Characterization of primed state
of CVD diamond by light and
alpha particles
C. Manfredotti
Experimental Physics Department
University of Torino
INFN- Sezione di Torino
Previous alpha particle measurements
Investigation of the primed state with
light:
ALPHA PARTICLE DETECTION
Time sequence of measurements
Experimental set up
Erasing previous
sample history
Priming
Alpha particle detection can discriminate
between hole and electron contribution
to Charge Collection Efficiency (CCE)
by changing bias polarity and by profiting of the short range
of 5.5 MeV alpha particles in diamond (13 mm)
Bias voltage (positive) and signal
+
eDx
L
Ramo’s theorem:
h+
a particle
Electrons cover the longest part of the
sample thickness and give the largest
contribution to charge signal.
CCE (for one carrier)= Dq/q = Dx/L
Holes almost do not
contribute: the range of a
particle is too short.
Charge collection efficiency as a function
of priming and of light sensitization
enhances
electron
response
Green, red and
infrared light
are not able to
affect the
primed state
and lowers
hole
response
Remember
TL?
X-ray priming
enhances holes
response
Blue light
bleaching
is the same
and affects
holes
Blue light
IBIC ( Ion Beam Induced Charge)
measurements
Effect of light on maps of Charge
Collection Efficiency ( CCE )
IBIC maps on De Beers detector primed (
10 Gy)
Response is still better for holes, but counting rate is very low
(counting efficiency is 5 – 10%) and detector polarizes
completely (no counts) after 1 – 2 hours of irradiation
35,00
250
250
31,25
27,50
23,75
200
200
20,00
50 ch = 280 um
16,25
12,50
150
150
8,750
5,000
holes
100
electrons
100
50
50
0
0
0
50
100
150
50 ch = 280 um
Bias – 600 V
200
250
0
50
100
150
200
250
280 mm
Bias + 600 V
IBIC maps after priming and during blue
light (450 nm) illumination
• Counting efficiency improves dramatically, particularly for electrons
• Response and uniformity for electrons are much better than for holes.
• Charge collection distance (CCD) for electrons can reach 600 mm !
Collection efficiency
(%)
250
35,00
31,25
27,50
23,75
20,00
16,25
12,50
8,750
5,000
200
150
250
200
150
100
100
50
50
0
0
50
100
150
200
250
280 mm
0
0
50
100
150
200
250
280 mm
Bias – 600 V
Bias + 600 V
Effects of priming and light ( 400 nm ) on
alpha spectra
Time behaviour of alpha spectra
Discrimination between electrons and holes
Preliminary data
Alpha spectra – Different primings
Holes
Holes
Holes
Priming 4 Gy
Counts
10
5
Counts
20
15
15
10
10
5
5
0
5
0
100
150
150
200
250
300
350
400
450
500
50
100
150
200
250
300
350
400
450
500
Counts
10
Fourth measure
15
5
0
0
450
500
0
50
100
150
200
250
300
350
400
450
500
25
25
20
20
15
100
150
200
250
300
Channel
30
350
400
450
25
20
20
100
150
200
250
300
350
400
450
0
Sixth measure
5
0
200
250
Channel
300
350
400
450
500
150
200
250
300
350
400
450
500
25
Counts
20
15
15
10
5
0
150
100
Sixth measure
10
0
100
500
10
5
50
450
Channel
Fiveth measure
5
0
400
30
25
15
50
Channel
Counts
Counts
Counts
10
350
15
500
20
15
300
0
50
500
Fiveth measure
25
250
5
0
50
200
10
5
0
Channel
150
Fourth measure
30
30
100
Channel
0
0
50
30
10
10
5
400
Third measure
20
15
350
30
25
20
300
Channel
30
Third measure
25
250
Channel
0
Channel
200
Counts
100
Counts
50
30
Counts
50
0
0
15
10
0
0
Second measure
Fistr measure
20
20
15
25
Second measure
25
First measure
20
Counts
25
30
25
Priming 0,57 Gy
( slight exposition to light )
30
Counts
30
Priming 4 Gy
30
0
0
0
50
100
150
200
250
Channel
300
350
400
450
500
50
100
150
200
250
Channel
300
350
400
450
500
0
50
100
150
200
Channel
250
300
350
400
450
500
Alpha spectra – Electrons
Priming – Priming + light
Electrons
Priming 650 mGy Under Light
Electrons
Priming 650 mGy
first measure
40
35
Second measure
30
40
30
Second measure
First measure
25
25
30
30
20
10
20
Counts
Counts
Counts
Counts
20
20
15
10
15
10
10
5
0
5
0
0
0
-5
0
50
100
150
200
250
0
Channel
50
100
150
200
0
250
100
150
50
100
150
200
250
Channel
Fourth measure
35
30
25
20
Counts
20
Counts
Y Axis Title
Third measure
25
30
20
0
Fourth measure
40
40
30
250
40
30
Third measure
200
Channel
Channel
50
50
Counts
50
15
10
20
15
10
10
10
5
5
0
0
0
0
-5
0
50
100
150
Channel
200
250
0
50
100
150
Channel
200
250
0
50
100
150
Channel
200
250
0
50
100
150
200
250
Channel
From these spectra, time evolution of both centroid and total counts ( integral ) has been derived
Decay of primed state - Short term
Priming and light
Holes
Holes
P 6.3 mGy
P 6.3 mGy
P 6.3 mGy
P 760 mGy
P 4.3 mGy
2500
Counts (integral)
2000
L under light (400 nm)
P after priming
NL No lignt
NP No Priming
1500
1000
NL NP
L NP
L P 114 mGy
L P 218 mGy
L P 2.4 Gy
500
0
0
1
2
3
4
5
6
7
8
9
Time ( 4 min )
10
11
12
13
14
15
Decay of primed state –Short term
Holes – Low doses priming
Holes
NP
P 1.9 mGy
P 1.9 mGy
P 3.8 mGy
P 3.8 mGy
P 5.7 mGy
P 5.7 mGy
P 630 mGy
2500
Counts ( integral )
2000
1500
1000
500
NP No Priming
P Priming
0
0
1
2
3
4
5
6
7
8
9
Time ( 4 min )
10
11 12
13
14 15
Decay of primed state – Short term
Electrons
130
125
120
115
110
105
100
95
90
85
80
75
70
65
60
55
50
45
NL NP
NL P 650 mGy
L P 650 mGy
L3
L2
L1
L under light (400 nm)
P after priming
NL No lignt
NP No Priming
Electrons
2200
NL NP
NL P 650 mGy
L P 650 mGy
L3
L2
L1
2000
1800
1600
Counts ( integral )
Centroid
Electrons
1400
1200
1000
800
600
L under light (400 nm)
P after priming
NL No lignt
NP No Priming
400
200
0
1
2
3
4
5
6
7
8
9
Time ( 4 min )
10
11
12
13
14
15
0
0
1
2
3
4
5
6
7
8
9
Time ( 4 min )
10
11 12
13
14 15
Decay of primed state – Different amounts of priming
Short term
Electrons
L P 6.3 mGy
L P 114 mGy
L P 650 mGy
L P 650 mGy
L P 840 mGy
L P 2.5 Gy
110
Centroid
100
1600
80
70
NL P 114 mGy
NL P 114 mGy
NL P 650 mGy
NL 650 mGy
60
NP
1400
L under light (400 nm)
P after priming
NL No lignt
NP No Priming
1200
1000
800
600
NL P 114 mGy
NL P 646 mGy
400
200
NP NL
0
40
0
2000
1800
90
50
NL
L P 6,3 mGy
L P 114 mGy
L P 646 mGy
L P 836 mGy
L P 2508 mGy
NP L
2200
Counts ( integral )
120
Electrons
L under light (400 nm)
P after priming
NL No lignt
NP No Priming
Electons
130
2
4
6
8
Time ( 4 min )
10
12
14
16
0
2
4
6
8
Time ( 4 min )
10
12
14
16
Decay of primed state – Long term
Electrons and holes
Electrons under light (400 nm)
Holes after priming (4,2 Gy)
130
2600
2500
2400
120
2300
2200
Counts ( integral )
110
Centroid
100
90
80
2100
2000
Electrons underr light (400 nm)
Holes after priming (4,2 Gy)
1900
1800
1700
1600
1500
1400
70
1300
1200
60
0
3
6
9
12
15
18
21
Time ( hours )
24
27
30
33
0
3
6
9
12
15
18
21
Time ( hours )
24
27
30
33
Effect of white light on electron collection
Tungsten lamp, no interf. filter
L under light (400 nm)
P after priming
NL No lignt
NP No Priming
Electrons
130
L P 6.3 mGy
L P 114 mGy
L P 650 mGy
L P 650 mGy
L P 840 mGy
L P 2.5 Gy
110
Centroid
100
L P 6.3 mGy
L P 114 mGy
L P 650 mGy
L P 650 mGy
L P 840 mGy
L P 2.5 Gy
Electrons
1600
1400
NL P 114 mGy
NL P 114 mGy
NL P 650 mGy
NL 650 mGy
90
80
70
Integral ( counts )
120
L under light (400 nm)
P after priming
2000
NL No lignt
1800
NP No Priming
1200
White light NP
1000
800
600
400
NL P 114 mGy
NL P 114 mGy
NL P 650 mGy
NL P 650 mGy
60
200
50
NL
NP
NL
0 NP
White light NP
40
0
0
2
4
6
8
10
Time ( 4 min steps)
12
14
2
4
6
8
10
Time ( 4 min steps)
12
14
Conclusions
•
•
•
•
Only blue light ( at least below 500 nm ) affects the primed state
Holes : blue light reduces the average CCE of the primed state
Electrons : blue light improves the average CCE
After a short transient ( 1 hr ) both X-ray primed state hole contribution to
CCE and blue light continuously primed state electron contribution to
CCE seem stable in time ( 30 hr )
• Blue light priming is different from sample to sample and it is
independent of amount of previous X-ray priming – even better with no
previous priming
• Hole reponse ( PC, alpha spectra ) is sensitive to low amounts of priming
( and it is linear with doses up to few tens of mGy )
Priming effects on photoconductivity
PPC Persistent PhotoConductivity
Dependence of PC on dose priming dose
Priming fills hole traps.
0,014
Normalised signal (a.u.)
After
annealing
at
360°C (restoring of the
unprimed state), the
band
at
2.4
eV
disappears.
It disappears also if the
BGPC measurements
are carried out starting
from higher energy
values.
Why a
Increasing
maximum
at 2.4 eV? photon energy
Increasing
number of
incident photons
Sample A
Unprimed
38 mGy
76 mGy
114 mGy
152 mGy
608 mGy
2724 mGy
0,012
0,010
0,008
0,006
0,004
0,002
0,000
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
Energy (eV)
BGPC increases
Deeper
levels are
involved
More detrapping
BGPC decreases
PC peak position and height depend on
time and on illumination intensity
The sample response depends on its “history”.
Annealing: heating at 360 °C for 60 seconds for three times,
performed in order to restore the starting conditions.
 Surface exposed to b-rays
(growth or substrate).
 Undesired exposure to
room light.
 Time
elapsed
before
starting the measurements.
 Total number of incident
photons (exposure time at
each wavelength).
0.0020
Normalised signal (a.u.)
Factors affecting the BGPC
value:
Without delay
Same conditions, imposing a delay of 60 s
before each measurement
0.0015
0.0010
0.0005
1.75
2.00
2.25
Energy (eV)
2.50
Transient photoconductivity
of primed and unprimed states
Normalised signal (a.u.)
Decay of the primed state under illumination
2,8x10
-3
2,4x10
-3
2,0x10
-3
1,6x10
-3
1,2x10
-3
8,0x10
-4
For energies of the incident
photons between 1.76 and
4.80 eV, the BGPC signal
after the priming decreases
with a decay that can be
fitted using the expression:
exp[(-t/τ)β]
with β < 1.
E = 2.72 eV
Signal
Fit
Effect of the priming:
filling hole traps.
0
1000
2000
3000
4000
5000
Total photon number (a.u.)
6000
Decay of the primed
state:
optical detrapping of
holes
Gain factor
Sample B virgin
Sample A virgin
Sample A 38 mGy
Sample B 38 mGy
0.010
400
300
0.006
200
Gain
0.008
0.004
Ratio Sprimed / Svirgin
Sample A
Sample B
100
0.002
0
2.0
2.5
3.0
3.5
Energy (eV)
4.0
4.5
5.0
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Energy (eV)
The photoconductive gain around 2.0 - 2.1 eV with respect to unprimed case is very
high, more than 300 in the case of sample B. This was observed also for other samples, with
no evidence of dependence on their electronic quality.
Applications in dosimetry
Diamond can be used as a passive solid state dosimeter for bio-medical applications. Its
attractiveness essentially stems from its radiation hardness, chemical stability against all the
body fluids and its absolute nontoxicity.
PC normalised signal (a.u.)
70
Moreover, diamond is to be
considered
as
a
tissue
equivalent material since its
atomic number is close to the
effective atomic number of soft
tissue (5.92 for fat and 7.4 for
muscle).
60
50
10
40
Linearity range
8
30
6
20
4
2
10
0
0
0
100
200
0
300
5
400
Dose (mGy)
10
500
15
600
20
700
Advantages
• both large-area detectors
or miniaturised detectors
• high sensitivity
• good spatial resolution