A new rapid neutron activation analysis system at Dalat nuclear research reactor

ology. Accordingly, shorter-
lived nuclides (half-life < 1 min) such as 
20
F, 
77m
Se, 
179m
Hf, 
46m
Sc, and 
110
Ag can be used for 
INAA at Dalat reactor, which the former 
system can not detect. 
The main purpose of this work is to test 
the system for both mechanical and analytical 
reliability. A systematic study has been carried 
out including measurements for timing 
parameters of the system and neutron flux at 
irradiation positions, and the application of this 
system to determining of Selenium in a number 
of biological reference materials for validation 
purpose. 
Fig. 1. Diagram of the auto-pneumatic transfer system installed at DNRR. 
A NEW RAPID NEUTRON ACTIVATION ANALYSIS SYSTEM AT  
86 
II. EXPERIMENT 
A. Timing measurements 
The accuracy of irradiation time of an 
irradiation facility should be checked and 
calibrated as a type of analytical qualify 
control [2]. In this experiment, the absolute 
irradiation time, the sample transferring time 
from entrance to irradiation position of 
aluminum tube (Tin) and the reverse movement 
(Tout) were determined. The experiments were 
done outside the reactor before installation of 
irradiation facilities inside the reactor. The 
arrangement of timing measurements is shown 
in Fig. 2. The setup includes two NaI(Tl) 
detectors placed at the top and bottom sides of 
the irradiation tube. The detectors detect 
gamma radiation pulses from a 
131
I source 
inserted in a capsule while moving inside. The 
counters are set for running in Multi-Channel 
Scaler (MCS) mode; MCS mode records the 
counting rate of events as a function of time. 
The return time from the irradiation to 
the measurement positions was determined by 
a series of irradiation (50 replicates) for a total 
weight (capsule, vial and sample) of about 4.4 
gram and air pressure of 3.1 bars over a 
distance of 40 m for 13-2 channel and 36 m for 
thermal column. The return times were 
recorded at the computer via a signal from the 
fast solenoid providing the return gas (START 
of return time) and from the optical sensor at 
the detector station recording the arrival of 
capsule (END of return time). 
B. Neutron spectrum parameters of irradiation 
position 
The neutron spectrum parameters 
including thermal neutron flux th, fast neutron 
flux f, the thermal flux to epithermal neutron 
flux (epi) ratio were measured at sample 
irradiation positions in channel No.13-2 and 
thermal column using Au, Zr, and Ni monitors. 
Monitors were inserted into a high purity 
polyethylene vial and loaded into rabbit 
(capsule) for irradiation. The activity 
measurements were carried out by a calibrated 
gamma-ray spectrometer combined with HPGe 
detector (GMX-30190). The measured 
spectrum were analyzed by using the k0-IAEA 
program. The irradiation, decay and counting 
times for each monitor are shown in Table I. 
Typically monitors with masses of 4 mg for 
Al-0.1%Au foil (IRMM-530R), 30 mg for pure 
Ni (wire), 10 mg for Zr (foil) were irradiated 
for 10 min at 13-2 channel (2 h at thermal 
column), and the decay time is 1 day for 
97
Zr 
and 3 days for 
198
Au, 
95
Zr and 
58
Co. 
 Fig 2. Arrangement for experiment of timing 
measurements. 
HO VAN DOANH et al. 
87 
Table I. The irradiation, decay and counting times for the monitors. 
Time/position 
irradiation (monitor, 
mass) 
Decay 
time 
Counting time 
(combination) 
Measured 
radionuclides 
(T1/2, -rays in keV) 
10 min/ 13-2 channel 
2 hours/ thermal column 
 (Al-0.1% Au, ~ 4 mg) 
(99.8% Zr, ~ 10 mg) 
(99.98% Ni, ~ 30 mg) 
~ 1 d 1  2 h 
97m
Nb (60 s, 743.4)
*
; 
97
Nb (16.7 h, 657.9) 
~ 3 d 0.5  3 h (5 h) 
198
Au (2.7 d, 411.8); 
95
Zr (64 d, 765.8); 
58
Co (70.8 d, 810.8) 
* Nuclide
 97m
Nb is decayed from nuclide 
97
Zr with half-life of 16.7h. 
C. Determination of Selenium 
A variety of reference materials (Tuna 
Fish IAEA-436, Oyster tissue NIST 1566b, 
Bovine Liver NIST 1577, Bovine Liver NIST 
1577b) were selected to assess reliability of 
this system on the short-time activation 
application. All of the samples were irradiated 
at a neutron flux of 4.2 1012 n.cm-2.s-1 in the 
13-2 channel and counted on the calibrated 
HPGe gamma-ray spectrometer (GMX40-76-
PL). 
In order to evaluate the limit of detection 
of Se in biological samples, two 200mg 
replicates of each material (IAEA 436 and 
NIST 1566b) were weighed and packed in high 
purity polyethylene bags. The samples were 
irradiated for 5, 10, 15, 20, 25, 30, 35 and 40 s. 
After a delay of 3.2 s (including both 
transferring time of sample from irradiation 
position to detector and the time required to 
start the detector). Each sample were counted 
for 20 s at a distance of 10 cm from detector. 
To test accuracy for the analysis of the 
Se concentration in biological reference 
materials, four 200 mg replicates of each 
material (IAEA 436, NIST 1566b, NIST 1577 
and NIST 1577b) were weighed. The samples 
were irradiated for 25 s, allowed 20 s delay 
time to eliminate interference of 
116m
In with a 
half-life of 2.18 s [3, 4]) and counted for 25 s 
at a distance of 10 cm from the detector 
(GMX40-76-PL). The concentrations of 
Selenium were determined by both k-zero and 
relative methods. 
III. RESULTS AND DISSCUSION 
A. Timing measurements 
The results for average transferring time 
of sample from the top to bottom of the 
aluminum irradiation tube (Tin) is (0.628 
0.021) s for the channel No.13-2 irradiation 
tube (a length of 6 m) and Tout is (0.323 
0.030) s (averaged for 90 runs over the three 
days). For thermal column irradiation tube (a 
length of 2.8 m), Tin is (0.248 0.019) s and 
Tout is (0.146 0.004) s, as shown in Table II. 
The result obtained for measuring the return 
time from the irradiation position to the 
measurement position was found to be (3.165 ± 
0.002) s for channel No.13-2. That for thermal 
column was (3.025 0.013) s. It should be 
noted that this timing parameters are included 
in the time required to start the detector after 
receiving the start signal. 
A NEW RAPID NEUTRON ACTIVATION ANALYSIS SYSTEM AT  
88 
Table II. The result of time measurements. 
Irradiation 
position 
The transferring time throughout 
aluminum irradiation tube (second) 
The return time from irradiation 
position to detector position (second) 
Tin Tout This word Manufacturer* 
13-2 channel 0.628 0.021 0.323 0.030 3.165 ± 0.002 3.301 0.013 
Thermal column 0.248 0.019 0.146 0.004 3.025 0.013 3.261 0.022 
* Sample weight:  8 g for thermal column tube and  6 g for 13-2 channel tube, 
operation air pressure:  3.1 bars, distance: 30 meters. 
There are significant differences 
between this work and that of the manufacturer 
in capsule sample weight and distance from 
irradiation position to measurement position. 
Hence, there are differences ( 7%) in the 
result of the return time from irradiation 
position to detector position. However, it is not 
a problem for analytical measurements. 
Results for absolute irradiation time at 
channel No.13-2 and thermal column were 
determined by a series of irradiations ranging 
from 1 to 30 s (3 replicates), as shown in Fig. 3 
and Fig. 4. The relative error of irradiation time 
in the first second is 16.02% for channel 
No.13-2 and 26.43% for thermal column, and 
those for irradiation time of 2 s is 1.5% for 
channel No.13-2 and 4.91% for thermal 
column. The relative error is less than 1% at 
irradiation time of 5 s for channel No.13-2, and 
10 s for thermal column. The large error for the 
first second is due to delay of the system in 
starting the irradiation timer and in ejecting the 
capsule once the “end of irradiation” signal has 
been received. 
This timing delay problem can be 
adjusted through the control unit and the 
software package for managing optimal 
operation and the analytical procedures. 
However, it is not a problem for INAA because 
the time parameters remain unchanged for all 
samples, standards, and control material
Fig. 3. The relative error of irradiation time 
for 13-2 channel. 
Fig. 4. The relative error of irradiation time for 
thermal column. 
0
2
4
6
8
10
12
14
16
18
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Irradiation time, second
R
e
la
ti
v
e
 e
rr
o
r,
 %
0
4
8
12
16
20
24
28
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Irradiation time, second
R
e
la
ti
v
e
 e
rr
o
r,
 %
HO VAN DOANH et al. 
89 
B. Neutron spectrum parameters of irradiation positions 
The results of the determination of 
neutron spectra parameters are shown in Table 
III. This table includes data obtained for the 
thermal, fast neutron flux, the ratio of thermal 
to epithermal neutron flux (th/epi). The 
thermal neutron flux at the irradiation position 
in the channel No. 13-2 is 4.2E+12 n.cm
2
.s
-1
, 
and associated with 0.5 times of epithermal. 
The integral fast neutron flux is 6.61E+12 
n.cm
-2
.s
-1
 for all neutrons above 2.9MeV in 
energy [5], measured using the 
58
Ni(n,p)
58
Co 
nuclear reaction. The thermal neutron flux at 
the irradiation position in the thermal column 
is 1.25E+11 n.cm
2
.s
-1
, associated with much 
lower fast and epithermal neutron flux. 
Hence, thermal column is a useful irradiation 
channel for eliminating interference reactions 
induced by fast neutron, in which sample is 
irradiated in an extremely well thermalized 
neutron field [6]. 
Table III. The results of neutron spectra parameters at irradiation positions in the channel No.13-2 
 and thermal column of DNRR. 
Irradiation position 
th (n/cm
2
/s) F (n/cm
2
/s) epith  / 
13-2 channel (4.2 0.1) x 1012 (6.6 0.9) x 1012 10.7 2.4 
Thermal column (1.24 0.03) x 1011 (8.4 0.5) x 108 195 4 
C. Determination of Selenium 
Finally, measurements of detection 
limits of Se in IAEA 436 and NIST 1566b 
samples were performed. The results for these 
measurements are presented in Fig 4. The 
obtained results confirm that in irradiation 
from 15 s to 25 s at irradiation position of the 
channel No.13-2 coupled with counting for 
roughly 20 s at 10 cm distance from detector, 
the detection limits for Se is within the range 
0.5  0.7 ppm, depending on the sample 
composition. It provides adequate analytical 
sensitivities for Se rapid determination in a 
variety of biological matrices. 
The accuracy for determination of 
Selenium using the short-lived nuclide 
77m
Se 
was evaluated by analyzing a number of 
certified reference materials with different 
levels of Se (IAEA 436, NIST 1566b, NIST 
1577 and NIST 1577b). The agreement 
between measured and certified values was 
generally very good with u-score < 1.64, as 
shown in Table IV. 
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0 5 10 15 20 25 30 35 40 45
Irradiation time, second
d
e
te
c
ti
o
n
 l
im
it
, 
p
p
m
NIST 1566b
IAEA 436
Fig. 4. The detection limits of Se in IEAE 436 and 1566b. 
A NEW RAPID NEUTRON ACTIVATION ANALYSIS SYSTEM AT 
90 
For the determination of the Selenium 
by the instrumental neutron activation analysis, 
the long-lived nuclide 
75
Se or the short-lived 
nuclide 
77m
Se can be used [7]. With the short-
lived nuclide, not only completion times are a 
distinct advantage but analytical sensitivities 
are also improved. The data for procedures are 
listed in Table V. 
Table IV. The results of concentration analysis for Se in biological reference materials. 
Reference 
material 
Certificated 
value 
(in ppm) 
k-zero method The relative method 
This work 
(in ppm) 
u-score 
This work 
(in ppm) 
u-score 
IAEA 463 4.63 0.48 4.55 0.50 0.12 4.19 0.46 0.66 
NIST 1566b 2.06 0.15 2.48 0.57 0.71 2.18 0.42 0.27 
NIST 1577 1.10 0.10 1.24 0.31 0.43 1.17 0.22 0.29 
NIST 1577b 0.73 0.06 0.70 0.11 0.24 0.80 0.17 0.39 
Table V. Parameters were used for INAA analysis of Selenium in biological sample by 
using 
77m
Se and 
75
Se isotopes. 
Radionuclide 
75
Se 
77m
Se 
Half-life 120 d 17.4 s 
Activation 20 h at 3.5 x 
10
12 
(n/cm
2
/s) 
1525 s at 4.2 
x 10
12 
(n/cm
2
/s) 
Decay time 20 d 20 s 
Counting time 23 h 25 s 
Detection limit 1.4 ppm 0.6 ppm 
Sample: IAEA 
436 
4.63 ± 0.48 
ppm 
4.63 ± 0.48 
ppm 
The results 4.35 ± 1.1 ppm 4.19 ± 0.46 
ppm 
IV. CONCLUSION 
A fast pneumatic sample transfer system 
for analyzing of extremely short-lived nuclides 
by neutron activation analysis has been 
installed and operated at Dalat nuclear research 
reactor. In this study, time parameters of the 
system were calibrated, thereby reducing 
irradiation time to seconds with precision. 
Neutron spectra parameters of the thermal 
column and channel No.13-2 were also 
determined in order to establish analytical 
procedures using the k0-NAA method. The 
system was applied to determine the 
concentration of Se in the biological sample by 
using the short-lived nuclide 
77m
Se. The results 
obtained through this research have opened a 
new possibility on using INAA technique for 
measurement of extremely short-lived nuclides 
at Nuclear Research Institute. 
HO VAN DOANH et al. 
91 
ACKNOWLEDGEMENTS 
This project was carried out under the 
nuclear research and development program of 
the Ministry of Science and Technology, 
Vietnam. 
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