In this post we describe a silicon photodiode coupled to a CsI(Tl) scintillator used as radiation detector. This application is well known and we have covered a similar field in this post : Si-PIN Photodiode with Micod CSA-SA, now we would like to test the apparatus built by Micod (a Russian company, that we thank) also with a scintillator.
In the cover image you can see the “kit” composed by a CsI(Tl) scintillator coupled to a photodiode and the evaluation board with a CSA (charge sensitive amplifier) and an SA (shaping amplifier).
A photodiode can be used as a radiation detector in two ways : directly or coupled with a scintillator crystal. When it is used directly it is efficient in the low-energy part of the spectrum, until 50-60 KeV. In this range it has a potentially high energy resolution. With a scintillator it can instead cover the whole gamma spectrum and mainly the high-energy range.
In this post we describe the tests we did with a photodiode coupled to a CsI(Tl) scintillator.
Photodiode & CsI(Tl) Scintillator
The image below shows the scintillator crystal, covered with high-reflecting white varnish, coupled to a silicon photodiode.
The photodiode has the following datasheet :
Dimensions (LxWxD) = 10.5x8x1.8 mm
Photo-sensitive surface = 25 mm2
Electrical Capacitance (Vb -30V) = 33 pF
Dark Current (Vb -50V, 20°C) = 0.37 nA
Vbias = 40 V – 60 V
Vbias max = -60 V
The most important feature of Cesium Iodide crystals doped with Thallium CsI(Tl) is their emission spectrum having the maximum at 550 nm, which allows photodiodes or SiPMs to be used to detect the emission. The use of a scintillator-photodiode pair makes it possible to diminish significantly the size of the detecting system (due to the use of photodiode instead of PMT), to do without high-voltage supply source, and to use detecting systems in magnetic fields. The high radiation resistance (up to 102 Gy) allows CsI(TI) to be used in nuclear, medium and high-energy physics. Special treatment ensures obtaining of CsI(TI) scintillators with a low afterglow (less than 0.1% after 5 ms) for the use in tomographic systems.
The Amplifier Chain
To measure the current pulse produced by the photodiode it is necessary to have an “amplifier chain” composed by a charge sensitive amplifier (CSA) followed by a shaping amplifier (SA). The matching between the detector and preamplifier defines the low noise of the spectroscopy system, therefore for these applications the best noise performance of the associated preamplifier is required.
Generally, a field effect transistor, FET, is used as the input device. The charge sensitive preamplifier uses a feedback capacitor Cf and, in this case, a feedback resistor Rf. The feedback capacitor is charged by the injected signal from the detector and discharged immediately through the feedback resistor. A drawback of the feedback resistor is that it is an additional source of noise.
When ionizing radiation or a particle strikes the CsI(Tl) scintillator it produces a little spark of light which in turn strikes the photodiode and produces a small pulse of current, the charge sensitive amplifier (CSA) receives a charge Q in its input, converts it to a voltage pulse and then amplifies it. The output signal from CSA has a sharp front and exponential decay. The amplification factor of the circuit is:
V = Q/Cf
while the exponential decay has this time constant τ = RfxCf
The Micod CSA has this values :
Cf = 0.1 pF
Rf = 1 GΩ
After the CSA it is necessary to send the signal to a shaper amplifier (SA) which processes the pulse to amplify it and reshape it to obtain a quasi-Gauss shaped output signals. The shaper amplifier enhances essentially the signal-to-noise ratio and enables the signal to be sent to a counting device or to an analog-to-digital converter to acquire a spectrum. The Micod SA has a shaping time of about 10 μs.
With the setup described above, we did some measurement tests of some sample sources (Am241, Na22, Cs137 and others). We placed the source directly on the scintillator crystal and connected the shaper output to our pulse counter (described in the post PSoC based Pulse Counter). The counting threshold has been configured to 80 – 100 mV, so as to exclude the pulses due to noise.
The images below show some sources put on the crystal to perform the measurement.
In the image below you can see traces of some impulses generated by the photodiode and subsequently amplified by the CSA (charge sensitive amplifier) and the SA (shaping amplifier). The pulse duration is about 15 μs, while the amplitude varies (depending on the energy of the radiation) from 200 mv to over 1000 mv. Superimposed on the signal there is a background ripple (due to the noise of the photodiode, amplified by the chain) having an amplitude of about 50 mV.
With the CsI(Tl) scintillator coupled to the photodiode we made the measurements of the activity of the sample sources and we compared the data with those obtained, under similar conditions, with a scintillation sensor NaI (Tl) 63×63 mm coupled to a PMT. The results are shown in the table below.
|PMT with NaI(Tl)
|Photodiode with CsI(Tl)
|1μCi – Am 241
|0.5μCi – Na 22
|0.25μCi – Cs 137
|0.1μCi – Sr 90
|235 CPS (only β)
In reading these data we must take into account that the PD plus scintillator is sensitive even to the β particles, while the PMT scintillation sensor is coated with an aluminum layer that stops them. Both the source of Cesium 137 and the Sodium 22 emit, with different intensities, β particles (electrons the Cesium and positrons sodium), the photodiode countings therefore also includes a share of β radiation which is practically absent in the measurements made with the PMT scintillator.
Even taking into account the contribution of β radiation, we can see how the photodiode with CsI(Tl) used as a gamma detector, included in the amplification chain with the pulse counter, is absolutely comparable to that obtained with a classic PMT scintillation detector.
With the pulses produced by the shaping amplifier we tried to acquire the gamma spectra of two gamma sources using the Theremino MCA (DIY Gamma Spectrometry). The pulse produced by the shaping has a duration of around 15μs, rather short for the audio ADC which works better with pulses of around 100μs.
In this paragraph we report the results obtained for the sample sources Cs137 and Na22.
The fist chart below shows the gamma spectrum of Cs137. The main peak and the compton continuum are evident, the low-energy part of the spectrum does not show any trace because of the difficulty to acquire, with our MCA, low amplitude pulses. Theremino MCA has the possibility to partially compensate the peak broadening, so in the second chart we show the Cs137 “compensated” spectrum, while the third chart shows the Na22 spectrum: in both cases the peaks are well defined and the resolution is pretty good.
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Katika chapisho hili tunaelezea photodiode silicon pamoja na CsI (Tl) scintillator kutumika kama detector mionzi. "Mfano wa ""Si-Pin Photodiode"" ni Microdesign (MISDA), ambayo ni teknolojia ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee ya kipekee." Katika picha ya jalada unaweza kuona kit iliyoundwa na scintillator ya CsI (Tl) iliyounganishwa na photodiode na ubao wa tathmini na CSA (kuongeza nguvu ya kuchochea) na SA (kuongeza nguvu ya kuunda). Photodiode inaweza kutumika kama detector mionzi kwa njia mbili: moja kwa moja au pamoja na kioo scintillator. Wakati ni kutumika moja kwa moja ni ufanisi katika sehemu ya chini ya nishati ya wigo, hadi 50-60 KeV. Katika mbalimbali hii ina uwezekano wa juu ya nishati azimio. Kwa scintillator inaweza badala yake kufunika spectrum nzima gamma na hasa high-nishati mbalimbali. Katika chapisho hili tunaelezea vipimo tulivyofanya na photodiode iliyounganishwa na CsI (Tl) scintillator. Photodiode & Csi (Tl) Scintillator: Picha hapa chini inaonyesha kioo scintillator, kufunikwa na high-reflecting rangi nyeupe, pamoja na photodiode silicon. Photodiode ina data yafuatayo: Dimensions (LxWxD) = 10.5x8x1.8mm, uso wa photosensitive = 25mm2, Capacitance ya umeme (Vb) = 33pF, Mzunguko wa giza (Vb) = 50v, 20v) = 0.37nA, Vb = 40v, Vb = 60v, Vb = 60v, Vb = 60v. Matumizi ya scintillator-photodiode jozi inawezesha kwa kiasi kikubwa kupunguza ukubwa wa mfumo wa kugundua (kwa sababu ya matumizi ya photodiode badala ya PMT), kufanya bila chanzo cha juu-voltage ya usambazaji, na kutumia mfumo wa kugundua katika mashamba ya sumaku. Kwa sababu ya upinzani wake wa juu wa mnururisho (hadi 102Gy), TSI inaweza kutumika katika fizikia ya nyuklia, ya kati na ya nishati ya juu. Utaratibu maalum unahakikisha kupata scintillators za CsI (TI) na afterglow ya chini (chini ya 0.1% baada ya 5 ms) kwa matumizi katika mifumo ya tomographic. Amplifier Chain: Kwa kupima msukumo wa sasa unaozalishwa na photodiode, ni muhimu kuwa na mnyororo wa amplifier ulioundwa na amplifier ya kujisikia (CSA) na kufuatwa na amplifier ya kuunda (SA). Kuunganisha kati ya detector na preamplifier hufafanua kelele ya chini ya mfumo wa spectroscopy, kwa hiyo kwa ajili ya maombi haya utendaji bora kelele ya preamplifier kuhusishwa inahitajika. Kwa ujumla, transistor athari uwanja, FET, ni kutumika kama kifaa cha pembejeo. malipo nyeti preamplifier anatumia feedback capacitor Cf na, katika kesi hii, feedback resistor Rf. feedback capacitor ni kushtakiwa na ishara injected kutoka detector na discharged mara moja kupitia feedback resistor. Hasara ya upinzani wa maoni ni kwamba ni chanzo cha ziada cha kelele. Wakati ionizing mionzi au chembe hits scintillator, ni kuzalisha kidogo cheche ya mwanga ambayo kwa upande hits photodiode na inazalisha pulse ndogo ya sasa, malipo nyeti amplifier (CSA) anapokea malipo Q katika pembejeo yake, hubadilisha kwa voltage pulse na kisha amplifies. Ishara ya pato kutoka CSA ina mbele mkali na exponential kuoza. "Kama vile, kwa mfano, ""mzunguko wa kupanua"" (V = Q = Cf) na ""mzunguko wa kupanua"" (Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf) ni pamoja na ""mzunguko wa kupanua"" (Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf = Rf) na ""mzunguko wa kupanua"" (Rf = Rf = Rf = Rf = Rf) na ""mzunguko wa kupanua"" (Rf = Rf = Rf) na ""mzunguko wa kupanua"" (Rf = Rf)." Shaper amplifier inaongeza kimsingi ishara-kwa-ngurumo uwiano na inawezesha ishara kutumwa kwa kifaa cha kuhesabu au kwa converter analog-kwa-digital kupata wigo. Microd SA ina muda wa kuunda wa karibu 10 μs. Kwa mfano, katika kesi ya mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani wa mtihani. "Kama vile, ""Sintet"" ni kifaa cha kuunganisha shapeshifter na pulse counter, ambayo ni sawa na shapeshifter ya PSC." Kiwango cha hesabu kimewekwa kwa 80 < 100 mV, ili kuondoa msukumo unaosababishwa na kelele. Picha zilizo hapa chini zinaonyesha baadhi ya vyanzo kuweka juu ya kioo kufanya kipimo. Katika picha ya chini unaweza kuona alama ya baadhi ya msukumo zinazozalishwa na photodiode na baadaye amplified na CSA (mshtuko amplifier) na SA (mshuko amplifier). Muda wa msukumo ni karibu 15 ms, wakati amplitude inatofautiana (kutegemea nishati ya mionzi) kutoka 200 mv hadi zaidi ya 1000 mv. Kuongeza kwa ishara kuna mviringo wa nyuma (kwa sababu ya kelele ya photodiode, iliyoimarishwa na mlolongo) na amplitude ya karibu 50 mV. Kwa scintillator ya TSI iliyounganishwa na photodiode, tulifanya vipimo vya shughuli za vyanzo vya sampuli na tukalinganisha data na zile zilizopatikana chini ya hali sawa na sensor ya scintillation ya NaI (TSI) 63mm na PMT. Matokeo yanaonyeshwa katika jedwali hapa chini. NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sintet: NaT-Sint: NaT-Sintet: NaT-Sint: NaT-Sint: Chanzo cha Cesium-137 na sodium-22 hutokeza chembechembe za beta (elektroni za cesium na positrons za sodium) kwa nguvu tofauti, kwa hivyo hesabu za photodiode pia zina sehemu ya mionzi ya beta ambayo haipo katika vipimo vilivyopangwa na scintillator ya PMT. Hata kwa kuzingatia mchango wa mionzi ya beta, tunaweza kuona jinsi photodiode na Csi (Tl) kutumika kama detector gamma, ni pamoja na katika mlolongo amplification na pulse counter, ni kabisa kulinganishwa na ile kupatikana na classic PMT scintillation detector. Kwa pulses zinazozalishwa na shaping amplifier, tulijaribu kupata gamma spectra ya vyanzo viwili vya gamma kwa kutumia Therminoscope MCA (DIY Gamma Spectrometry). Pulses zinazozalishwa na shaping ina muda wa karibu 15ms, badala ya muda mfupi kwa ajili ya ADC sauti ambayo kazi bora na pulses ya karibu 100ms. Katika sehemu hii, sisi ripoti matokeo kupatikana kwa ajili ya sampuli vyanzo Cs137 na Na22. Chati ya chini ya picha inaonyesha spectrum ya gamma ya C137. "Mkomo mkuu na mfululizo wa compton ni dhahiri, sehemu ya nishati ya chini ya spectrum haionyeshi alama yoyote kwa sababu ya ugumu wa kupata, na MCA yetu, pulses ya amplitude ya chini. """ Kwa mfano, katika chati ya pili tunaonyesha spectrum ya Cs137 "iliyopunguzwa", wakati chati ya tatu inaonyesha spectrum ya Na22: katika kesi zote mbili kilele ni vizuri kufafanuliwa na azimio ni nzuri. Kama wewe kama chapisho hili unaweza kushiriki juu ya <unk>social<unk> Facebook, Twitter au LinkedIn na vifungo chini. Kwa njia hii unaweza kutusaidia! Asanteni! Kama wewe kama tovuti hii na kama unataka kuchangia katika maendeleo ya shughuli unaweza kufanya mchango, asante!