I am very new too Matlab and how it all works but I am having trouble understanding at what axis the numerical integration is occurring from on the graph that I plotted.(adsbygoogle = window.adsbygoogle || []).push({});

So I am currently doing an experiment in gamma ray spectroscopy and due to issue with the software we found it hard to integrate under a specific photo peak so it was suggest to use Matlab to integrate under the area, but the photo peak is off the x-axis so which I have highlighted in my picture below.

But my issue is I don't know how to integrate that specific area in Matlab, so what I have done so far is cherry pick the data and fit a Gaussian to it which produced the given function and graph.

Firstly here is my data set I picked for the curve

1117.4 7

1120.4 9

1123.4 7

1126.4 13

1129.4 6

1132.4 14

1135.4 15

1138.4 11

1141.4 20

1144.4 16

1147.5 16

1150.5 18

1153.5 33

1156.5 20

1159.5 19

1162.5 24

1165.5 35

1168.5 35

1171.5 43

1174.5 42

1177.5 40

1180.5 38

1183.5 38

1186.5 37

1189.5 34

1192.5 39

1195.5 34

1198.5 25

1201.6 23

1204.6 23

1207.6 19

1210.6 19

1213.6 13

1216.6 10

1219.6 16

1222.6 10

1225.6 6

1228.6 2

1231.6 11

1234.6 11

1237.6 5

1240.6 7

Next here is my code to plot the data

Ignore comment out parts Im currently experimentingCode (Text):dataset = xlsread('Lab 3 Results 11.10.18 (1).xlsx','Sheet3','C390:D431');

x=dataset(:,1);

%y=dataset(:,2);

%plot(x,y,'-')

a1=37.43 ;

b1=1178 ;

c1=39.28 ;

d1=0

y=a1*exp(-((x-b1)/c1).^2)-d1;

%int1=trapz(x,y)

%int2=trapz(y)

plot(x,y)

%xlabel('Energy : KeV', 'FontSize', 15)

%ylabel('Number Of Interactions','FontSize', 15)

this produced the following graph

so now this is the part hat get confusing for me I need to calculate the area in the red but when i used the trapz(x,y) I think it is finding the area of both red and green, my only option at the moment I think is to shift the graph down so it aligns with the x=0 axis and it should integrate just the red area is this correct?

**Physics Forums | Science Articles, Homework Help, Discussion**

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# MATLAB Numerical approximation of the area under curve

Have something to add?

Draft saved
Draft deleted

**Physics Forums | Science Articles, Homework Help, Discussion**