from the A/D converter from Figs, land 2, an example of signal discrimination. The red curve represents a proton response; the green curve is a photon response.

Each particle transferred the same amount of energy, the pulse area between the curve and the horizontal axis is the same. Two protons and five photons are overlaid in the chart.

Fig. 5 shows a block diagram of another example how to implement the invention using a peak detector and a pulse-length to digit converter. Fig. 6 contains a block diagram showing how to implement the invention using analogue circuits only; two analogue values appear in the output - one corresponds to the amplitude, the other is the pulse area.

Detailed description of the invention

Fig. 1 shows a block diagram based on the invention, where a semiconductor PIN diode is in the reverse direction and its output is connected to a charge amplifier, connected to a computer via a filter (low-pass) and A/D converter.

It is a silicon PIN diode setup with a low/zero negative bias (from zero to a few volts, compared to tens or hundreds of volts commonly used for PIN diodes hundreds-of- micrometres thick). The block diagram in Fig. 1 consists of an amplifier (charge to voltage converter - charge amplifier) which converts the charge on the diode to a measurable voltage pulse. This amplifier/converter can be substituted by a current to voltage amplifier; however, these amplifiers are less stable than a solution based on the charge amplifier. The pre-amplifier/converter type does not play a major role in the invention principle. Any pre-amplifier capable of creating a pulse that can be processed using an AID converter can be used. The block diagram also contains a low-pass filter. A low-pass filter is an element used to increase a device's noise immunity and suppress high-frequency noise. A high-pass filter can be integrated into the circuit in the same way and used to filter out low-frequency noise. High/low-pass filter parameters depend on dynamic parameters of a usable signal produced in the pre-amplifier. The last element is an appropriate A D converter which converts a pulse to a series of values over time. The resulting digital signal can be discriminated using digital filters or directly, using digital analysis, as described below.

The PIN diode's output in the circuit produces a charge proportional to energy deposited by primary particles of the radiation field or by secondary particles of the ionizing radiation in the sensitive volume of the detector - diode. When the charge is converted to a voltage pulse in the charge amplifier, its area corresponds to energy passed on to the diode by the ionising radiation. Simultaneously, a positive ion response will have higher amplitude than a photon response for the same pulse area (i.e. for the same energy passed on). A positive ion can be discriminated from a photon using shape discrimination. In other words - for zero/low bias on the diode, collecting a charge created by photon ionisation is slower than collecting a charge created by a positive ion. A custom software application for shape discrimination is not the subject matter of the invention. Particle type discrimination can also be achieved using analogue/digital pulse integration (i.e. determining the pulse area) and comparing pulse amplitudes for the same pulse area (see Fig. 4) or comparing pulse areas for the same pulse amplitude (see Fig. 3), or calculating amplitude-pulse area ratio or comparing pulse duration over a specific, pre-set discrimination level to the pulse amplitude. The discrimination level is above the signal noise level, approximately below one-third of the pulse amplitude, optimum is double the noise level.

Fig. 2 shows a functional implementation of the invention. The charge created by ionisation in the D1 PIN diode is converted to voltage using the U1A charge amplifier. This way, a voltage pulse is created, which is, in turn, reshaped in the C2, R2 high- pass filter and U1B low-pass filter. The filters mentioned above do not substantially influence the core of the invention, since they only .increase circuit endurance to interference and its own noise. To enable determination of changes in both the leading and trailing edge, the resulting band-pass filter cannot be too narrow for the described method of determining the ionising radiation type. Subsequently, the analogue signal is converted to a series of values over time using an ADC

(Analogue/Digital converter). The used A/D converter has to feature a sufficiently high sampling rate to enable the determination of changes in the leading and trailing edges of the signal. The sampling frequency used in the above mentioned example of implementation is 20 MHz. The output signal undergoes discrimination, e.g., the one method shown in Figs. 3 or 4. The specific signal discrimination method is not the subject matter of the invention. Considering, for example, discrimination based on the pulse area, we can determine two particles of the same energy passed on to the PIN diode material. A pulse that will have the same area and higher amplitude will be related to a positively charged ion, whereas a pulse with the same area and lower amplitude will be attributed to a photon.

Digital processing of the signal can be replaced by analogue circuits, or special digital circuits. Figures 5 shows an example of implementation using a peak detector. A pulse from the charge amplifier comes to the peak detector, where its amplitude is recorded. Simultaneously, the pulse duration is measured using the comparator and time-to-digit converter. These two values, amplitude and pulse duration, are used to discriminate the particle type, the same way as used with continual sampling above. This circuit's advantage is that there is no need to perform continual signal conversion to digital values, and it is suitable for devices where low power consumption is required. The comparator's START output is connected to a computer and can trigger both ADC and TDC conversions. Having finished the conversions, the computer resets the peak detector (RESET signal) and waits until another pulse arrives.

Another example of implementation in Figure 6 uses a pure analogue approach to process the signal. A pulse from the charge amplifier comes to the peak detector and, simultaneously, to the integrator. The peak detector is used as a memory for pulse amplitude and the integrator as a memory for the pulse area. The resulting analogue values can then be compared using a suitable procedure. Pulse amplitude and area in the above mentioned example are converted to numbers which are, in turn, compared using software in the computer.

Claims

1. A method to determine the ionising radiation type using a semiconductor PIN diode in the reverse direction, characterized by usage of a PIN diode with a low or zero negative bias from zero to a few volts, whose output contains a charge proportional to energy passed on by ionising radiation to the diode, the charge is then amplified and converted to a voltage pulse whose area corresponds to energy passed on by ionising radiation to the diode, the signal is evaluated using shape discrimination to determine the ionising radiation type where the response of a positive ion has a higher amplitude than that of a photon for the same pulse area, i.e. the same energy passed on.

2. The method according to claim 1 , characterized by discriminating the particle type by determining the pulse area using analogue or digital integration of the pulse and comparing pulse amplitudes for the same pulse area.

3. The method according to claim 1 , characterized by discriminating the particle type by determining the pulse area using analogue or digital integration of the pulse and comparing pulse areas for the same pulse amplitude.

4. The method according to claim 1 , characterized by discriminating the particle type by comparing the pulse duration over a pre-set discrimination level against the pulse amplitude.

5. The method according to claim 1 , characterized by discriminating the particle type by comparing the pulse duration over a pre-set discrimination level against the pulse area.

6. The circuit for carrying out the method according to any of claims 1 to 5,

characterized in that the output of a semiconductor PIN diode in the reverse direction is connected to an amplifier connected to a computer via an A/D converter.