- Email: [email protected]
Ultrafast detection in particle physics and positron emission tomography using SiPMs
Author’s Accepted Manuscript Ultrafast Detection in Particle Physics and Positron Emission Tomography Using SiPMs R. Dolenec, S. Korpar, P. Križan, R. Pestotnik
www.elsevier.com/locate/nima
PII: DOI: Reference:
S0168-9002(17)30499-0 http://dx.doi.org/10.1016/j.nima.2017.04.036 NIMA59824
To appear in: Nuclear Inst. and Methods in Physics Research, A Received date: 10 March 2017 Revised date: 18 April 2017 Accepted date: 21 April 2017 Cite this article as: R. Dolenec, S. Korpar, P. Križan and R. Pestotnik, Ultrafast Detection in Particle Physics and Positron Emission Tomography Using SiPMs, Nuclear Inst. and Methods in Physics Research, A, http://dx.doi.org/10.1016/j.nima.2017.04.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultrafast Detection in Particle Physics and Positron Emission Tomography Using SiPMs R. Dolenec∗,a,b , S. Korparb,c , P. Kriˇzana,b , R. Pestotnikb a Faculty
of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia b Joˇ zef Stefan Institute, Ljubljana, Slovenia c Faculty of Chemistry and Chemical Engineering, University of Maribor, Maribor, Slovenia
Abstract Silicon photomultiplier (SiPM) photodetectors perform well in many particle and medical physics applications, especially where good efficiency, insensitivity to magnetic field and precise timing are required. In Cherenkov time-of-flight positron emission tomography the requirements for photodetector performance are especially high. On average only a couple of photons are available for detection and the best possible timing resolution is needed. Using SiPMs as photodetectors enables good detection efficiency, but the large sensitive area devices needed have somewhat limited time resolution for single photons. We have observed an additional degradation of the timing at very low light intensities due to delayed events in distribution of signals resulting from multiple fired micro cells. In this work we present the timing properties of AdvanSiD ASD-NUV3SP-40 SiPM at single photon level picosecond laser illumination and a simple modification of the time-walk correction algorithm, that resulted in reduced degradation of timing resolution due to the delayed events. Key words: Silicon photomultipliers, PET, Time-of-flight, Cherenkov radiation
∗ Corresponding author at: Joˇ zef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia. Tel.: +386 1 477 3157, Fax.: +386 1 477 3166. Email address: [email protected] (R. Dolenec)
Preprint submitted to Nuclear Instruments and Methods A
April 22, 2017
1
2
1. Introduction By measuring the time-of-flight (TOF) of the annihilation gammas in positron
3
emission tomography (PET) the quality of activity distribution images can be
4
improved. Despite many recent efforts to develop new scintillator materials,
5
the time resolution in PET is still mainly limited by the time evolution of scin-
6
tillation light production. To completely avoid this limitation, the promptly
7
produced Cherenkov light can be used instead. We previously demonstrated
8
a coincidence resolving time below 100 ps FWHM using lead fluoride (PbF2 ),
9
a pure emitter of Cherenkov light, coupled to microchannel plate photomulti-
10
plier (MCP PMT) photodetectors [1]. We also tested silicon photomultipliers
11
(SiPMs) as photodetectors due to their relatively high photon detection effi-
12
ciency (PDE), low cost and the possibility to operate them inside the magnetic
13
field of magnetic resonance scanners. SiPMs have two drawbacks for use in
14
Cherenkov PET: they need to be cooled so their dark count rate is sufficiently
15
reduced and their time resolution is slightly limited. In our previous study with
16
four SiPMs, produced by different manufacturers, we achived the best coinci-
17
dence resolving time of 300 ps FWHM with AdvanSiD ASD-NUV3S-P-40 SiPM
18
[2]. This timing improved to 190 ps FWHM if only the events, corresponding to
19
single fired micro cell (m.c.) on both sides of the coincidence were used. In our
20
latest work [3], we have observed that the multiple m.c. events have the timing
21
degraded by an additional contribution, detected with a delay of approximately
22
0.2 ns, and suggested that the most likely source for this effects is the optical
23
crosstalk. The delayed contribution degrades the timing in two ways: the spread
24
in time of the events increases and the accuracy of the time-walk correction is
25
degraded.
26
In this work we present the response of the AdvanSiD SiPM to single pho-
27
ton level laser pulses and a simple modification of the time-walk correction
28
algorithm, with which we achieved an improvement in time resolution without
29
discarding the multiple m.c. events. The experiment and the standard time-
30
walk correction algorithm are presented in Section 2. In Section 3 the results
2
31
obtained with the standard and improved time-walk correction algorithm are
32
shown. This is then followed by the summary in Section 4.
33
2. Methods
34
2.1. Experimental setup
35
An AdvanSiD ASD-NUV3S-P-40 SiPM [4] with 3×3 mm2 active area and
36
40 µm cell size was connected to a custom electronic board featuring a NEC
37
uPC2710TB preamplifier (upper limit operating frequency 1.0 GHz) [5]. The
38
photodetector was illuminated by pulses, generated by a PiLas diode laser sys-
39
tem EIG1000D with blue (404 nm) laser head. Neutral density filters were used
40
to attenuate the laser pulses to very low intensity, corresponding to approxi-
41
mately one detected photon per pulse on average. The light was guided inside
42
the light tight, temperature controlled chamber via an optical fiber and con-
43
nected to an optical system, used to illuminated the whole SiPM active surface
44
approximately uniformly.
45
The preamplified SiPM signals were lead into an ORTEC FTA820 NIM
46
amplifier with 350 MHz bandwidth and then split into a CAEN Mod.V965
47
charge-to-digital converter (QDC) and Phillips Scientific Mod.752 leading edge
48
discriminator. The measurement was triggered by the laser control unit. Time
49
relative to the trigger signal was obtained by leading the logic signal from dis-
50
criminator into a Kaizu Works KC3781A time-to-digital converter (TDC). The
51
discriminator threshold was set to 0.5 single m.c. pulse height for every mea-
52
surement.
53
All of the presented measurements were performed at a temperature of
54
−25◦ C. The SiPM breakdown voltage at this temperature was determined by
55
measuring the signal heights at different overvoltages and extrapolating to zero
56
signal height.
57
2.2. Standard time-walk correction
58
For each laser trigger, the time and charge of the signal was measured.
59
By plotting time vs. charge correlation plot (Fig. 1, top) the time-walk effect 3
60
becomes apparent. This 2-D correlation plot was divided into a large number
61
of projections along the time axis, which were individually fitted with Gaussian
62
functions. The mean values of the Gaussian functions from each projection
63
64
were forming the basis for the standard time-walk correction function fit. The √ correction function used was of the form: f (x) = p0 + p1 / x − p2 . Here, p0,1,2
65
are the fit parameters which were then used to calculate the corrected time
66
on event per event basis by subtracting the value of the correction function at
67
measured charge from the measured time.
68
2.3. Time resolution
69
The time-walk corrected time distributions were fitted with a sum of a Gaus-
70
sian and a product of an exponential and error functions. The time resolution
71
was then calculated as the FWHM of the fit function.
72
With the time-walk correction method, described above, a time resolution of
73
58 ps FWHM was measured when an ultrafast reference photodetector, Hama-
74
matsu R3809U-52 MCP PMT, was used in the experiment. According to pro-
75
ducers test sheets, the MCP PMT and the laser contribute about 25 ps FWHM
76
and 35 ps FWHM respectively. This leaves any other contributions (e.g. elec-
77
tronics, time-walk correction) at roughly 40 ps FWHM.
78
3. Results
79
In Fig. 1, top, the measured time vs. charge correlation plot is shown.
80
The peaks corresponding to one, two and three fired micro cells are clearly
81
separated while the events with a higher measured charge are visibly affected
82
by the saturation of the amplifier. The time-walk correction function follows the
83
single m.c. events well, but the situation is more complicated for the multiple
84
m.c. signals, which are composed of two contributions. In Fig. 1, bottom,
85
the two contributions to the double m.c. events are more clearly visible. The
86
first contribution, detected at shorter time, fits the expectations of true double
87
photon detection. The second contribution is registered with a delay of about
4
88
0.2 ns and also has a slightly larger spread in time than the first. As discussed
89
in our previous work [3], the most likely cause of the delayed contributions
90
is the SiPM optical crosstalk. This conclusion is also supported by previous
91
investigations of SiPMs similar to the one used in this work. The measured
92
crosstalk probabilities were reported in [6], while the degrading effect on the
93
time resolution, produced by the delayed contribution, was already observed in
94
[7]. The AdvanSiD device tested in this work is optimized for low afterpulse rates
95
and seems to have only a limited isolation of neighboring microcells. Optical
96
crosstalk could be reduced by improved trenches between the microcells.
97
The time distribution obtained from this data and the contributions from
98
single and double m.c. events are shown in Fig. 2. A time resolution of 218 ps
99
and 149 ps FWHM was obtained for all and single m.c. events, respectively.
100
The resolution for double m.c. events was degraded to 314 ps FWHM due to
101
the delayed contribution. It is also clearly visible that the delayed contribution
102
caused an suboptimal time-walk correction function fit which resulted in an
103
improper shift in time for the double m.c. events.
104
In an attempt to improve the time resolution in the presence of the delayed
105
contribution to the multiple m.c. signals, events from each measurement were
106
split into subsets, corresponding to a certain number of fired micro cells (for
107
example, such subset corresponding to two fired cells was shown in Fig. 1, bot-
108
tom). The time-walk correction function fit and time resolution calculation was
109
then performed separately for each subset. In Fig. 3, top, the so obtained time
110
resolutions for subsets corresponding to only single and only double m.c. signals
111
are shown. For only single m.c. signals the time resolution continuously im-
112
proves with the overvoltage and the best time resolution achieved at the highest
113
overvoltage was 136 ps FWHM. For only double m.c. signals the delayed contri-
114
bution significantly influences the shape of the corrected time distribution and
115
degrades the resolution.
116
The time distributions including only events, corresponding to a certain
117
number of fired micro cells, corrected with parameters obtained from the sub-
118
set of such events, can be combined. In Fig. 3, bottom, the time resolutions 5
119
obtained with such improved time-walk correction are compared to those ob-
120
tained with the standard time-walk correction, presented in Section 2.2. The
121
time resolutions obtained with the improved approach are slightly better than
122
those obtained from the standard time-walk correction over the whole data set
123
and continue to improve also at higher overvoltages. The best result achieved
124
was 173 ps FWHM with the improved time-walk correction, compared to the
125
best result with the original algorithm of 218 ps FWHM.
126
4. Summary
127
The true single photon (single fired cell) response of SiPMs depends mainly
128
on the intrinsic properties of individual micro cell and the variations in signal
129
travel time from individual cells to the common output. Some of the older large
130
area SiPMs seemed to be limited in timing especially by the latter, but the latest
131
generations of devices are much improved. We measured a time resolution of
132
136 ps FWHM with AdvanSiD ASD-NUV3S-P-40 SiPM when only single m.c.
133
signals were used. When also the multiple m.c. signals were included, the time
134
resolution degraded to 218 ps FWHM. For best possible timing at single photon
135
level illumination, multiple m.c. signals should therefore be excluded. However,
136
using all detected events is important to keep the gamma detection efficiency of
137
the Cherenkov TOF PET method as high as possible.
138
We observed a delayed contribution to the multiple m.c. signals which not
139
only degraded the time resolution but also degraded the performance of the time-
140
walk correction. We achieved a small improvement in the time resolution, while
141
keeping also the multiple m.c. signals, by performing the time-walk correction
142
separately for subsets of data corresponding to a certain number of fired micro
143
cells and combining the obtained time distributions. In this way we achieved a
144
time resolution of 173 ps FWHM.
145
The presented modification of the time-walk correction algorithm is actu-
146
ally minimal and represents just the very first step of improvement. It should
147
be possible to improve the time resolution even further by developing a time-
6
148
walk algorithm even better suited to the SiPM response. Also, discrimination
149
methods based on pulse shape (multi-threshold measurement or analysis of in-
150
dividual waveforms) might be able to separate the two contributions and reduce
151
the degrading effect the delayed contribution has on the time resolution. All of
152
this is the subject of our current investigations, which will be presented in our
153
future works.
154
References
155
[1] S. Korpar, et al., Nuclear Instruments and Methods in Physics Research
156
Section A: Accelerators, Spectrometers, Detectors and Associated Equip-
157
ment 654 (2011) 532.
158
159
160
161
162
163
164
165
166
167
168
169
[2] R. Dolenec, et al., IEEE Transactions on Nuclear Science, Vol. 64, No. 5, October 2016, 2478. [3] R. Dolenec, et al., to be published in IEEE Nuclear Science Symposium Conference Record (NSS/MIC) (2016). [4] AdvanSiD ASD-NUV3S-P Low Afterpulse Datasheet Rev. 9; 10.2014 (www.advansid.com). [5] NEC Bipolar Analog Integrated Circuit µPC2710TB Data Sheet, NEC Corporation, January 2001. [6] P.W. Cattaneo, et al., IEEE Transactions on Nuclear Science, Vol. 61, No. 5, October 2014, 2657. [7] F. Acerbi, et al., IEEE Transactions on Nuclear Science, Vol. 61, No. 5, October 2014, 2678.
7
Time [ns]
0.6
1000
0.5 800
0.4 0.3
600
0.2 400
0.1 0
200 -0.1 -0.2
0 200
300
400
500
600
700
800
900
1000
1100
Time [ns]
Charge [ADC Ch.]
350
0.4
300 0.3 250 0.2
200
0.1
150
0
100 50
-0.1
0 460
480
500
520
540
560
580
600
Charge [ADC Ch.]
Figure 1: The time vs. charge correlation plot obtained at an overvoltage of 6 V (top) and a close-up of the double micro cell events (bottom). Also shown is the time-walk correction function obtained from the fit over all events.
8
Counts
All: 218 ps
50000
1 m.c.: 149 ps 40000
2 m.c.: 314 ps
30000
20000
10000
0 -0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
Time[ns]
Figure 2: The standard time-walk corrected time distribution and resulting fits at an overvoltage of 6 V for all, single and double m.c. events.
9
Timing FWHM [ps]
350 300 250 200 150 100
1 m.c.
50 0 3
2 m.c. 3.5
4
4.5
5
5.5
6
6.5
7
Timing FWHM [ps]
Overvoltage [V] 350 300 250 200 150 100 50 0 3
Standard Improved 3.5
4
4.5
5
5.5
6
6.5
7
Overvoltage [V]
Figure 3: The time-walk corrected time resolutions obtained for single and double m.c. signals (top) and for all signals with the standard and improved time-walk corrections (bottom) at different overvoltages.
10
www.elsevier.com/locate/nima
PII: DOI: Reference:
S0168-9002(17)30499-0 http://dx.doi.org/10.1016/j.nima.2017.04.036 NIMA59824
To appear in: Nuclear Inst. and Methods in Physics Research, A Received date: 10 March 2017 Revised date: 18 April 2017 Accepted date: 21 April 2017 Cite this article as: R. Dolenec, S. Korpar, P. Križan and R. Pestotnik, Ultrafast Detection in Particle Physics and Positron Emission Tomography Using SiPMs, Nuclear Inst. and Methods in Physics Research, A, http://dx.doi.org/10.1016/j.nima.2017.04.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultrafast Detection in Particle Physics and Positron Emission Tomography Using SiPMs R. Dolenec∗,a,b , S. Korparb,c , P. Kriˇzana,b , R. Pestotnikb a Faculty
of Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia b Joˇ zef Stefan Institute, Ljubljana, Slovenia c Faculty of Chemistry and Chemical Engineering, University of Maribor, Maribor, Slovenia
Abstract Silicon photomultiplier (SiPM) photodetectors perform well in many particle and medical physics applications, especially where good efficiency, insensitivity to magnetic field and precise timing are required. In Cherenkov time-of-flight positron emission tomography the requirements for photodetector performance are especially high. On average only a couple of photons are available for detection and the best possible timing resolution is needed. Using SiPMs as photodetectors enables good detection efficiency, but the large sensitive area devices needed have somewhat limited time resolution for single photons. We have observed an additional degradation of the timing at very low light intensities due to delayed events in distribution of signals resulting from multiple fired micro cells. In this work we present the timing properties of AdvanSiD ASD-NUV3SP-40 SiPM at single photon level picosecond laser illumination and a simple modification of the time-walk correction algorithm, that resulted in reduced degradation of timing resolution due to the delayed events. Key words: Silicon photomultipliers, PET, Time-of-flight, Cherenkov radiation
∗ Corresponding author at: Joˇ zef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia. Tel.: +386 1 477 3157, Fax.: +386 1 477 3166. Email address: [email protected] (R. Dolenec)
Preprint submitted to Nuclear Instruments and Methods A
April 22, 2017
1
2
1. Introduction By measuring the time-of-flight (TOF) of the annihilation gammas in positron
3
emission tomography (PET) the quality of activity distribution images can be
4
improved. Despite many recent efforts to develop new scintillator materials,
5
the time resolution in PET is still mainly limited by the time evolution of scin-
6
tillation light production. To completely avoid this limitation, the promptly
7
produced Cherenkov light can be used instead. We previously demonstrated
8
a coincidence resolving time below 100 ps FWHM using lead fluoride (PbF2 ),
9
a pure emitter of Cherenkov light, coupled to microchannel plate photomulti-
10
plier (MCP PMT) photodetectors [1]. We also tested silicon photomultipliers
11
(SiPMs) as photodetectors due to their relatively high photon detection effi-
12
ciency (PDE), low cost and the possibility to operate them inside the magnetic
13
field of magnetic resonance scanners. SiPMs have two drawbacks for use in
14
Cherenkov PET: they need to be cooled so their dark count rate is sufficiently
15
reduced and their time resolution is slightly limited. In our previous study with
16
four SiPMs, produced by different manufacturers, we achived the best coinci-
17
dence resolving time of 300 ps FWHM with AdvanSiD ASD-NUV3S-P-40 SiPM
18
[2]. This timing improved to 190 ps FWHM if only the events, corresponding to
19
single fired micro cell (m.c.) on both sides of the coincidence were used. In our
20
latest work [3], we have observed that the multiple m.c. events have the timing
21
degraded by an additional contribution, detected with a delay of approximately
22
0.2 ns, and suggested that the most likely source for this effects is the optical
23
crosstalk. The delayed contribution degrades the timing in two ways: the spread
24
in time of the events increases and the accuracy of the time-walk correction is
25
degraded.
26
In this work we present the response of the AdvanSiD SiPM to single pho-
27
ton level laser pulses and a simple modification of the time-walk correction
28
algorithm, with which we achieved an improvement in time resolution without
29
discarding the multiple m.c. events. The experiment and the standard time-
30
walk correction algorithm are presented in Section 2. In Section 3 the results
2
31
obtained with the standard and improved time-walk correction algorithm are
32
shown. This is then followed by the summary in Section 4.
33
2. Methods
34
2.1. Experimental setup
35
An AdvanSiD ASD-NUV3S-P-40 SiPM [4] with 3×3 mm2 active area and
36
40 µm cell size was connected to a custom electronic board featuring a NEC
37
uPC2710TB preamplifier (upper limit operating frequency 1.0 GHz) [5]. The
38
photodetector was illuminated by pulses, generated by a PiLas diode laser sys-
39
tem EIG1000D with blue (404 nm) laser head. Neutral density filters were used
40
to attenuate the laser pulses to very low intensity, corresponding to approxi-
41
mately one detected photon per pulse on average. The light was guided inside
42
the light tight, temperature controlled chamber via an optical fiber and con-
43
nected to an optical system, used to illuminated the whole SiPM active surface
44
approximately uniformly.
45
The preamplified SiPM signals were lead into an ORTEC FTA820 NIM
46
amplifier with 350 MHz bandwidth and then split into a CAEN Mod.V965
47
charge-to-digital converter (QDC) and Phillips Scientific Mod.752 leading edge
48
discriminator. The measurement was triggered by the laser control unit. Time
49
relative to the trigger signal was obtained by leading the logic signal from dis-
50
criminator into a Kaizu Works KC3781A time-to-digital converter (TDC). The
51
discriminator threshold was set to 0.5 single m.c. pulse height for every mea-
52
surement.
53
All of the presented measurements were performed at a temperature of
54
−25◦ C. The SiPM breakdown voltage at this temperature was determined by
55
measuring the signal heights at different overvoltages and extrapolating to zero
56
signal height.
57
2.2. Standard time-walk correction
58
For each laser trigger, the time and charge of the signal was measured.
59
By plotting time vs. charge correlation plot (Fig. 1, top) the time-walk effect 3
60
becomes apparent. This 2-D correlation plot was divided into a large number
61
of projections along the time axis, which were individually fitted with Gaussian
62
functions. The mean values of the Gaussian functions from each projection
63
64
were forming the basis for the standard time-walk correction function fit. The √ correction function used was of the form: f (x) = p0 + p1 / x − p2 . Here, p0,1,2
65
are the fit parameters which were then used to calculate the corrected time
66
on event per event basis by subtracting the value of the correction function at
67
measured charge from the measured time.
68
2.3. Time resolution
69
The time-walk corrected time distributions were fitted with a sum of a Gaus-
70
sian and a product of an exponential and error functions. The time resolution
71
was then calculated as the FWHM of the fit function.
72
With the time-walk correction method, described above, a time resolution of
73
58 ps FWHM was measured when an ultrafast reference photodetector, Hama-
74
matsu R3809U-52 MCP PMT, was used in the experiment. According to pro-
75
ducers test sheets, the MCP PMT and the laser contribute about 25 ps FWHM
76
and 35 ps FWHM respectively. This leaves any other contributions (e.g. elec-
77
tronics, time-walk correction) at roughly 40 ps FWHM.
78
3. Results
79
In Fig. 1, top, the measured time vs. charge correlation plot is shown.
80
The peaks corresponding to one, two and three fired micro cells are clearly
81
separated while the events with a higher measured charge are visibly affected
82
by the saturation of the amplifier. The time-walk correction function follows the
83
single m.c. events well, but the situation is more complicated for the multiple
84
m.c. signals, which are composed of two contributions. In Fig. 1, bottom,
85
the two contributions to the double m.c. events are more clearly visible. The
86
first contribution, detected at shorter time, fits the expectations of true double
87
photon detection. The second contribution is registered with a delay of about
4
88
0.2 ns and also has a slightly larger spread in time than the first. As discussed
89
in our previous work [3], the most likely cause of the delayed contributions
90
is the SiPM optical crosstalk. This conclusion is also supported by previous
91
investigations of SiPMs similar to the one used in this work. The measured
92
crosstalk probabilities were reported in [6], while the degrading effect on the
93
time resolution, produced by the delayed contribution, was already observed in
94
[7]. The AdvanSiD device tested in this work is optimized for low afterpulse rates
95
and seems to have only a limited isolation of neighboring microcells. Optical
96
crosstalk could be reduced by improved trenches between the microcells.
97
The time distribution obtained from this data and the contributions from
98
single and double m.c. events are shown in Fig. 2. A time resolution of 218 ps
99
and 149 ps FWHM was obtained for all and single m.c. events, respectively.
100
The resolution for double m.c. events was degraded to 314 ps FWHM due to
101
the delayed contribution. It is also clearly visible that the delayed contribution
102
caused an suboptimal time-walk correction function fit which resulted in an
103
improper shift in time for the double m.c. events.
104
In an attempt to improve the time resolution in the presence of the delayed
105
contribution to the multiple m.c. signals, events from each measurement were
106
split into subsets, corresponding to a certain number of fired micro cells (for
107
example, such subset corresponding to two fired cells was shown in Fig. 1, bot-
108
tom). The time-walk correction function fit and time resolution calculation was
109
then performed separately for each subset. In Fig. 3, top, the so obtained time
110
resolutions for subsets corresponding to only single and only double m.c. signals
111
are shown. For only single m.c. signals the time resolution continuously im-
112
proves with the overvoltage and the best time resolution achieved at the highest
113
overvoltage was 136 ps FWHM. For only double m.c. signals the delayed contri-
114
bution significantly influences the shape of the corrected time distribution and
115
degrades the resolution.
116
The time distributions including only events, corresponding to a certain
117
number of fired micro cells, corrected with parameters obtained from the sub-
118
set of such events, can be combined. In Fig. 3, bottom, the time resolutions 5
119
obtained with such improved time-walk correction are compared to those ob-
120
tained with the standard time-walk correction, presented in Section 2.2. The
121
time resolutions obtained with the improved approach are slightly better than
122
those obtained from the standard time-walk correction over the whole data set
123
and continue to improve also at higher overvoltages. The best result achieved
124
was 173 ps FWHM with the improved time-walk correction, compared to the
125
best result with the original algorithm of 218 ps FWHM.
126
4. Summary
127
The true single photon (single fired cell) response of SiPMs depends mainly
128
on the intrinsic properties of individual micro cell and the variations in signal
129
travel time from individual cells to the common output. Some of the older large
130
area SiPMs seemed to be limited in timing especially by the latter, but the latest
131
generations of devices are much improved. We measured a time resolution of
132
136 ps FWHM with AdvanSiD ASD-NUV3S-P-40 SiPM when only single m.c.
133
signals were used. When also the multiple m.c. signals were included, the time
134
resolution degraded to 218 ps FWHM. For best possible timing at single photon
135
level illumination, multiple m.c. signals should therefore be excluded. However,
136
using all detected events is important to keep the gamma detection efficiency of
137
the Cherenkov TOF PET method as high as possible.
138
We observed a delayed contribution to the multiple m.c. signals which not
139
only degraded the time resolution but also degraded the performance of the time-
140
walk correction. We achieved a small improvement in the time resolution, while
141
keeping also the multiple m.c. signals, by performing the time-walk correction
142
separately for subsets of data corresponding to a certain number of fired micro
143
cells and combining the obtained time distributions. In this way we achieved a
144
time resolution of 173 ps FWHM.
145
The presented modification of the time-walk correction algorithm is actu-
146
ally minimal and represents just the very first step of improvement. It should
147
be possible to improve the time resolution even further by developing a time-
6
148
walk algorithm even better suited to the SiPM response. Also, discrimination
149
methods based on pulse shape (multi-threshold measurement or analysis of in-
150
dividual waveforms) might be able to separate the two contributions and reduce
151
the degrading effect the delayed contribution has on the time resolution. All of
152
this is the subject of our current investigations, which will be presented in our
153
future works.
154
References
155
[1] S. Korpar, et al., Nuclear Instruments and Methods in Physics Research
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Section A: Accelerators, Spectrometers, Detectors and Associated Equip-
157
ment 654 (2011) 532.
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159
160
161
162
163
164
165
166
167
168
169
[2] R. Dolenec, et al., IEEE Transactions on Nuclear Science, Vol. 64, No. 5, October 2016, 2478. [3] R. Dolenec, et al., to be published in IEEE Nuclear Science Symposium Conference Record (NSS/MIC) (2016). [4] AdvanSiD ASD-NUV3S-P Low Afterpulse Datasheet Rev. 9; 10.2014 (www.advansid.com). [5] NEC Bipolar Analog Integrated Circuit µPC2710TB Data Sheet, NEC Corporation, January 2001. [6] P.W. Cattaneo, et al., IEEE Transactions on Nuclear Science, Vol. 61, No. 5, October 2014, 2657. [7] F. Acerbi, et al., IEEE Transactions on Nuclear Science, Vol. 61, No. 5, October 2014, 2678.
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Time [ns]
0.6
1000
0.5 800
0.4 0.3
600
0.2 400
0.1 0
200 -0.1 -0.2
0 200
300
400
500
600
700
800
900
1000
1100
Time [ns]
Charge [ADC Ch.]
350
0.4
300 0.3 250 0.2
200
0.1
150
0
100 50
-0.1
0 460
480
500
520
540
560
580
600
Charge [ADC Ch.]
Figure 1: The time vs. charge correlation plot obtained at an overvoltage of 6 V (top) and a close-up of the double micro cell events (bottom). Also shown is the time-walk correction function obtained from the fit over all events.
8
Counts
All: 218 ps
50000
1 m.c.: 149 ps 40000
2 m.c.: 314 ps
30000
20000
10000
0 -0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
Time[ns]
Figure 2: The standard time-walk corrected time distribution and resulting fits at an overvoltage of 6 V for all, single and double m.c. events.
9
Timing FWHM [ps]
350 300 250 200 150 100
1 m.c.
50 0 3
2 m.c. 3.5
4
4.5
5
5.5
6
6.5
7
Timing FWHM [ps]
Overvoltage [V] 350 300 250 200 150 100 50 0 3
Standard Improved 3.5
4
4.5
5
5.5
6
6.5
7
Overvoltage [V]
Figure 3: The time-walk corrected time resolutions obtained for single and double m.c. signals (top) and for all signals with the standard and improved time-walk corrections (bottom) at different overvoltages.
10