Originally posted by physicsopenlab, the link is Detection of beta and alfa radiation with KC761B
Abstract: in this article, we continue the presentation of the new KC761B device. In previous posts, we described the device in general terms and its functionality as a gamma spectrometer. In this post, we describe its use as a beta and alpha radiation detector. To detect beta and alpha particles, the device uses a PIN-type semiconductor sensor positioned on the back of the device.
Introduction
The Deepace KC761B instrument contains two radiation sensors. The main sensor is a CsI(Tl) scintillator crystal coupled to solid-state photomultipliers (SiPM). The scintillator crystal is mainly sensitive to gamma radiation, which allows it to obtain an excellent energy resolution of about 7.5%. The excellent performance of the scintillator crystal/SiPM enables the instrument to be used both as a sensitive gamma radiation detector (dosimeter) and as a spectrometer.
In addition to the scintillator crystal, the instrument houses a second solid-state sensor, based on a PIN diode, sensitive only to beta radiation and alpha radiation. Even with this second sensor, the KC761B instrument, in addition to recording the interaction event with the particle, measures the amplitude of the signal and therefore allows the acquisition of the energy spectrum of the radiation.
The PIN Detector
A PIN diode (p-type, intrinsic, n-type diode) is a diode with a large intrinsic (undoped) semiconductor region contained between a p-type and an n-type region. The advantage of a PIN diode is that the depletion region of the junction is located almost entirely within the intrinsic region, which has a constant (or nearly constant) width regardless of the voltage applied to the diode.
The intrinsic region can be made as large as desired, thus increasing the volume in which electron-hole pairs can be generated.
The generation of charge carriers within the intrinsic region can occur thanks to incident light radiation. For these reasons, many photosensors use a PIN diode. In addition to light radiation, charge carriers can also be generated (but with low efficiency) by gamma radiation and X-ray radiation, but especially by beta and alpha particles, for which the detection efficiency is practically 100%. For these reasons, a PIN diode can also be used as an efficient solid-state radiation detector.
In the KC761B instrument, the PIN sensor is located on the back of the device, protected by a rubber cover. Under the rubber cover is the capsule that encloses the sensor, which is in turn obscured by a silver mylar film. The PIN sensor is sensitive to light, for this reason, it is necessary to cover it with a film that must be completely opaque to light but, at the same time, must allow the passage of beta and alpha radiation with minimal absorption. The images in Fig. 1 and Fig. 2 show the back of the instrument and the housing of the PIN sensor.
Fig 1 – The back side of the instrument with the PIN sensor housing for beta and alpha radiation
Fig 2 – Detail of the PIN sensor with its mylar protection
Beta radiation detection with the scintillation crystal
Before showing the beta and alpha radiation detection results made with the PIN sensor, we show how beta radiation can also be efficiently detected and measured with the scintillation sensor. The scintillation sensor consists of a scintillator crystal that is also sensitive to beta radiation, which is made up of energetic electrons. In the KC761B, the scintillation sensor is placed on the front side of the instrument, protected by the plastic case of the instrument. The electrons of the beta radiation easily pass through the plastic protection, with minimal energy loss, and reach the scintillator crystal where they produce a light pulse detected by the SiPM sensor. In Fig. 3 we show the setup for the detection of beta radiation produced by a Strontium 90 source. Strontium 90 is a pure beta emitter, so the spectrum we measure is due only to beta radiation (to be precise, there will also be a portion of bremsstrahlung radiation, which we however ignore because we are not using strongly absorbing materials). The energy spectrum of Sr-90, obtained from online resources, is shown in the image on the side. The spectrum of beta radiation is never characterized by monoenergetic lines but rather consists of a continuum that extends, with gradually decreasing intensity, up to a maximum energy value, for this reason, beta spectrometry is not, in general, very significant, unlike gamma and alpha spectrometry which instead show monoenergetic lines corresponding to emissions. We can however see that the spectrum measured by our instrument is qualitatively similar to the real spectrum of the source, with energy values that are in the range 0.5 – 1.5 MeV.
Fig 3 – Detection of beta radiation from a Sr-90 source with the scintillation sensor
Beta radiation measurement with the PIN detector
When the source under examination emits both gamma and beta radiation, the use of the scintillation sensor is problematic because the sensor does not distinguish between the two types of radiation. In this case, we can use the PIN sensor of the KC761B which is practically insensitive to gamma radiation. The measurement we made is described in Fig.4. In practice we brought our Sr-90 beta source close to the PIN sensor, taking care to position the source at a close distance to maximize the counting and decrease the attenuation caused by the air.
Fig 4 – Detection of beta radiation with the PIN sensor
Fig.5 shows the energy spectrum of the Sr-90 source. The spectrum has a minimum threshold value of about 400 keV, below which no signal is detected, this is normal considering the type of sensor and the protections positioned in front of the sensor itself which absorb part of the incident radiation. The spectrum however extends towards energy values of about 1 MeV, in qualitative agreement with the energy range of the beta emission of the Sr-90 source (reported in the previous paragraph).
Fig 5 – Beta radiation spectrum of a Sr-90 source
Alfa radiation detection with the PIN sensor
The measurement of alpha radiation is done by positioning the source near the PIN sensor. Remember that alpha particles are strongly absorbed by practically any material, including gases. For this reason, the sensor protection film must be very thin, to allow the passage of alpha particles with minimal energy loss. In addition to this, we must also take into account the absorption by the air. In the air, alpha particles have an average path of a few centimetres, for this reason, the source must be very close to the sensor. Naturally, the energy spectrum that we will obtain will be strongly influenced by the absorption of the air and by the mylar film. The source used is an Americium 241 source that emits alpha particles at an energy of about 5.5 MeV. In Fig.6 we show the setup for the measurement of alpha radiation.
Fig 6 – Detection of alpha radiation from an Am-241 source
In the following Fig.7, we report the energy spectrum of the alpha radiation emitted by Am-241 obtained from resources available online. It can be seen that the emission is practically monoenergetic with a very pronounced peak at about 5.5 MeV. Of course, it must be taken into account that these spectra are obtained with professional sensors, used in the absence of light and vacuum regimes.
Fig 7 – Alpha spectrum of the Am-241 source
In Fig.8 we report the spectrum of the Am 241 source obtained with the PIN sensor of our KC761B instrument. The result is very satisfactory because the presence of the peak at high energy, about 2 MeV is evident. Of course, the measurement made in air and with the mylar film protecting the sensor significantly reduces the energy of the alpha particles, furthermore, the reduction of energy has the additional effect of broadening the peak. In addition to this, it should also be considered that the source we used is protected by a metal layer that contributes to the reduction of energy and the broadening of the peak.
Fig 8 – Measured alpha spectrum of the Am-241 source
Alfa radiation measurements at increasing distances
A simple test that can be done with our instrument and the Am-241 alpha source is the measurement of the energy spectrum as the distance of the source from the sensor varies. This is a qualitative test that is easily performed by initially placing the alpha source in direct contact with the case of the instrument that protects the PIN sensor, and then gradually moving it away. At each position of the source, a measurement is made with the acquisition of the energy spectrum. Fig.9 shows the spectra obtained at 4 increasing distances.
Fig 9 – Alpha spectra of the Am-241 source taken at increasing distances
The spectra above show a decrease in energy: in fact, it goes from 1980 keV of maximum energy in the case of a source in contact with the case, to the value of 1242 keV in the case of a distance of a couple of centimetres. In addition to the decrease in energy, there is also a broadening of the energy peak. Even if it is not noticeable in the spectra shown, because they are calibrated vertically to occupy the width of the display, even the result of the counting, i.e. the intensity of the detected radiation, decreases as the distance increases, as is expected.
Detection of gamma and beta radiation from a uraninite sample
The KC761B, being portable and battery-operated, is perfect for use as a radioactivity detection instrument in the field. As an example, we show the results obtained by examining a rock sample that shows a nice mineralization of uraninite (or pitchblende).
In Fig.10 we show the gamma detection of the sample. Since it is a rather active sample, a couple of minutes is enough to detect the spectrum. In the gamma spectrum, we can recognize all the main emissions of uranium and its decay products, among which the three peaks of radium are particularly evident.
Fig 10 – Measurement of a uraninite sample with the scintillation sensor
We also subjected the same rock sample to detection with the PIN sensor, which we recall is sensitive to beta and alpha radiation. With this detection the number of counts is clearly lower than with gamma radiation, this is mainly due to the smaller size of the PIN sensor compared to the scintillator. The energy spectrum is however interesting because it provides evidence of both beta and alpha emission, as shown in Fig.11.
Fig 11 – Measurement of a uraninite sample with the PIN sensor
Conclusions
The KC761B instrument is a gamma spectrometer rich in features. The sensor’s energy resolution is excellent and the detection efficiency is also good. However, the instrument’s features do not end with the gamma spectrometer and dosimeter functions. The KC761B is equipped with a PIN sensor sensitive to beta and alpha radiation. Even with this sensor, all the spectrometer and dosimeter functions are available. The detection efficiency is high, but the small size of the sensor and the need to operate in the air require the source to be placed close to the sensor. These are features common to all solid-state sensors and do not prevent the detection of alpha radiation energy spectra with good resolution.